R_inacon - Hsdpa - Dd & Se v2.1

HSDPA Principles

The Physical Layer of HSDPA

Forward and Backward Error Correction in HSDPA

HSDPA Protocol Enhancements and Extensions

HSDPA Mobility Procedures

ObjectivesAfter this Lecture the Student will be able to:

Describe the important changes and characteristics of HSDPA

State the new channels of HSDPA and how they operate in principle

Describe the extended UTRAN protocol stack with HSDPA

State the advantages and disadvantages of HSDPA and future enhancements

Describe the concept of HSUPA

INACON GmbH 1999 - 2006. All rights reserved. Reproduction and/or unauthorized use of this material is prohibited and will be prosecuted to the full extent of German and international laws.
http://www.inacon.de/http://www.inacon.de/

HSDPA (High Speed Downlink Packet Access) for WCDMA

HSDPA considers the trend that the volume of IP-based traffic has already exceeded that for circuit-switched traffic in most fixed networks. The same change can be anticipated in mobile networks because new IP-based mobile services become available and are used by increasing number of people in their daily communication. Current estimates show that in advanced mobile communication markets, packet-switched traffic will overtake circuit-switched traffic in the near future. Delivery of digital content over mobile networks will generate additional traffic and revenue.

Feature Study

The HSDPA feature in 3GPP Release 5 is the result of a study carried out in the Release 4 time frame. This study considered a number of techniques in order to provide instantaneous high speed data in the downlink.

Some of the considerations and goals taken into account in the evaluation of the different techniques were:

To focus on the streaming, interactive and background services: services which require a constant and/high throughput or low error rate.

To prioritize urban environments and then indoor deployments (but not limited to these environments and supporting full mobility). To enable compatibility with advanced antenna and receiver techniques: transmit and receive diversity methods are used and

might be enhanced To take into account mobiles processing time and memory requirements: ( UEs limitations are taken into account by the network To minimize changes on existing techniques and architectures: modest changes to NodeB hardware and UTRAN software

Compatibility with Release 99

HSDPA is designed to co-exist on the same carrier as the current Release 99 WCDMA services, enabling a smooth and cost-efficient introduction of HSDPA into existing WCDMA networks.

Demand for Packet Switched Traffic

The increasing demand for capacity in order to provide high data rate multimedia services in wireless environments necessitates enhanced radio transmission techniques and network protocol functionality. Such techniques have to be added to already existing mobile cellular networks. For 3rd generation UMTS networks based on WCDMA, the HSDPA is being introduced to meet this demand and improve spectral efficiency by higher order modulation using 16-QAM.

Note: HSDPA achieves gross data rates in downlink up to 14 Mbit/s under ideal conditions. The reverse link (uplink) may remain on 64 kbit/s until at a later stage new features are introduced with HSUPA (High Speed Uplink Packet Access). However, HSUPA will require new mobile terminals and PC-cards to operate with. Except where otherwise indicated, the description of this course only applies to the FDD mode of UMTS.


HSDPA Targets

Higher Data Rates for Streaming-, Interactive- and Background Services

HSDPA is a feature based on a downlink shared channel that allows user net-data rates of up to 10 Mbit/s. It is designed to support services that require instantaneous high rates in the downlink and lower rates on the uplink. This feature also decreases the level of retransmissions (at the radio link and hence higher layers), in turn allowing the reduction of delivery time. Examples of end-user services targeted by HSDPA are internet browsing and video on demand.

Consideration of UE Processing Time and Memory Requirements

HSDPA takes UE limitations like available physical memory for transmission and especially for combining with retransmission into account. Also the physical channel processing capability is considered. (Examples: Minimum inter-TTI interval, maximum number of transport channel bits per TTI)

The terminals can be grouped in categories from 1 to 12. The UEs can firstly be distinguished by the amount of physical channels they support, further distinction is possible by the maximum number of TB size and total number of soft channel bits available for soft combining.

Higher Spectrum Efficiency

With 16-QAM applied in downlink, throughput rates can be doubled compared to QPSK which is used for Rel. 99 and Rel. 4 physical channels. The amount of bits/Hz is increased with 16-QAM as one modulation symbol corresponds to 4 bits whereas in QPSK one modulation symbol represents 2 bits. Even when HSDPA uses QPSK modulation the spectrum efficiency increases as HSDPA exploits good downlink C/I conditions. This is achieved by reducing the amount of FEC (( increasing the code rate) and thus having more capacity for the application data.

Small Changes to existing Techniques and Architectures

HSDPA minimizes the necessary upgrades and changes in UTRAN and UE. Nevertheless some protocol additions are necessary in NodeB and UE as well some enhancements of existing procedures and protocols. They are mentioned in the following pages and are explained in full detail in chapter 4.

Efficient Resource Sharing in Downlink among Users

HSDPA introduces a new transport channel type that makes efficient use of valuable radio frequency resources. Beside this, it takes into account the bursty nature of packet switched data by sharing the channelization codes, transmission power and infrastructure hardware among users.

Statistical multiplex gain is obtained as HSDPA shares the physical resources in a time and/or code multiplex manner among the users. If a user requires a high peak throughput, all available physical resources can be allocated for a short time.


HSDPA Characteristics

High Speed Downlink Packet Access comes with certain characteristics which distinguishes it clearly from Rel. 99 UTRAN.

Modulation Types

QPSK is already known from Rel. 99 UTRAN. Besides QPSK, HSDPA incorporates the 16-QAM modulation to increase the peak data rates for users served under favorable radio conditions. Support for QPSK is mandatory, though the support for 16-QAM is optional for the network and UE.

16-QAM (Quadrature Amplitude Modulation) was newly introduced in Rel .5 with HSDPA. It is a so called higher order modulation which basically doubles the data rate in good radio conditions. Thus it increases the spectrum efficiency of WCDMA.

Higher Throughput Rates

HDSPA supports peak throughput rates far beyond 2 Mbit/s when radio conditions are suitable and therefore it satisfies the demand for instantaneous high throughput of packet switched services e.g. streaming or interactive traffic class. Theoretically, under optimum condition (code rate R of 1:1) 14.4 Mbit/s can be achieved. However more realistic are throughput rates in the order of 10.8 Mbit/s or lower if radio conditions are unfavorable.

AMC (Adaptive Modulation and Coding)

AMC is a key feature of HSDPA allowing adjustment of modulation between QPSK and 16-QAM according to radio conditions and retransmission ratio. In addition a variable code rate is used to flexibly adapt the data rate to the physical channel capacity depending on the UEs downlink C/I.

Hybrid ARQ

HARQ functionality combines retransmissions with the original transmissions. There a two different ways for HARQ to operate. Either identical retransmissions of the data block are sent or retransmissions are not identical and differ in data and parity bits compared to the original transmission. The latter method is referred to as incremental redundancy and the first one as chase combining. HARQ operates on a N-SAW (N-channel Stop and Wait) principle. The N-SAW is defined by the amount of processes or HARQ-processes set up by the NodeB.

Transmission and Retransmission Scheduling in NodeB

All Rel. 99 transport channels are terminated at the RNC, except BCH; hence the retransmission procedure for packet data is located in the SRNC. In order to maximize throughput and reduce delays when retransmitting, additional intelligence is put into the NodeB. In this way, retransmissions in HSDPA are controlled by the NodeB, leading to faster retransmission and therefore shorter delay for packet data services.

A packet scheduler in NodeB evaluates for different users what the radio channel conditions are, how much data is pending for each user, how much time has passed since a particular user was last served and for which user retransmissions are pending etc. From this input data the scheduler in NodeB may derive a decision when and how many resources should be assigned to certain users.

[3GTR 25.213 (5.1), 3GTR 25.848 (5.5)]


QPSK versus 16-QAM Modulation

The figure illustrates the I/Q Plane for QPSK and 16-QAM modulation technique. The bit to symbol mapping is done according to the Gray Code. In case of wrong estimation of the symbol, only one bit is wrong. However 16-QAM requires a more accurate phase estimation in the first place.

An increase of the transmission rate in bandwidth limited frequency spectrum can be achieved through digital transmission systems where both the amplitude and the phase of a high frequency carrier are modulated ( amplitude-phase-keying ( 16-QAM.

16-QAM combines phase and amplitude keying ==> quadrature amplitude modulation

QPSK

Each symbol corresponds to 2 consecutive input bits. The four symbols are represented by different phase shifts in the I/Q plane.

Spectrum efficiency: 2 bits / (modulation symbol and Hz) All constellation points lie on the same circle, thus a constant amplitude for all 4 modulation symbols.

16-QAM

Each symbol corresponds to four consecutive input bits. Thus the data rate can be doubled with 16-QAM compared to QPSK. The 16 symbols are represented in the I/Q plane by different phase shifts and amplitudes.

Spectrum efficiency: 4 bits / (modulation symbol and Hz) The amplitude-phase keying 16-QAM requires compared e.g. 16-PSK less power in order to achieve the same BER probability.

However this is compromised by increased complexity due to the amplitude modulation information. 16-QAM achieves the same BER probability like 16-PSK but needs about 3.5 dB less power because in 16-QAM the constellation

points are further apart in the I-Q-plane compared to 16-PSK.

In 16-QAM modulation the symbol value is determined by phase and amplitude. Compared to that, in QPSK the phase is only modulated and variation in amplitude have only minor influence on the decision space in the I/Q diagram. However with 16-QAM the decision space is heavily influenced by amplitude variations, thus higher constraints are put on the transmitter linearity. Note, a more accurate phase estimate is necessary with 16-QAM since constellation points have smaller differences in phase domain compared to QPSK.

Note: The number of constellation points in the I/Q-diagram can be calculated with 2m, where m represents the number of bits or chips per modulation symbol. QPSK modulation has four constellation points in the I/Q-diagram: 2^m = 4 ( m = 2. 16-QAM modulation has 16 constellation points in the I/Q-diagram: 2^m = 16 ( m = 4

[3GTS 25.213 (5.1), Erich Pehl (digitale und analoge Nachrichtenbertragung)]


Maximum Throughput Rates

UMTS Rel. 99 / Rel. 4

Downlink:

2 x [3.84 Mchips/s / (4 chips/bit)] x (3 OVSFs) = 5.76 Mbit/s

Uplink:

1 x [3.84 Mchips/s / (4 chips/bit)] x (2 x 3 OVSFs) = 5.76 Mbit/s

HSDPA Rel. 5

QPSK:


16-QAM:


The formulas above consider the physical maximum throughput rates available in FDD mode of WCDMA according to releases and modulation. In Rel. 99 and Rel. 4 only QPSK is used whereas Rel. 5 allows also to user higher order modulation scheme 16-QAM.

The standard chip rate is 3.84 Mchips/s across all releases. The slot duration is in all releases 0.67ms corresponding to 2560 chips.

Note that the physical maximum chip rate achievable on I and Q plane with QPSK modulation is: 2 x 3.84 Mchips/s as QPSK.

UMTS Rel. 99 / Rel. 4

Downlink: In downlink DPDCH and DPCCH are time multiplexed onto I and Q plane so the DPDCH data rate also depends on the DPCCH data

rate. The physical maximum bit rate (ignoring losses due to DPCCH) using spreading factor 4 is 5.76 Mbit/s. The downlink slot format # 16 allows for 1248 DPDCH bits/slot and per physical channel. The maximum DPDCH data rate considering 3 spreading codes @ sf4 is: 1248 bits/slot x 15 slots x (3 OVSFs) = 5.616 Mbit/s Uplink: In uplink each channel DPDCH and DPCCH is assigned an orthogonal channelization code. As both physical channels are I/Q

multiplexed, means separated onto I and Q, the maximum physical data rate has to be calculated with 1 bit/symbol. The uplink slot format # 6 allows for 640 bits/slot. The maximum DPDCH data rate considering 6 spreading codes @ sf4 is therefore: 5.76 Mbit/s.

HSDPA Rel. 5

In HSDPA the spreading factor for the user plane is fixed to 16. Thus up to 15 physical channels can be allocated at maximum per UE.

QPSK:

The downlink slot format # 0 allows for 320 bits/slot and per physical channel. This results in 960 bits/ 2 ms subframe. The maximum physical data rate considering 15 spreading codes @ sf16 is: 7.2 Mbit/s.

16-QAM:

The downlink slot format #1 allows for 640 bits/slot and per physical channel. This results in 1920 bits/ 2 ms subframe. The maximum physical data rate considering 15 spreading codes @ sf16 is: 14.4 Mbit/s.

[3GTS 25.211 (5.2.1, 5.3.2, 5.3.3.13)]


Hybrid ARQ Techniques

ARQ (automatic repeat request) is basically a mechanism which allows the receiver to request retransmissions of erroneous received packets.

Hybrid ARQ schemes (later only called HARQ) can be classified as type I, type II and type III depending on the level of complexity employed in their implementation.

Type I Hybrid ARQ Low UE Complexity

The ARQ method used in Rel. 99 / Rel. 4 specifications is referred to as HARQ type I. In this basic HARQ type I a CRC is added to the TB and the data is encoded with a forward error correction (FEC) code. In the receiver the FEC code is decoded and the quality of the packet is checked (CRC check). If there are errors in the packet, a retransmission of the packet (RLC-PDU) is requested. The erroneous packet is discarded and retransmissions use the same coding as the first transmission. In EGPRS for instance LA (link adaptation) is used beside IR for retransmission of erroneous packets. LA means adjusting the MCS in case of retransmission. Basically a more robust MCS could be used to increase the likelihood of successful transmission.

Type II Hybrid ARQ High UE Complexity

In this HARQ scheme, also denoted as full IR (incremental redundancy), retransmission requests only consist of parity bits. The original transmission must be self-decodable, but the retransmissions are not and typically not identical with the original transmission. The receiver combines additional parity bits from retransmission with bits of the first transmission resulting in lower code rates, before decoding is attempted. The basic difference between type I and type II is that when retransmitting a coded block with HARQ type II, each retransmission is punctured using a different puncturing pattern than the original coded block. Thus the level of redundancy is increased for each retransmission. Type III is a variant of type II wherein each retransmission is self-decodable.

Type III Hybrid ARQ Medium UE Complexity

Like type II, type III hybrid ARQ also belongs to the IR ARQ schemes. This means that retransmissions concerning one PDU are not discarded but kept at the receiver for combination with additional information before decoding. In situations where the original transmitted PDU is severely damaged, for example, due to interference, it is desirable to have a scheme where any additional information sent is self decodable. In type III HARQ each retransmission is self-decodable but different redundancy bits are retransmitted in various transmissions of the same PDU. Thus, the data can be recovered from the retransmitted packet even without combining with previous transmission instances if it is transmitted with sufficient quality.

Chase combining is yet a special form of type III HARQ where each transmission instance contains the same bits like in the original transmission.

Note: Both methods chase combining and IR perform combining of original transmitted and retransmitted packets based on soft decision bits. This means the combining is made by simply adding soft bits after equalization. Chase combining involves the retransmission of the same coded block in each retransmission, whereas in IR, instead of simple repetitions, additional redundancy (parity bits) information is transmitted with each retransmission until the base code rate is reached.


Turbo Coding Systematic Bits, self-decodable and non-self-decodable Transmission

In case of turbo coding one speaks about a self-decodable and non-self-decodable transmission. Turbo coding follows the systematic block coding as the systematic bits (input bit = transport block bits) are part of the output. A transmission is self-decodable whenever the information word (transport block bits) is part of the code word. The rest of the turbo coded bits consists of the added redundancy bits. One speaks of a non-self-decodable transmission when the systematic bits are missing, thus the code word only consists of redundancy bits.

Code Rate R = input bits before turbo coding / output bits after turbo coding and transmitted

Full IR

1.) The first transmission must be always a transmission with the systematic bits inside. Therefore the 100 systematic bits are contained in numerator and denominator.

Code Rate R = 100 / (100 + 25 + 25) = 100/150 = 2/3

2. The retransmission contains only of parity P1 and parity P2 bits. Thus the denominator is increased by 75 P1 bits and 75 P2 bits.

Code Rate R = 100 / (100 + 25 + 75 + 25 + 75) = 100/300 = 1/3

Partial IR

1. Code Rate R = 100 / (100 + 25 + 25) = 2/32. Compared to Full IR, further retransmission with partial IR contain each time the systematic bits in the denominator. However, from

coding rate point of view the systematic bits must not be counted more than once as they do not change with increasing number of retransmission.

Code Rate R = 100 / (100 + 50 + 25 + 25) = 100 / 200 = 1/2

3. Code Rate R = 100 / (100 + 50 + 50 + 25 + 25) = 100 / 250 = 2/54. Code Rate R = 100 / (100 + 50 + 50 + 50 + 25 + 25) = 100 / 300 = 1/3

Chase Combining

The code rate R never changes as every retransmission is identical to the original transmission thus neither parity information is increased nor the systematic bits change. The receiver simply benefits from the fact that the fading changes with retransmissions and the likelihood values of retransmissions step by step increase the probability of turbo decoding.

Code Rate R = 100 / (100 + 25 + 25) = 2/3


Important Changes with HSDPA

HSDPA involves significant changes in the UTRAN providing a high flexibility to react upon changing air-interface conditions or variable user QoS.

New 2 ms Subframe for HSDPA

The TTI (transmission time interval) in HSDPA has been shortened to 2 ms in order to be faster in retransmitting erroneous data blocks compared to the minimum TTI of 10 ms in genuine UTRA-FDD. Another advantage of the shorter TTI in HSDPA is that NodeB can adapt literally every data block to fast changing radio conditions by the means of AMC. Thus it is possible to counteract the fading on the air-interface by adjusting modulation and coding almost every 2 ms depending on NodeBs processing delay and packet scheduling algorithm.

New Physical Channels and Transport Channel with HSDPA

New channels are introduced for HSDPA: HS-PDSCH, HS-SCCH, HS-DPCCH and HS-DSCH.

No Fast Power Control and variable Spreading Factor

With HSDPA, two of the most fundamental features of WCDMA, fast power control and variable spreading factor are disabled and replaced by AMC and HARQ. Note: AMC uses extensive multicode operation ((the UE can use more than one channelization code in parallel) in order to increase the data rate for a certain user and adapts the code rate to the air-interface quality. By these means AMC is able to improve the user throughput or at least keep it constant even the downlink channel quality deteriorates between subsequent transmissions.

New UE Capabilities / Categories

The HSDPA feature is optional for both UE and network in Rel. 5. The UE indicates its HSDPA support and its HS-DSCH physical layer category within the radio access capability parameter. The physical layer category defines among other parameters the maximum number of channelization codes the UE supports in parallel for multicode operation. A UE may support up to 5, 10 or at max. 15 channelization codes in parallel.

New MAC-hs in NodeB and UE

The implementation of MAC-hs (MAC high speed) in NodeB and UE is a pre-requisite for allowing the NodeB to schedule transmissions and retransmissions, to maintain the HSDPA specific channels and to operate with AMC and HARQ.

Impact on NBAP and Frame Protocol Procedure

NBAP procedures need to support HSDPA capability and HSDPA related IEs. The increased bandwidth needs to be supported by a new frame protocol. Among other parameters the frame protocol needs to cater for HSDPA flow control information, priority queue handling and UE capability information.

[3GTS 25.211 (7.1), 3GTS 25.306 (5.1), 3GTS 25.308 (5.1)]


New Channels with HSDPA

The support of HSDPA is based on several new physical channels and one new transport channel.

Physical Channels

HS-PDSCH (High Speed Physical Downlink Shared Channel)

The HS-PDSCH has a fixed spreading factor of value 16. Thus, it provides for multicode operation using up to 15 channelization codes in parallel. Of course the UE must support the use of up to 15 channelization codes which depends on its category. The HS-PDSCH adopts the shortened TTI of 2 ms.

HS-SCCH (High Speed Shared Control Channel)

The HS-SCCH has a fixed spreading factor of value 128 and is configured only in the downlink direction. It also adopts the shortened TTI of 2 ms. In theory, up to 127 HS-SCCHs can be configured in a cell. However, the UE is required only to be able to listen to up to four HS-SCCH in parallel.

The HS-SCCH allows the efficient sharing of one or more HS-PDSCHs among different users. Nevertheless every UE needs to be informed on the DCCH via RRC messages about the specific HS-SCCH-set that it shall monitor in order to receive data via the HS-PDSCHs.

HS-DPCCH (High Speed Dedicated Physical Control Channel)

The HS-DPCCH has a fixed spreading factor of value 256 and is only configured in uplink direction. The HS-DPCCH also follows the shortened TTI of 2 ms. Its purpose is to provide feedback information about the downlink receive quality and whether the packet data received by the UE are error-free or need to be retransmitted. Thus the NodeB is quickly notified of unsuccessful transmissions and/or changing radio conditions in downlink direction.

Transport Channel

HS-DSCH (High Speed Downlink Shared Channel)

The HS-DSCH is the actual transport resource carrying the packet data of the user applications. As it also follows the shortened TTI of 2 ms, it allows for short round trip delay in the operation between NodeB and UE. The 2 ms TTI is short when compared to 10, 20, 40 or 80 ms TTIs supported by Rel. 99 and Rel. 4 transport channels. HS-DSCH describes the physical layer processing by MAC-hs of a HSDPA transport block.

Dynamic part: TB size = TBS size {1 to 200 000 bits with 8 bit granularity}; modulation scheme {QPSK, 16-QAM}; redundancy / constellation version {1 8}.

Static part: TTI {2 ms for FDD}; type of channel coding {turbo coding}; mother code rate {1/3}, CRC size {24 bits} No semi-static attributes are defined for HS-DSCH.

[3GTS 25.211 (4.1.2.7, 5.2.1, 5.3.3.12, 5.3.3.13), 3GTS 25.213 (4.2.1, 4.3.1.2), 3GTS 25.302 (7.1.6a)]


Multicode Operation in HSDPA

The figure shows two UEs operating in HSDPA.

HS-SCCH-set Decoding

The graphic demonstrates that both UE's have to decode their assigned HS-SCCH-set first, before they can attempt to decode the HS-PDSCHs. For simplicity reasons, both UEs have the same HS-SCCH-set assigned. A HS-SCCH-set consists of up to a maximum of four HS-SCCH channelization out of a possible range from codes C(ch,128,1) C(ch,128,127) under e.g. the primary scrambling code. As depicted, only one of the four HS-SCCHs contains valid information per UE per TTI. This is indicated by the appropriate color coding for each UE. All the information necessary for demodulating the related HS-DSCH subframe which follows always 2 slots later after HS-SCCH, is transmitted to the UEs within part 1. It can be seen that every HS-SCCH is (logically) divided into two parts. The second part contains the necessary information on how to decode the HS-DSCH. So part 1 and part 2 serve different purposes. Part 1 allows the demodulation of the HS-PDSCH subframe and part 2 is responsible for layer 2 decoding of the HS-DSCH.

Note: The HS-SCCH is intended for the very UE once it recognizes its UE-id inside part 1 of the HS-SCCH subframe. If the UE detected consistent control information intended for it in the immediately preceding subframe, it is sufficient to only monitor the same HS-SCCH used in the immediate succeeding subframe. In the graph this is indicated by e.g. user 1 in slot 0 to slot 6 where user 1 gets two consecutive valid HS-SCCHs. Therefore UE 1 only needs to decode the same HS-SCCH from slot 3 onwards. From slot 8 onwards the complete HS-SCCH code-set has to be monitored again by UE 1 (only part 1).

HS-DSCH Demodulation

If a UE detects that one of the monitored HS-SCCHs contains its encoded UE-id (implicitly included) and consistent control information intended for this UE, the UE prepares to receive the HS-PDSCHs. Consistent control information hereby means that modulation scheme and HS-PDSCH channelization code-set info are valid according to the UEs capability. The UE has about one slot duration time after receiving part 1 to prepare for HS-PDSCHs reception. As already mentioned, the UE indicates via the category parameter if it supports up to 5, 10 or 15 HS-PDSCH channelization codes in parallel. The color coding used in the figure for the HS-SCCH and their related HS-DSCH shows that HSDPA allows for time multiplexing and code multiplexing of the HS-PDSCHs. Time multiplexing means that user 1 and user 2 get the HS-PDSCHs assigned one after the other in different subframes. Code multiplexing or multicode operation means that several user, here user 1 and user 2, use different HS-PDSCHs within the same subframe. The various HS-PDSCHs are separated by different channelization codes.

[3GTS 25.212 (4.6.2), 3GTS 25.213 (5.2.1)]


HSDPA Basic Operation

The figure consists of three functional parts. The upper part shows the basic communication between UE and NodeB via MAC-hs for packet transmission and retransmission. The lower left part depicts the Uu-interface together with the newly introduced physical channels and transport channel. Note for simplicity reasons the red colored arrows represent the individual HS-DPCCHs per UE.

In the lower right corner we sketched a basic scheduling principle how the HS-DSCH resources can be assigned among several UEs.

MAC-hs

MAC-hs located in NodeB receives user packets from the SRNC. MAC-hs is responsible for transmission and in case of erroneous reception also for retransmission of user packets. The retransmission of user packets is commanded by NodeBs MAC-hs which represents the fundamental change in HSDPA compared to Rel. 99 or Rel. 4. In legacy UMTS releases retransmissions are always performed between the RLC peers in UE and SRNC. With HSDPA the NodeB retransmits the user packets if the UE indicates a Nack on HS-DPCCH. Via this physical channel the UE also sends feedback information about the downlink channel quality in regular intervals to the NodeB.

New Channels

Two UEs receiving user packets via the HS-DSCH. The physical channels in uplink and downlink are necessary to firstly signal which UE shall decode the very HS-DSCH. This is indicated by implicitly encoding the UE-id in the very HS-SCCH. Secondly after decoding of the user packet, the respective UE has to signal the successful or unsuccessful transmission to the MAC-hs entity in NodeB via the so called Ack/Nack description. Therefore HSDPA employs the uplink HS-DPCCH to signal the downlink reception quality and the Ack/Nack description. The downlink reception quality informs the NodeB about the current radio condition which serves as a vital input for the NodeB. The NodeB is therefore able to derive the proper modulation scheme and code rate for transmission and retransmission. This process is denoted as AMC in HSDPA.

UE Scheduling

The NodeB also contains a scheduling/priority handling function which determines whether a new transmission or retransmission shall be performed. The green and blue lines represent the changing downlink channel quality reported by UE in the uplink. One scheduling method which could be implemented in NodeB is to serve each UE according to the reported downlink channel quality and therefore always exploit best radio conditions. This method maximizes user throughput as it allows to use 16-QAM and an aggressive code rate if C/I is high. Another option is to serve each user proportionally fair despite unfavorable downlink radio conditions. This benefits especially UE 2 which indicates for several TTIs a bad radio quality, but this second method assures at least a minimum guaranteed throughput for UE 2.


HSDPA Packet Switched Protocol Stack

The protocol stack of HSDPA can be divided like it is done for legacy UMTS releases in control plane and user plane function. Note that HSDPA is only meant to operate in the packet domain of UMTS because of the use of shared resources preventing real time traffic. However, real time service could be handled in parallel via the circuit switched domain beside HSDPA. Note that the transport layer in UTRAN might be based solely on IP only in Rel. 5.

HSDPA Control Plane

The packet-switched control plane is used for the exchange of control information related to packet-switched services between UE and UTRAN as well between UE and Core Network. In addition, the packet-switched control plane can be used for the exchange of short messages. With the introduction of HSDPA the Rel. 99 / Rel. 4 protocol stack remains nearly unchanged except that a new MAC-hs (high speed MAC) is introduced in NodeB.

Access Stratum Protocols

The Access Stratum (AS) protocols are all protocols within the rectangle that is defined by the RRC-protocol (Radio Resource Control) at the upper left corner, the MAC-protocol (Medium Access Control) at the lower left corner and the RANAP-protocol (Radio Access Network Application Part) at the upper right corner.

Non Access Stratum Protocols

The Non Access Stratum (NAS) protocols are GMM (GPRS Mobility Management), SM (Session Management), and GSMS (SMS over packet-switched domain).

Note: Note that for HSDPA, the control plane signaling has to be extended in order interact and inter-connect with MAC-hs. HSDPA brings new IEs and procedures (e.g. HS-DSCH cell change) thus NBAP, RNSAP and RRC signaling have to be extended.

MAC-hs

The high speed MAC controls the operation of the shared resources through new the signaling control channels HS-SCCH and HS-DPCCH. Please note that the HS-SCCH is a point to multipoint control channel received by all UEs which are in HSDPA standby and having the same HS-SCCH channelization codes assigned. On the other side the HS-DPCCH is a point to point signaling channel which is unique per UE as it is scrambled by UEs uplink scrambling code. MAC-hs is located in NodeB and gets configured for each UE through NBAP messages only extended by HSDPA specific IEs (Rel. 5 information elements). The SRNC setups HSDPA in UE via RRC signaling messages. Both UE and SRNC communicate via Rel. 99 / Rel. 4 RRC messages which are extended by HSDPA specific IEs (Rel. 5 information elements).

[3GTS 25.308 (5.1), 3GTS 25.412]


HSDPA User Plane with MAC-c/sh

The support of the new HSDPA capabilities requires updates of the protocol architecture of Rel. 99. These changes will be implemented as additional protocol capabilities in order to preserve backwards compatibility. This and the next page explain the relationship between MAC-hs, MAC-c/sh and MAC-d.

Protocol structure with HSDPA

The HS-DSCH should be able to operate in an environment where some cells are not updated with HS-DSCH functionality. The PDCP, RLC and MAC-d layers remain unchanged from the Rel. '99 and Rel. 4 architecture. However, they need to be extended in order to support HSDPA.

RLC can operate in either AM (Acknowledged Mode) or UM (Unacknowledged Mode) but not in TM (Transparent Mode) due to ciphering. In TM mode ciphering needs to be performed by MAC-d. MAC-hs is located in the NodeB and thus would require the transfer of the ciphering key via the Iub-interface. Additionally, retransmissions of the same RLC-SDU would be ciphered differently and complicate the backward error correction methods.

PDCP can be configured either to perform or not to perform header compression.

The new functionalities of Hybrid ARQ and HS-DSCH scheduling are included in the MAC layer. In UTRAN, these functions are included in a new MAC-entity called MAC-hs which is located in the NodeB.

Configuration with MAC-c/sh

The MAC-hs is implemented in NodeB and communicates with the MAC-c/sh in the CRNC. MAC-c/sh shall provide functions to the HS-DSCH identical to those provided for the DSCH in the Release '99. The HS-DSCH FP will handle the data transport from SRNC to CRNC (if the Iur interface is involved) and between the CRNC and the NodeB.

MAC-c/sh entity UTRAN Side

MAC-c/sh covers functionalities like scheduling/priority handling, target channel type field multiplexing, UE-id multiplexing, transport format combination selection in downlink. A flow control function exists towards MAC-hs to limit buffering between MAC-hs and MAC-c/sh entities. It reduces discarded and retransmitted data as a result of HS-DSCH congestion or HS-DSCH cell change in UM Mode as in any moment in time only the necessary amount of data are being transferred to MAC-hs by flow control. The flow control between MAC-c/sh and MAC-hs operates independently from MAC-d. Therefore a separate flow control also exists between MAC-c/sh and MAC-d.

Note: The configuration with MAC-c/sh allows for flow control functionality in the CRNC. Its implementation depends on the network setup. In any case, it has been specified due to compatibility reasons with Rel. 99 DSCH. If Iur interface is not present (not needed), CRNC and SRNC are identical. Then MAC-c/sh and MAC-d are located in the same RNC.

[3GTS 25.321 (4.2), 3GTS 25.308 (5.1, 6.1, 6.2)]

http://www.inacon.de/file:///F|/out/linker/PDCP/4296495796214.htmfile:///F|/out/linker/PDCP/4296495796214.htmhttp://www.inacon.de/

HSDPA User Plane without MAC-c/sh

Configuration without MAC-c/sh

In this case, the CRNC does not have any user plane function for the HS-DSCH. MAC-d in the SRNC is located directly above MAC-hs in the NodeB, i.e. in the HS-DSCH user plane the SRNC is directly connected to the NodeB via the HS-DSCH Frame Protocol, thus bypassing the CRNC.

MAC-d entity UE Side

MAC-d has a connection to the MAC-hs entity. This connection is used to receive data from the HS-DSCH transport channel which is handled by MAC-hs.

MAC-hs entity UE Side

MAC-hs handles the HSDPA specific functions like HARQ, data queue distribution, reordering of MAC-hs PDUs and disassembly of MAC-hs PDUs.

MAC-d entity UTRAN Side

MAC-d covers the following functionality: transport channel type switching, channel type field multiplexing, ciphering/deciphering for RLC TM mode, DL scheduling / priority handling and flow control.

A flow control function exists towards MAC-hs transparently passing through a CRNC, if present.

MAC-d is responsible for the mapping of dedicated logical channels onto the available dedicated transport channels or routing the data received on a DCCH or DTCH towards MAC-hs.

MAC-hs UTRAN Side

There is one MAC-hs entity in the NodeB for each cell that supports HS-DSCH transmission. The MAC-hs is responsible for handling the data transmitted on the HS-DSCH as well for the management of the physical resources (HS-PDSCH, HS-SCCH, HS-DPCCH) allocated to HSDPA.

There is a companion flow control function between MAC-hs and MAC-d. Both entities provide a controlled data flow taking the transmission capabilities of the air interface into account in dynamic manner. This reduces discarded and retransmitted data as a result of HS-DSCH congestion or HS-DSCH cell change in RLC-UM (less RL-UM PDUs have to be discarded in case of HS-DSCH cell change).

Note: If CRNC and SRNC are identical, then actually nothing changes for MAC-d and NodeBs MAC-hs except that the Iur interface is not present.

[3GTS 25.321 (4.2), 3GTS 25.308 (5.1, 6.1, 6.2)]


HSDPA Control and User Plane Extensions

Control Plane Extensions

New NBAP, RRC & RNSAP IEs

Theses protocols are extended in Rel. 5 with new IEs in order to support the HSDPA procedures like e.g. Intra-NodeB Serving HS-DSCH Radio Link Change, HS-DSCH MAC-d Flow Addition/Deletion etc. Examples are: the AUDIT procedure is extended by the HS-DSCH resources Information IE for every cell which has been configured with HS-DSCH resources. RRC messages contain new information elements like for example the HS-DPCCH feedback cycle IE in uplink. RNSAP-messaging must also be extended similar to NBAP to cope with the additional HSDPA functionality.

Radio Resource Sharing

Radio Resource Sharing has to be extended in order to support the shared operation of dedicated and shared channels in a cell. For example, a simple algorithm can be used for best effort services in HSDPA to always use the capacity (e.g. available transmission power) that remains unused by the DCH channels in a cell for HS-DSCH and HS-SCCH. However, streaming services require an algorithm with a dynamic behavior when sharing dedicated and HSDPA channels to assure a constant throughput. Both transport channel types HS-DSCH and DCH share the available downlink transmission power in the NodeB.

Admission / Congestion Control

The additional transmission output power and higher throughput rates have to be taken into account by the admission control algorithm in order to protect existing services from quality degradation due to too high interference generated by HSDPA transmission. Updated thresholds for overload defense strategies have to be implemented. Ultimately, congestion control has to be updated with new thresholds and mechanisms for denying services, downlink power up commands, throughput reduction and last but not least deliberate dropping if the overall QoS is jeopardized. A vendor dependent solution can be assumed for such capacity control issues in conjunction with HSDPA support.

User Plane Extensions

PDCP/RLC/MAC-d

These user plane protocols have to be (re-)designed to meet the higher bandwidth needs for user allocated to HSDPA channels. PDCP may perform header TCP/IP header compression or not, RLC parameters (mode, timers, TTI etc.) are configured like in Rel. 99 / Rel. 4 but now much higher throughput rates must be supported by processing boards in RNC.

Frame Protocol

A new variant of frame protocol, called HS-DSCH FP, has to be implemented with a suitable flow control in order to support the higher throughput rates. The TTI of RLC is of Rel. 99 / Rel. 4 (10 ms, 20 ms, 40 ms or 80 ms) whereas the TTI of MAC-hs is 2 ms.

http://www.inacon.de/file:///F|/out/linker/PDCP/4296495796214.htmhttp://www.inacon.de/

HSDPA and DPCH Operation HSDPA Setup

HSDPA Setup via DPCH

The resource allocation of HSDPA requires the previous setup of a DCCH mapped on DCH. The DCH transport channel runs on a Rel. 99 DPCH. This means, in order to setup and maintain HSDPA operation, there is always a DL DPCH and UL DPCH needed. Upon RRC connection request sent by UE, the SRNC may request the UEs HSDPA capabilities. The HSDPA related configuration supported in a cell is signaled to the UE via a DCCH mapped onto a DCH which is carried by DPDCH+DPCCH. By decoding the so called High Speed Information on the DCCH the UE obtains is informed about the physical layer configuration for HSDPA in the Cell. This physical layer configuration allows the UE to decode the HS-SCCH which informs the UE about available user data on the shared transport channel HS-DSCH.

Note: Note there are no parameters broadcast on BCCH about a cells HSDPA capability.

The basic HSDPA setup is explained below:

1. Before moving in CELL_DCH state the UE is told by the RNC to reveal its HSDPA capabilities and category e.g. via RRC Connection Setup message.

2. Once the RNC has obtained the UEs HSDPA capabilities, the subsequent RRC configuration messages (e.g. Radio Bearer Setup) contain the high speed information telling the UE about the HSDPA configuration in the cell. High speed information contains e.g. the UE identity (( called H-RNTI) which is implicitly encoded on HS-SCCH to identify the very UE getting HS-DSCH resources allocated, the scrambling code to be applied for HS-PDSCH and HS-SCCH and the HS-SCCH channelization code-set. This is necessary to inform the UE about the decoding of the HS-SCCH which contains specific information about the HS-PDSCHs format carrying the HS-DSCH. From this moment on the UE is in a so called HSDPA standby mode ready to decode HS-SCCHs. Note: The UE might be told to monitor up to four HS-SCCHs.

3. A DPCH must always exist before a UE is able to operate in HSDPA as it carries the DCCH. The DPCH handles the real-time services, e.g. AMR 12.2 voice channel and the RRC signaling via SRBs. On SRBs the necessary RRC signaling messages are exchanged e.g. to reconfigure the physical link for HSDPA or prepare for HS-DSCH cell change. Please remember that HSDPA is not (yet) intended for services with real-time QoS requirements.

4. If there are packet data to be transferred to the UE in downlink, the NodeB will relay the necessary information for decoding the HS-DSCH to the UE on HS-SCCHs. The UE must be capable of decoding up to four HS-SCCH in parallel. This is a so called HS-SCCH-set. From the HS-SCCHs the UE obtains information how to decode the subsequent HS-PDSCHs and finally obtains the user data from the HS-DSCH.

5. If the UE detects consistent control information intended for it, the UE shall start receiving the HS-PDSCHs. Consistent control information means e.g. that the UE decodes its H-RNTI in one of the assigned HS-SCCHs of the HS-SCCH code-set.

[3GTS 25.302 (8.2), 3GTS 25.331 (8.6.3.1b, 8.6.6.32, 10.3.6.23a, 10.3.6.36a)]
http://www.inacon.de/

http://www.inacon.de/

HSDPA Transmission and Retransmission

6. Whenever there are packet data to be transmitted to a certain UE, the HS-SCCH indicates to the respective UE that the successive HS-DSCH transport channel contains user data destined for it. If the decoding of the HS-DSCH transport block fails because of CRC error, it is going to report a NACK in the corresponding uplink HS-DPCCH channel.

7. The UE shall transmit acknowledgement information received from the MAC-hs-entity to inform the NodeB whether the HS-DSCH subframe was decoded correctly or incorrectly.

8. If the HS-DSCH was received incorrectly and this result is reported by the UE on uplink HS-DPCCH, the NodeB schedules retransmissions in case of RLC-AM. The MAC-hs in NodeB informs the UE with the next possible HS-SCCH whether a retransmission or a new transmission of HS-DSCH transport block will be performed.

Note, while the UE is in HSDPA standby or receiving data, it may send CQI reports about the downlink reception quality to the serving NodeB.

Physical Channel Combinations supported with HSDPA on the same FDD Frequency

[3GTS 25.302 (8.1, 8.2), 3GTS 25.331 (8.6.3.1b, 8.6.6.32, 10.3.6.23a, 10.3.6.36a)]


Channel Type Switching with HSDPA

The figure shows only the important RRC connected states of a UE in UTRAN. We would like to show here that even with HSDPA transport channel type switching is feasible which saves resources in UTRAN and battery in UE. A prerequisite is that HSDPA retains MAC-d in SRNC.

In order to ease the understanding we distinguish the Rel. 99 CELL_DCH state between CELL_DCH & HSDPA and CELL_DCH ONLY. Note the CTS description does not intend to be exhaustive. For detailed information about the various RRC states and transitions between them the reader kindly is referred to [8].

CELL_DCH & HSDPA

Here the UE is actually in HSDPA standby and ready to receive data on HS-DSCH. A DCCH and possibly also DTCHs (e.g. for real time traffic) exist and are transmitted via DCHs.

Soft and softer handover are possible with DCH. ( UE is known on Cell level The UE has been configured for HSDPA (e.g. through Radio Bearer Setup or Physical Channel Reconfiguration message) and is

monitoring the assigned HS-SCCH code-set. For HSDPA there are DTCHs configured and ready to use. High speed user data are transmitted to the UE within 2 ms subframes. If the UE changes from a HSDPA capable cell to a cell without HSDPA capability, then the packet switched radio bearers need to be

reconfigured to e.g. a 384 kbit/s bearer. This allows to continue user data exchange within the packet domain. Changes of the HSDPA serving cell (serving HS-DSCH radio link change) are performed through hard handover through e.g.

Transport Channel Reconfiguration or Radio Bearer Reconfiguration messages. Of course, if the UE changes to a HSDPA capable cell back, bearer reconfiguration puts the UE back into HSDPA standby. If the UE is idle (not receiving any user data on HS-DSCH) for a while being in Cell_DCH with HSDPA setup, the SRNC may put the

UE into CELL_FACH state.

CELL_DCH ONLY

This RRC state is identical with the CELL_DCH in Rel. 99. Here we would like to highlight that whenever the UE moves from a HSDPA capable cell to a non-capable cell, the radio bearer(s) have to be reconfigured to e.g. a ordinary 384 kbit/s bearer known since Rel. 99.

CELL_FACH

The advantage of being in CELL_FACH state is twofold. Firstly the UE does not have to monitor the HS-SCCH code-set anymore which extends its battery capacity. Secondly the UTRAN saves channelization code resources as the UE does not longer occupy DCHs.

Through traffic volume measurements the UE reports its RLC transmit buffer occupancy and a rise in uplink or downlink (RLC transmit buffer in SRNC) above a certain threshold triggers the reconfiguration of UE into CELL_DCH and possibly into HSDPA standby mode.


HSDPA High Speed Information

When the UE is in CELL_DCH state, RRC-messages are exchanged e.g. for setting up radio bearers. If the UE has indicated beforehand its HSDPA capability, the RB_SETUP-message may contain HSDPA related information such as:

H-RNTI

The H-RNTI (High Speed Radio Network Transaction Identifier) is firstly used to identify the very UE which shall receive data from a HS-DSCH. Therefore the UEs H-RNTI is implicitly encoded in the HS-SCCH. Implicitly encoded means that part 1 of HS-SCCH uses H-RNTI as input for UE specific masking. For part 2 of HS-SCCH, the H-RNTI is used for UE specific CRC attachment. Thus the UE is able to distinguish which out of a max. of four HS-SCCHs contains decoding information for the following HS-PDSCHs in the sub-sequent HS-DSCH subframe. This means on the other hand, that the actual HS-DSCH TB does not contain any UE-id.

Thus the HS-DSCH resource allocation in HSDPA is entirely done without higher layer involvement. The reasons for this are twofold. Leaving the resource assignment at layer 1 speeds up the entire process (avoiding higher layer processing delays) and avoids to transmit UE specific RNTIs and sequence numbers.

The timely relationship between HS-SCCH and HS-DSCH avoids the need of attaching an UE-id within HS-DSCH transport block to identify the recipient.

Note: For each HS-DSCH TTI of 2 ms, each related HS-SCCH occurring 2 slots ahead of HS-DSCH carries HS-DSCH related downlink signaling for one UE only. The H-RNTI encoded in the HS-SCCH identifies therefore an UE having a HS-PDSCH assignment within a cell. Therefore the UE needs at first to decode the very HS-SCCH (part 1) before it can attempt to demodulate the allocated HS-PDSCHs.

The HS-SCCH signaling message is divided into two parts, with part 1 containing the time critical information on channelization code set and modulation scheme. Part 2 of HS-SCCH consists of transport block size and HARQ-related (indicating new transmission or retransmission, HARQ process id, redundancy and constellation version). A 16-bit UE specific CRC is computed over part 1 and part 2 and attached to part 2. The UE ID is only implicitly included in the CRC. In order to allow the very UE quickly to determine whether HS-PDSCHs are going to be allocated, the part 1 alone of HS-SCCH is scrambled with the H-RNTI as well.

If more than one UE shall be served within a subframe, more than one HS-SCCH need to be transmitted as one HS-SCCH can only allocate HS-DSCH resources for one UE only. However code multiplexing may be limited between 2-3 users per subframe as it becomes less efficient and processing is quite high for NodeB.

[3GTS 25.213 (4.3.1.2), 3GTS 25.331 (8.6.3.1b, 8.6.6.32, 10.3.6.23a, 10.3.6.36a, 10.3.6.40a)]


DL-HSPDSCH-Info

The downlink HS-PDSCH information consists of HS-SCCH Info and measurement feedback info. (Measurement feedback info will explain at a later stage in chapter 3.

HS-SCCH Info

Downlink Scrambling Coding

This information element informs the UE about the DL scrambling code to be applied for HS-PDSCHs and HS-SCCHs. Default is same scrambling code as for the P-CPICH. If dl-Scrambling Code IE is present, then secondary scrambling code is used in the cell under which HSDPA operates.

HS-SCCH Channelization Code info

This information element contains the maximum number of HS-SCCH codes that can be assigned to a UE. In Rel. 5 this parameter is set to four.

HS-SCCH Codes

This information element tells the UE the specific channelization codes used for the HS-SCCH code set. As a HS-SCCH channel has a fixed spreading factor of 128, the channelization code ranges from 0127.

Note: The channelization code C(ch,128,0) cannot be used as the related branch is already occupied by the P-CPICH and P-CCPCH. The same is valid for C(ch,16,0). And also if secondary scrambling codes are used.

Note HSDPA may operate on a different frequency than legacy UMTS traffic so almost the entire resources of the second carrier could be dedicated to HSDPA.

[3GTS 25.213 (4.3.1.2), 3GTS 25.331 (8.6.3.1b, 8.6.6.32, 10.3.6.23a, 10.3.6.36a, 10.3.6.40a)]


HSDPA Timing Relations

Timing between HS-SCCH and HS-PDSCH

As illustrated in the figure, the HS-SCCH is split into two parts, part 1 and part 2 which are independently coded from each other. Part 1 carries the TFRI (transport format and resource related information. Part 2 carries Hybrid ARQ related information. Every HS-SCCH subframe employs UE specific masking with the H-RNTI for both parts allowing the UE to determine whether the detected control channel is actually intended for this particular UE.

It can be seen that the part2 of HS-SCCH carries a CRC which is build over part1 and part2 information together.

Modulation Type and Number of HS-PDSCHs

HS-PDSCH channelization-code informs UE about how many HS-PDSCHs in parallel shall be decoded. Modulation scheme informs UE if QPSK or 16-QAM is employed for HS-PDSCH.

HARQ Info (Hybrid-ARQ-related Information)

Transport block size used for HS-DSCH. HARQ protocol related information for the corresponding HS-DSCH TTI and information about the redundancy version necessary for

retransmission combining. HARQ parameters inform the UE if the HS-DSCH subframe contains either a new TB or a repetition of a Nack'd TB and how original

transmission and retransmissions have to be combined based on redundancy parameter.

Note: The very HS-PDSCH subframe can be de-spread and demodulated by the UE without having to wait for complete reception of the preceding HS-SCCH subframe.

The very HS-PDSCHs subframe carrying the HS-DSCH is delayed by 5120 chips compared to the respective HS-SCCH subframe. Therefore the UE has approximately one slot duration time to prepare for HS-PDSCH demodulation. As already mentioned, the maximum number of HS-SCCHs that a single UE shall be able to monitor is four. This maximum number is referred to as HS-SCCH set. However, in case there is data for the same UE in consecutive TTIs, then the HS-SCCH shall be kept the same to increase signaling reliability and to ease the UEs decoding tasks. It is sufficient for the UE to monitor the same HS-SCCH as used in the immediately preceding subframe. This means the UE can stick with the same HS-SCCH channelization code. This approach is necessary for two reasons. Firstly the UE does not have to buffer other HS-SCCHs data (at least part 1 of up to four HS-SCCHs have to be decoded in the worst case). Secondly, there could be more than four HS-SCCHs configured in the cell which would exceed UEs terminal capability.

[3GTS 25.211 (7.8), 3GTS 25.212 (4.6)]


HSDPA Downlink Timing

The green color in the figure should indicate signaling/control channel only and the peach color indicates physical channels transferring user data.

The HSDPA HS-SCCH is aligned with the P-CPICH and the P-CCPCH on which the cell SFN (System Frame Number) is transmitted. The HS-DSCH follows the HS-SCCH with a fixed offset. The HS-PDSCHs carrying the HS-DSCH is/are always transmitted with a fixed offset of 5120 chips after the related HS-SCCH subframe has started. Note that the users Rel. 99/Rel. 4 DPCH and HS-SCCH/HS-DSCH are not slot aligned but differ always only of a multiple of 256 chips in order to maintain downlink orthogonality. Therefore some additional timing constraints have to be considered in uplink.

DPCH (Dedicated Physical Channel)

The DL-DPCH consists of DPDCH and DPCCH which are time multiplexed. The DPDCH is the physical bearer for the DCH transport channel which carries DCCH and may be also DTCH (e.g. real time services). The DPCH timing may be different for different DPCHs, but the offset from the CPICH frame timing is always a multiple of 256 chips, i.e. t(DPCH) = n x 256 chips, n = {0, 1, 2, 3 149} resulting in maximum chip offset of 38,144 chips. The DPCH is divided into 15 slots per radio frame allowing for a fast power control in every slot ((1500 times/s) via DPCCH.

HS-SCCH (High Speed Shared Control Channel

The HS-SCCH is time aligned with the CPICH. It has a fixed spreading factor of 128 which results in a fixed rate of 2 x 38400/128 = 60 kbit/s. The HS-SCCH is transmitted in downlink only and is used to carry HS-DSCH related signaling information. In difference to the DPCH, the HS-SCCH is divided into five subframes per radio frame resulting into a duration of 2 ms per subframe. One subframe consists therefore of three slots resulting in 2560 chips/slot x 3 = 7680 chips.

HS-DSCH (High Speed Downlink Shared Transport Channel)

The HS-PDSCH is the physical bearers for the HS-DSCH. The HS-PDSCH has a fixed spreading factor of 16. Like the HS-SCCH the HS-DSCH is transmitted in subframes. However the start of HS-DSCH subframe is always delayed by 5120 chips compared to the related HS-SCCH allowing the UE to obtain the most important information on HS-SCCH before attempting to decode the very HS-DSCH subframe.

Note: The HS-SCCH subframe structure is aligned with the CPICH. Default is the P-CPICH. The downlink DPCH timing is not tied to the HS-SCCH or consequently HS-DSCH timing. However, the difference between HS-SCCH and DPCH is always a multiple of 256 chips which ensures downlink orthogonality.

[3GTS 25.211 (7.1, 7.8)]


HSDPA Uplink Timing

The color coding of the figure should be interpreted like this. The green color of the HS-DSCH shall only indicate the downlink transmission direction. Of course user data are transferred via this transport channel. The red color shall indicate uplink channels. It can be seen that the HS-DPCCH is not slot aligned with uplink DPCH of Rel. 99/Rel. 4. Therefore the correct timing to start transmission on HS-DPCCH must be derived from the timing of the uplink DPCH. Only a proper timing of HS-DPCCH transmission ensures uplink orthogonality with uplink DPCCH. For simplicity reasons we provide here only a principle drawing. Exact details are shown in chapter 2.

The HSDPA operation procedure has strictly specified timing values for the UE operation starting from HS-SCCH reception via HS-DSCH decoding to the uplink Ack/Nack transmission on HS-DPCCH.

7.5 slots delay of uplink HS-DPCCH

The key timing from UE point of view is the app. 7.5 slots (= 7.5 x 2/3 ms = 5 ms) from the end of the HS-DSCH TTI to the start of the Ack/Nack transmission on the HS-DPCCH in the uplink as can be seen in the figure. The uplink DPCH is shown for reference purpose as the related downlink DPCH is not necessarily slot-aligned to the HS-SCCH as can be obtained from the previous slide. Therefore the uplink HS-DPCCH may start in the middle of the uplink slot and this needs to be taken into account in the uplink power setting process ensuring a proper decoding in the uplink in NodeB.

The UE capabilities do not impact the timing of individual TTI transmission but do define how often the NodeB can schedule transmission to a particular UE. The UE capabilities include information about the minimum inter-TTI interval that tells whether consecutive TTIs may be used or not for transmitting HS-DSCHs to the same UE. Thus this provides a relieve for certain UE's which do not have to receive in downlink in consecutive subframes neither transmit in the uplink on consecutive subframes.

Note: TTI interval in HSDPA always refers to the 2 ms subframe duration.

[3GTS 25.211 (7.6.3, 7.7)]


DCH / HS-DSCH Comparison

Variable Spreading Factor

The SF does not vary for the DCH on a frame by frame basis but is chosen upon setup according to the max. expected data rate. The data rate variation on the DCH during operation is taken care of through either rate matching or through DTX. Compared to that, the SF for the HS-DSCH is always fixed at SF=16, the data rate variation is achieved by AMC and therefore no explicit RRC signaling is needed.

Fast Power Control

The DPCH DL power control is based on the inner-loop power control commands sent by the UE on the associated uplink DPCCH. Since the HS-DSCH uses AMC, fast power control in downlink is on the one hand not allowed when 16-QAM is used. Note, 16-QAM requires amplitude estimations by the receiver. On the other hand it is not needed as the DL power control dynamics is quite limited (about 20 dB) due to intra-cell interference and by NodeB implementation. Intra-cell interference occurs between users on parallel code channels. This interference is caused by various propagation delays in downlink. Thus former transmitted orthogonal channelization codes at NodeB side are no longer orthogonal when they arrive at the UE.

For a user close to NodeB, the power control cannot reduce power maximally, and on the other hand reducing the power beyond 20 dB dynamics would have only marginal impact on the DCHs capacity. With HSDPA, this property is now utilized by the link adaptation function and AMC may select a coding and modulation combination that requires higher Ec/Ior, which is only available for user close to the cell. To enable a large dynamic range of the HSDPA link adaptation and to maintain a good spectral efficiency, a user may simultaneously utilize up to 15 HS-DSCH channelization codes in parallel. The use of more robust coding, Hybrid ARQ and multicode operation removes therefore the need for a variable SF and fast power control.

AMC

The MAC-hs in NodeB is applying adaptive modulation and coding only for the HS-DSCH. Note, the detailed function of AMC is explained in chapter 2.

Multicode Operation

Both HS-DSCH and DCH may use more than one channelization code, but for HS-DSCH the UE can go for the maximum of either 5, 10 or 15 parallel channelization codes with SF = 16 depending on the UEs category. Therefore HSDPA uses the code multiplexing extensively compared to DCH operation.

Fast Layer 1 Hybrid ARQ

The significant difference between DCH and HS-DSCH is that MAC-hs allows for retransmissions in NodeB directly at the air interface once the UE indicates a negative reception via HS-DPCCH. Therefore the higher layer RLC located in SRNC is not confronted with retransmissions in the first place, whereas for DTCH operation, RLC takes care of retransmissions. Note that retransmissions are only performed in RLC-AM operation.


DCH / HS-DSCH Comparison contd

16-QAM Modulation / Interleaving / Channel Coding Scheme

DCH uses the Rel. 99 QPSK modulation with interleaving depth between 10-80 ms depending on the configured TTI. Turbo and convolutional coding can be configured for the DCH, whereas HS-DSCH exclusively uses turbo coding. In order to reduce the latency for retransmission, HS-DSCH uses a shortened TTI of 2 ms and can interchange between QPSK and 16-QAM modulation depending on UE capability and radio conditions.

TrCH Multiplexing

The HS-DSCH is a transport channel with only one TB (transport block) per TTI per UE, so there is no need for static rate matching, transport block concatenation or balancing of quality. This complex matter shall be explained here in detail now:

Static rate matching DCH versus HS-DSCH

On transport channels, transport blocks are exchanged between MAC layer and the physical layer. For each transport block there is a CRC checksum calculated which provides for error detection. After that, channel coding is performed. After channel coding the code blocks become encoded blocks and are treated in rate matching. Static rate matching ensures that there are firstly only as many coded bits to be transmitted as there are bits available on the physical channel but also it balances the coding rate between different transport channels according to their relative importance. In rate matching, the bits of encoded blocks are repeated or punctured according to the available bandwidth and according to the RMAs (rate matching attributes). Those RMAs define the priorities between the coded bits of different transport channels, and are set by layer 3 (semi-static attribute). For instance, by setting RMA of transport channel x to twice the value of the RMA of a transport channel y, the coded bits of the TrCH x are made twice more important than the coded bits of TrCH y. This mechanism allows for balancing the coding rate between transport channels. As HSDPA uses only one transport channel per TTI, there is no need to match this only one transport channel to the capacity of the available physical channels. HSDPA uses a fixed SF = 16 for HS-DSCH and there is only one transport block per TTI, thus the entire transport block can be mapped onto the HS-DSCH. Only one transport channel per TTI is allowed in HSDPA, so there is never the case that two transport channels of type HS-DSCH must be mapped onto a CCTrCH and need to apply rate matching.

Transport block Concatenation DCH versus HS-DSCH

In HSDPA there occurs only one transport block per HS-DSCH TTI, thus no case where 2 or more transport blocks need to be mapped onto the same HS-DSCH. As HSDPA uses only one transport channel, the balancing of quality is not applicable as quality balancing makes only sense if more than one TrCH are in use.

Soft(er) Handover

For HS-DSCH there is no macro diversity possible as it is a common transport channel. Of course the associated DCH supports SHO.

[3GTS 25.212 (4.2.9, 4.2.13.8, 4.5), 3GTS 25.213 (5.1), 25.214 (5.2.11)]


Practical Exercise:

How does the FACH transport channel compare to DCH and HS-DSCH?

Answer:


Advantages and Disadvantages of HSDPA

Reduced Latency for Retransmissions

Higher DL Throughput and Peak Throughput Rates up to 14 Mbit/s

Support of Non-Real Time and Streaming Applications

Higher Spectrum and Code Efficiency due to 16-QAM

Data services are anticipated to have an enormous growth rate over the coming years and will likely become the dominating source of traffic load in 3G mobile cellular networks. As more sophisticated services evolve, a major challenge of cellular systems design is to achieve a high system capacity and simultaneously facilitate a mixture of diverse services with very different QoS-requirements. HSDPA tries to meet those requirements and brings quite significant benefits for the subscribers:

Advantages of HSDPA

Reduced Latency for Retransmissions

With MAC-hs installed in NodeB and UE, the uplink feedback channel HS-DPCCH and the shortened TTI of 2 ms, reception of erroneous packet data results in a faster retransmission. On top of this, no transmission is wasted due to the use of incremental redundancy or chase combining.

Higher DL Throughput and Peak Throughput Rates up to 14 Mbit/s

As HSDPA makes extensive use of code multiplexing in conjunction with higher order modulation, the downlink throughput can be increased beyond 10 Mbit/s. If the radio conditions are favorable, AMC allows a very aggressive code rate of 1:1, thus offering peak data rates up to 14 Mbit/s.

Support of Non-Real Time and Streaming Applications

Through a sophisticated packet scheduler implementation in the NodeB it is possible to manage stringent QoS requirements of streaming applications and of course less time critical services as well. The packet scheduler is able to adjust the code rate, modulation type, number of channelization codes and the transmission power within every subframe. Thus the NodeB is highly flexible in assuring various QoS requirements of subscribers services.

High Spectrum and Code Efficiency due to 16-QAM

Spectrum efficiency increases when the modulation scheme changes from QPSK to 16-QAM as literally more kbit/s respectively Mbit/s can be transmitted in the 5 MHz bandwidth. Code efficiency increases when the UE is able to dynamically make use of the available channelization codes.

In downlink, the WCDMA power control dynamics is in the order of about 20 dB. It is limited by the intra-cell and inter-cell interference compared to the uplink power control dynamics of about 70 dB. This means that for a particular user, very close to the NodeB, the DL power control cannot reduce the power maximally, and on the other hand reducing power beyond 20 dB dynamics may only have marginal impact on the downlink capacity. HSDPA now utilizes this available DL capacity though link adaptation and through AMC. The higher Ec/Io signal quality (when user is close to NodeB) is exploited by selecting a very aggressive code rate and 16-QAM. Vice versa, if radio conditions become worse, HSDPA switches down to the more robust QPSK, applies lower code rate (meaning more protection) but may use increasing number of channelization codes to support the users QoS demands (e.g. guaranteed throughput rate).


Disadvantages of HSDPA

Higher Processing power in UE

More Memory in UE due to HARQ

Advanced Receiver for 16-QAM

Higher Vulnerability using 16-QAM

Higher Processing Power in UE

The higher throughput rates in the downlink require that the UEs processors and DSP's are designed accordingly. UEs battery consumption will increase, too. This imposes serious challenges on the UE vendors.

More Memory in UE due to HARQ

The HARQ is making use of every previous transmission instance of a data packet. In case of incremental redundancy, the UE is required to buffer previously transmitted data blocks for soft combining. Compared to that, chase combining allows the UE to add the soft decision values after every transmission instance of a data packet, thus less memory is needed. Incremental redundancy requires the maximum UE memory. Therefore the terminal default memory requirements are set on the basis of chase combining at the maximum data rate which is supported by the terminal. Hence, at the highest data rate, only chase combining may be used, while with lower data rates, also incremental redundancy can be used.

The UE category value informs the RNC about the so called total number of soft channel bits which may limit the IR performance if it comes to peak data rates.

Advanced Receiver for 16-QAM

With QPSK only a phase estimate is necessary for the demodulation process. However, with 16-QAM an amplitude estimation is additionally required to separate the constellation points on the I-Q plane. Furthermore, accurate phase information is needed since constellation points have smaller decision space in the phase domain compared to QPSK.

Higher Vulnerability using 16-QAM

For peak data rates using 16-QAM, higher C/I (Eb/No) values are required and thus the HS-DSCH is more vulnerable to interference including inter-symbol interference. As 16-QAM needs both the phase and the amplitude to determine the symbol value, amplitude distortions have a big impact. Compared to that in QPSK where only the phase is considered for determining the symbol value, amplitude distortions are negligible. Beside this, the HSDPA transmitters with 16-QAM need an improved linearity of the power amplifier chain in terms of EVM performance (Error Vector Magnitude).


Disadvantages of HSDPA contd

Software and possibly Hardware Upgrade

No Macro Diversity for HS-PDSCH and HS-SCCH

Power / Interference Increase in uplink and downlink

Signaling Overhead due to HS-SCCH (dl) and HS-DPCCH (ul)

Software and possibly Hardware Upgrade

Depending on the very UTRAN vendor, a hardware upgrade might be necessary for the NodeB in order to support 16-QAM. Additional software upgrades in NodeB and RNC complement the HSDPA support.

No Macro-Diversity for HS-PDSCH and HS-SCCH

As HS-PDSCH and HS-SCCH are shared channels, no soft handover is possible and thus the performance decreases particularly at cell edge. Nevertheless mobility procedures like intra-NodeB and inter NodeB cell changes are supported which will mitigate this disadvantage.

Power / Interference Increase in uplink and downlink

HSDPA operation may require quite large transmission power in downlink in order to operate at reasonable BLER (considering 1st transmission) and may therefore increase the inter cell interference. Especially HSDPA operation at cell edge is only feasible at high downlink transmission power for HS-SCCHs and HS-PDSCHs. It should be noted, that the amount of HS-SCCHs increases with code multiplexing of HS-PDSCHs and can add considerably to the downlink interference when users are subjected to unfavorable radio conditions.

In the uplink the HS-DPCCH may increase uplink interference if e.g. operated at maximum power value. Proper decoding of HS-DPCCH is on the one hand necessary to avoid unnecessary retransmissions but on the other hand high uplink HS-DPCCH transmission power increases the interference among the various users.

Signaling Overhead caused by HS-SCCH (dl) and DPCCH (ul)

The necessary signaling in order to support HS-DSCH consumes valuable channelization code resources. In downlink a spreading factor of 128 is needed per HS-SCCH and in uplink a spreading factor of 256 is needed per UE. Of course, the channelization code consumption in uplink is negligible as every UE has its dedicated uplink scrambling code so there is almost no resource limitation. However, in downlink every HS-SCCH occupies a spreading factor of 128 which is also necessary for one AMR 12.2 kbit/s speech conversation.


Future Enhancements of HSDPA

UMTS UTRA FDD aims to support a variety of multiple antenna transmission techniques in order to enhance coverage, system throughput and spectral efficiency of HSDPA. A major aim of using multiple antenna transmission in macro-cellular environments is to increase the coverage ratio at medium and higher data rates, lets say 2 Mbit/s and beyond. In a typical deployment, two to four or more transmit antennas might be used per sector.

Beamforming

Beamforming makes use of adaptive antennas and can therefore provide a better C/I to UEs in downlink. At the same time beamforming allows to re-use the scarce downlink channelization codes as the individual UEs are separated in space and possibly through different downlink scrambling codes, thus making use of secondary scrambling codes. The signals toward different UE's from the same cell are typically transmitted under the same primary scrambling code and separated by means of orthogonal channelization codes. However, some of the beams may be transmitted under a secondary scrambling code with its associated channelization code tree, thereby increase the resources in the cell. Note that the loss of the reduced orthogonality between primary and secondary scrambling code can be partly mitigated in the case of beamforming by splitting the cell into multiple scrambling code regions, so the spatial isolation between beams using different scrambling codes helps to compensate the lack of orthogonality.

Transmit Diversity

The downlink capacity could also be improved by using receive antenna diversity in the UE. However for small and cheap mobiles it is not feasible to use two antennas and receiver chains. Therefore, the WCDMA standard supports the use of base station transmit diversity already in Rel. 99. There exist two modes: open loop (TSTD and STTD) and closed loop mode (mode 1 with phase adjustment only and mode 2 with phase and amplitude adjustment).

The open loop mode simply transmits the coded information from two antennas, but on the diversity antenna the bits are time reversed and complex conjugated.

The STTD method provides two kinds of diversity. The physical separation of the antennas provides space diversity and the time difference derived from a bit-reversing process provides for time diversity, thus the decoding in the receiver becomes more reliable.

The closed loop mode can only be applied to the downlink channel, if there is an associated uplink channel. Thus this mode can only be used with dedicated channels (DPCH, PDSCH or HS-PDSCH with an associated uplink DPCCH).

MIMO

With MIMO (Multiple Input Multiple Output) at the transmitter, x independent data streams are transmitted out of the x antennas on the same frequency band. At the receiver, each antenna receives all of the transmitted sub-streams superimposed, not separately.

If multipath scattering is sufficient, these x data streams have different spatial signatures to each of the e.g. p receive antennas and they are separable, the signals arrive with different phases. When a transmitter has x antennas and the receiver has p antennas, the link speed increases linearly with min (x,p) given the same power and bandwidth budget.

[3GTR 25.848]


Preview to HSUPA

There are fundamental differences between uplink downlink to consider:

Total available Transmission Power

Much larger Dynamic of the UL Power Control

UL does not suffer from Channelization Code Shortage

Fast Power Control cannot be abandoned in UL

The aim of the HSUPA is to enhance the uplink DCH operation and performance, using several techniques in order to support services like video-clips, multimedia, e-mail, telematics, gaming and video-streaming. A study showed that various techniques such as Node-B controlled scheduling, shorter TTI and a hybrid ARQ layer in the Node-B can enhance the uplink packet transfer performance significantly compared to Release-99/Rel-4/Rel-5. The targeted improvements are a higher air interface capacity in the uplink and a better end user experience by increasing both the cell throughput and the coverage of higher bit rates in uplink.

There are fundamental differences between uplink and downlink data transmission, making it impossible to simply introduce the same HSDPA solutions for the uplink.

Total available Transmission Power

The key difference between uplink and downlink is the handling of the total transmission power available. In downlink, the power is centralized whereas in the uplink the power available for an individual UE is limited by the terminal amplifier capabilities. Therefore it can be said, that a pure time division approach, being in place with the maximum data rate in HSDPA, would not work for the uplink.

Much larger Dynamic of the UL Power Control

Furthermore in uplink the power control has much larger dynamics compared to downlink.

UL does not suffer from Channelization Code Shortage

Higher order modulation like 16-QAM alleviates the problem of scarce channelization codes available in downlink, which is not the case for uplink.

Fast Power Control cannot be abandoned in UL

Additionally the fast power control cannot be abandoned in the case of continuous uplink transmission due to the near-far problem.

Simulation results have shown a significant improvement compared to Rel. 5, in the order of 50%-70% increase in system capacity, 20%-55% reduction in end-user packet call delay and around 50% increase in user packet call throughput, when simultaneously applying NodeB scheduling, hybrid ARQ with soft combining, and a shortened TTI. Hence, significant technical benefits have been found for a system using these techniques in conjunction.

Complexity of hybrid ARQ has been studied in terms of buffering and timing requirements. The enhancements can be introduced into the FDD specifications without impacting the backwards compatibility with Rel. 5 and earlier releases.


HSUPA Areas of Improvement

NodeB controlled scheduling for uplink

Hybrid ARQ in Uplink

Shorter TTI

Based on research, the following enhancements for HSUPA are recommended:

NodeB controlled scheduling

With the physical layer scheduling in NodeB, the scheduling period could be shorter and the physical layer measurement information readily available in Node could be used as a basis for scheduling. This enables more up-to-date scheduling decisions and thus, better use of the available uplink air interface capacity. Significant reduction of the scheduling periods allows for more dynamic control over the uplink air interface capacity. Whenever one UE stops transmitting or reduces its transmission data rate, the released capacity can quickly and efficiently be allocated to other UE's.

Hybrid ARQ in Uplink

The principle of fast HARQ is to allow the NodeB to ask the UE to retransmit an uplink packet if it was received incorrectly. Additionally, the NodeB can use different methods for combining the multiple transmissions of a single packet, thus reducing the required Ec/No of individual transmissions. Therefore, the BLER target of the outer loop power control could be significantly set to higher values. A higher BLER target reduces the UE transmission power required for a given service/data rate. Hence, for the same cell loading, the cell capacity can be increased. How the uplink HARQ should operate when the UE is in soft handover is still under discussion, b

R_inacon - Hsdpa - Dd & Se v2.1

Documents

Transcript of R_inacon - Hsdpa - Dd & Se v2.1