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WHITEPAPER

Third-generation wireless communication systems based on W-CDMA (wideband code-divisionmultiple access) are being deployed all over the world. To ensure that these systems remain competi-

tive, the 3GPP (3rd Generation Partnership Project) initiated a project in late 2004 for the long-termevolution (LTE) of 3GPP cellular technology.

This article focuses on the physical layer (“Layer 1”) characteristics of the LTE uplink, describing thenew Single-Carrier Frequency Division Multiple Access (SC-FDMA) transmission scheme and some

of the measurements associated with it. Understanding the details of this new transmission schemeand measurements is a vital step towards developing LTE UE designs and getting them to market.

Written by: Moray Rumney BSc, C. Eng, MIET

Lead Technologist, Agilent Technologies

new transmission scheme called SC-FDMA. This new scheme borrowsrom both traditional single-carrier schemes as well as rom OFDM.

OFDM and OFDMAOFDM has been around since the mid 1960s and is now used ina number o non-cellular wireless systems such as Digital VideoBroadcast (DVB), Digital Audio Broadcast (DAB), Asymmetric DigitalSubscriber Line (ADSL) and some o the 802.11 amily o Wi-Fistandards. OFDM’s adoption into mobile wireless has been delayed

or two main reasons. The rst is the sheer processing power whichis required to perorm the necessary FFT operations. However, thecontinuing advance o signal processing technology means that thisis no longer a reason to avoid OFDM, and it now orms the basiso the LTE downlink. The other reason OFDM has been avoided inmobile systems is the very high peak to average ratio (PAR) signalsit creates due to the parallel transmission o many hundreds oclosely-spaced subcarriers. For mobile devices this high PAR is prob-lematic or both power amplier design and battery consumption,and it is this concern which led 3GPP to develop the new SC-FDMAtransmission scheme.

Multiple access in the LTE downlink is achieved by usingan elaboration o pure OFDM called orthogonal requency divisionmultiple access (OFDMA). This method allows subcarriers to beallocated to dierent users. This acilitates the trunking o manylower-rate users as well as enabling the use o requency hopping tomitigate the eects o narrowband ading.

SC-FDMASC-FDMA is a hybrid transmission scheme which combines the lowPAR characteristics o single-carrier transmission systems - such asthose used or GSM and CDMA - with the long symbol time andfexible requency allocation o OFDM. The principles behind SC-

The LTE specications are being documented in Release 8 othe 3GPP standard. The core specications are scheduled to becompleted by mid-2008 with the conormance test specicationsollowing approximately six months later.

With early system deployment expected in the 2010timerame, LTE provides a ramework or an evolved 3G network,and aims specically to achieve the ollowing:• Increased uplink peak data rates up to 86.4 Mbps in a 20 MHz

bandwidth with 64QAM (quadrature amplitude modulation)

• Increased downlink peak data rates up to 172.8 Mbps in a 20MHz bandwidth with 64QAM and 2x2 SU-MIMO (single-usermultiple input/multiple output)

• Maximum downlink peak data rates up to 326.4 Mbps using 4x4SU-MIMO

• Spectrum fexibility with scalable uplink and downlink channelbandwidths rom 1.4 MHz up to 20 MHz

• Improved spectral eciency, with a 2-4 times improvement overRelease 6 HSPA (high speed packet access)

• Sub-5 ms latency or small IP (internet protocol) packets• Mobility optimized or low mobile speed rom 0 to 15 km/h;

higher mobile speeds up to 120 km/h will be supported withhigh perormance with the system operating up to 350 km/h• Co-existence with legacy systems while evolving towards an all-IP

network The LTE Air InteraceThere are two primary duplexing modes used in LTE which arerequency division duplex (FDD) and time division duplex (TDD).Variants including hal-rate FDD are also anticipated. The integrationo the FDD and TDD modes o LTE is much closer than was the casewith UMTS. The downlink transmission scheme is based on orthogo-nal requency division multiplexing (OFDM) and the uplink uses a

De-mystiying Single Carrier FDMAThe New LTE Uplink



FDMA signal generation are shown in Figure 1. This is taken romFigure 1 o the study phase report or the LTE physical layer 3GPPTR 25.814.

On the let hand side o Figure 1 the data symbols are depictedin the time domain. The symbols are converted to the requencydomain using an FFT, and then in the requency domain they aremapped to the desired location in the overall carrier bandwidth.They must then be converted back to the time domain in order tohave the cyclic prex inserted prior to transmission. An alternativename or SC-FDMA is Discrete Fourier Transorm Spread OFDM(DFT-SOFDM).

An alternative description is provided in Figure 2 whichshows, in requency and time, how OFDMA and SC-FDMA wouldeach transmit a sequence o 8 QPSK data symbols. For thissimplied example, the number o subcarriers (M) is set to our.For OFDMA, our (M) symbols are taken in parallel, each o themmodulating its own subcarrier at the appropriate QPSK phase. Eachdata symbol occupies 15 kHz or the period o one OFDMA symbolwhich lasts or 66.7µs. At the start o the next OFDMA symbol, theguard interval containing the cyclic prex (CP) is inserted. The CP isa copy o the end o a symbol prepended to the start o the symbol.

Due to the parallel transmission, the data symbols are the samelength as the OFDMA symbols.

In the SC-FDMA case, the data symbols are transmitted

Figure 1 SC-FDMA signal generation

Figure 2 Comparison o OFDMA and SC-FDMA transmitting a series o QPSK data symbols

Figure 3 Creating the time-domain waveorm o anSC-FDMA symbol

sequentially. Since this example involves oursubcarriers, our data symbols are transmittedsequentially in one SC-FDMA symbol period.The SC-FDMA symbol period is the same lengthas the OFDMA symbol at 66.7µs but due tosequential transmission, the data symbols areshorter being 66.7/M µs. A consequenceo the higher data rate symbols means more

bandwidth is required, so each data symbol occupies 60 kHz ospectrum rather than the 15 kHz or the slower data symbols usedor OFDMA. Ater the our data symbols have been transmitted, theCP is inserted.

Following this graphical comparison o OFDMA and SC-FDMA,the detail o the SC-FDMA signal generation process is shown inFigures 3 and 4. A time domain representation o the data symbolsequence is rst generated as shown in Figure 3.

For this our subcarrier example a sequence o our datasymbols is required to generate one SC-FDMA symbol. Using therst our o the color-coded QPSK data symbols rom Figure 2, theprocess creates one SC-FDMA symbol in the time domain by com-puting the trajectory traced by moving rom one QPSK data symbol

to the next. This is done at M times the rate o the SC-FDMAsymbol such that one SC-FDMA symbol contains M consecutiveQPSK data symbols. For simplicity, we will not discuss time-domain

ltering o the data symboltransitions even though suchltering will be present in anyreal implementation.

Having created an IQrepresentation in the time do-main o one SC-FDMA symbol,the next stage is to represent this in the requency domain us-ing a discrete Fourier transorm(DFT; Figure 4).

The DFT sampling re-quency is chosen such that thetime-domain waveorm o oneSC-FDMA symbol is ully repre-sented by M DFT bins spaced15 kHz apart, where each binrepresents one subcarrier withamplitude and phase held



constant or the 66.7µs SC-FDMA symbol period. There is always aone-to-one correlation between the number o data symbols to betransmitted during one SC-FDMA symbol period and the number oDFT bins created — which in turn becomes the number o occupiedsubcarriers. When an increasing number o data symbols aretransmitted during one SC-FDMA period, the time-domain waveormchanges aster, generating a higher bandwidth and hence requiringmore DFT bins to ully represent the signal in the requency domain.

Multipath Resistance With Short Data

Symbols?At this point it is reasonable to ask, “How can SC-FDMA still be resis-tant to multipath when the data symbols are still short?” In OFDMA,the modulating data symbols are constant over the 66.7 µs OFDMAsymbol period but an SC-FDMA symbol is not constant over timesince it contains M data symbols o much shorter duration. Themultipath resistance o the OFDMA demodulation process seems torely on the long data symbols that map directly onto the subcarriers.Fortunately, it is the constant nature o each subcarrier— not thedata symbols — that provides the resistance to delay spread. Asshown earlier, the DFT o the time-varying SC-FDMA symbol gener-ated a set o DFT bins constant in time during the SC-FDMA symbol

period even though the modulating data symbols varied over thesame period. It is inherent to the DFT process that the time-varyingSC-FDMA symbol - made o M serial data symbols - is representedin the requency domain by M time-invariant subcarriers. Thus,

even SC-FDMA with its short data symbols benets rom multipathprotection. Figure 2 shows the SC-FDMA subcarriers all at the sameamplitude but in reality each will have its own amplitude and phaseor any one SC-FDMA symbol period.

To conclude SC-FDMA signal generation, the process ollowsthe same steps as or OFDMA. Perorming an inverse FFT convertsthe requency-shited signal to the time domain and inserting the CPprovides OFDMA’s undamental robustness against multipath.

Figure 5 shows the close relationship between SC-FDMA andOFDMA. The orange blocks represent OFDMA processing and theblue blocks represent the additional time domain processing requiredor SC-FDMA.

Figure 5 Simplifed model o SC-FDMA generation and reception

Figure 4. Baseband and shited requency domainrepresentations o an SC-FDMA symbol

UL Signals Full Name Purpose

DMRS (Demodulation)Reerence Signal

Used by the base stationor synchronization to theUE and or UL channelestimation.Associated with PUCCH or

PUSCHSRS Sounding Reerence

SignalUsed or channel estima-tion when there is noPUCCH or PUSCH

UL Channels Full name Purpose

PRACH Physical RandonAccess Channel

Call setup

PUCCH Physical UplinkControl Channel

Scheduling, ACK/NACK

PUSCH Physical UplinkShared Channel

Payload

Table 1 Uplink signals and channels



The key point to note is that the signal which isconverted rom the requency domain back to the time domainis no more than a requency shited version o a series o QPSKsymbols. This example illustrates the main reason SC-FDMA wasdeveloped: that is, the PAR o the nal signal is no worse thanthat o the original data symbols, which in this case was QPSK.This is very dierent to OFDMA where the parallel transmissiono the same QPSK data symbols creates statistical peaks - much

like Gaussian noise - ar in excess o the PAR o the data symbolsthemselves. Limiting PAR using SC-FDMA signicantly reducesthe need or the mobile device to handle high peak power. Thislowers costs and reduces battery drain.

Physical Layer StructureThe LTE physical layer comprises two types o signals known asphysical signals and physical channels. Physical signals are gener-ated in Layer 1 and used or system synchronization, cell identica-tion, and radio channel estimation. Physical channels carry datarom higher layers including control, scheduling, and user payload.Table 1 shows the uplink physical signals and channels.

Uplink Frame StructureThere are two uplink rame structures, one or FDD operationcalled type 1 and the other or FDD operation called type 2. Framestructure type 1 is 10 ms long and consists o ten subrames, each

Figure 6 Frame Structure 1 or uplink showing mapping or DMRS and PUSCH

Figure 7 Frame Structure 1 or the uplink showingone subrame vs.requency



comprising two 0.5 ms slots. Figure 6 shows how the DMRS andPUSCH map onto the rame structure. The number o symbols ina slot depends on the CP length. For a normal CP, there are sevenSC-FDMA symbols per slot. For an extended CP used or when thedelay spread is large, there are six SC-FDMA symbols per slot.

Demodulation reerence signals are transmitted in theourth symbol (that is, symbol number 3) o every slot. ThePUSCH can be transmitted in any other symbol.

Figure 7 shows the uplink rame structure type 1 in bothrequency and time. Each vertical bar represents one subcarrier.Transmissions are allocated in units called resource blocks (RB)comprising 12 adjacent subcarriers or a period o 0.5 ms. Inaddition to the DMRS and PUSCH the gure also shows thePUCCH which is always allocated to the edge RB o the channelbandwidth alternating rom low to high requency on adjacent slots. Note that the requency allocation or one UE is typicallyless than the system bandwidth. This is because the number oRB allocated directly scales to the transmitted data rate whichmay not always be the maximum. The DMRS is only transmittedwithin the PUSCH and PUCCH requency allocation-unlike thereerence signals on the downlink which are always transmittedacross the entire channel bandwidth even i the channel is not

ully occupied.I the base station needs to estimate the uplink channel

conditions when no control or payload data is scheduled thenit will allocate the SRS which is independent o the PUSCH andPUCCH. The PUSCH can be modulated at QPSK, 16QAM or64QAM. The PUCCH is only QPSK and the DMRS is BPSK with a45 degree rotation.

Analyzing an SC-FDMA SignalFigure 8 shows some o the measurements that can be made ona typical SC-FDMA signal using the Agilent 89601A Vector Signal

Analyzer sotware. The IQ constellation in trace A (top let) showsthat this is a 16QAM signal. The unity circle represents the DMRSoccurring every seventh symbol, which are phase-modulatedusing an orthogonal Zado-Chu sequence.

Trace B (lower let) shows signal power versus requency.The requency scale is in 15 kHz sub-carriers numbered rom-600 to 599, which represents a bandwidth o 18 MHz or 100RB. The nominal channel bandwidth is thereore 20 MHz andthe allocated signal bandwidth is 5 MHz towards the lower end.The brown dots represent the instantaneous subcarrier amplitudeand the white dots the average over 10 ms. In the center o thetrace, the spike represents the local oscillator (LO) leakage - IQoset - o the signal; the large image to the right is an OFDMartiact deliberately created using 0.5 dB IQ gain imbalance inthe signal. Both the LO leakage and the power in non-allocatedsub-carriers will be limited by the 3GPP specications.

Trace C (top middle) shows a summary o the measuredimpairments including the error vector magnitude (EVM),requency error, and IQ oset. Note the data EVM at 1.15 percent is much higher than the DMRS EVM at 0.114 percent. This isdue to a +0.1 dB boost in the data power as reported in traceE, which or this example was ignored by the receiver to create

data-specic EVM. Also note the DMRS power boost is reportedas +1 dB, which can also be observed in the IQ constellationbecause the unity circle does not pass through eight o the 16QAMpoints. Trace D (lower middle) shows the distribution o EVM bysubcarrier. The average and peak o the allocated signal EVM is inline with the numbers in trace C. The EVM or the non-allocatedsubcarriers reads much higher, although this impairment will bespecied with a new “in-band emission” requirement as a powerratio between the allocated RB and unallocated RB. The ratio orthis particular signal is around 30 dB as trace B shows. The bluedots in trace D also show the EVM o the DMRS, which is very low.

Figure 8 Analysis o a 16QAM SC-FDMA signal



Trace E (top right) shows a measurement o EVM by modula-tion type rom one capture. This signal uses only the DMRS phasemodulation and 16QAM so the QPSK and 64QAM results are blank.Finally, trace F (lower right) shows the PAR — the whole point oSC-FDMA — in the orm o a complementary cumulative distribu-tion unction (CCDF) measurement. It is not possible to come upwith a single gure o merit or the PAR advantage o SC-FDMA overOFDMA because it depends on the data rate. The PAR o OFDMA isalways higher than SC-FDMA even or narrow requency allocations;

however, when data rates rise and the requency allocation getswider, the SC-FDMA PAR remains constant but OFDMA gets worseand approaches Gaussian noise. A 5 MHz OFDMA 16QAM signalwould look very much like Gaussian noise. From the white trace it can be seen at 0.01 percent probability the SC-FDMA signal is 3dB better than the blue Gaussian reerence trace. As every amplierdesigner knows, shaving even a tenth o a decibel shaved rom thepeak power budget is a signicant improvement.

Agilent Design and Test Solutions to HelpYou Take LTE ForwardAs a world leader in test and measurement solutions, Agilent

Technologies is at the oreront o emerging wireless and broadbandmarkets, such as LTE. Agilent provides the most complete suite oLTE tools, oering design and test solutions or the entire product development cycle -- rom RF and digital early design & test throughprotocol development to network deployment.

Useful Resources from Agilent

To order your free copy of Agilent’s new LTE Poster, as well as other valuable resources, please visit www.agilent.com/fnd/lte-mwj

Figure 9 Agilent’s MXG Vector Signal Generator with LTE Signal Studio sotware and the MXA Signal Analyzer with89600 LTE VSA sotware provides the most comprehensive solution or physical layer testing.

Reerences:“Long Term Evolution o the 3GPP radio technology,” 3GPP website, www.3gpp.org/Highlights/LTE/LTE.htm. LTE 36-series specication documentswww.3gpp.org/tp/Specs/html-ino/36-series.htm

“3GPP: Introducing Single Carrier FDMA”Agilent Measurement Journal, Issue 4 2008

Included in this comprehensive suite o LTE tools are solutionsto design and simulate LTE signals, create and measure LTE encodedsignals with sources and analyzers, and test mixed analog & digitalsignals – see gure 9. Just added to Agilent’s suite o LTE solutionsis a one-box tester that provides the platorm or protocol design andtest solutions, in partnership with Anite. This platorm will provide RFand protocol conormance test systems when they are needed. And,the newly introduced signaling analyzer enables analysis o the newLTE/SAE network.

So as you take LTE orward, Agilent will continue to clear the way.

5989-8230EN April 2008

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