Hybrid CTDMA

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568 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 12, NO. 4, MAY 1994

Performance of a Cellular Hybrid C/TDMA

Mobile Radio System Applying Joint Detection

and Coherent Receiver Antenna Diversity

Josef Blanz, Anja Klein, Student Member, IEEE, Markus NaBhan, and Andreas Steil

Absfruct-For future mobile radio systems, an appropriatelychosen mu ltiple access technique is a critical issue. Multiple accesstechniques presently under discussion are code division multipleaccess (CDMA), time division multiple access (TDMA), and hy-brids of both. In this paper, a hybrid CR DM A system using jointdetection (JD-CD'DMA) with coherent receiver antenna diversity(CRAD) at the base station (BS) receiver is proposed. Someattractive features of the JD-C/TDMA system are the possibilityto flexibly offer voice and data services with different bit rates,soft capacity, inherent frequency and interferer diversity, andhigh system capacity due to JD. Furthermore, due to JD , a clustersize equal to 1 can be realized without needing soft handover. Thesingle cell E ~ / N o erformance and the interference situation in

a cellular environment of the uplink of a JD-CD'DMA mobileradio system with CRAD is investigated in detail. It is shownthat the cellular spectrum efficiency is remarkably high, takingvalues up to 0.2 bit/s/Hz/BS in the uplink, depending on the actualtransmission conditions.

I . INTRODUCTION

OR future mobile radio systems, the selection of appro-F riate multiple access techniques is a critical issue having

major impact on the system capacity or cellular spectrum

efficiency, the radio network planning, the services being

supported, and the flexibility with respect to further system

evolution [11. Multiple access techniques presently under

discussion are code division multiple access (CDMA), time

division multiple access (TDMA), and hybrids of both [2]-[4].Fast frequency hopping (FFH) seems to be a less attractive

candidate, mainly due to the sophisticated code management

and synthesizers required [l]. Therefore, FFH will not be

discussed in this paper. Each conceivable system will most

likely include a frequency division multiple access (FDMA)

component since the total available system bandwidth has to

be split into subbands for technological reasons, to set up

umbrella cells or to support multioperator networks [11.

An advanced TDMA system approach could evolve from

the present day standards of the GSM (Global System for

Mobile Communications), the ADC (American Digital Cel-

lular), and the JDC (Japanese Digital Cellular) systems [5].

In a TDMA system, the users transmit in bursts, and the

bursts of different users, spread in duration due to multipath

reception, are separated by setting time windows at the re-

ceiver. This requires multiple access management entailing

overheads for protocolling, channel estimation, guard intervals,

and synchronization. On the other hand, a TDMA component

makes it easily possible to offer voice and data services with

different bit rates. TDMA systems are operated with cluster

sizes larger than 1 requiring either frequency planning or

adaptive channel allocation (ACA) [ 5 ] . By ACA, channels

are allocated depending on the actual traffic and interference

situation leading to higher capacity and more flexibility [5].

The CDMA multiple access technique has been especially

promoted by the proposal of a CDMA mobile radio system

by Qualcomm, Inc., claiming a large capacity increase over

TDMA [6]. In a CDMA system, a number of user signals ar-

rive at the receiver simultaneously in the same frequency band,

only separable by different user-specific signature sequences.

Due to time variance and multipath propagation of the mobile

radio channel, both intersymbol interference (ISI) and multiple

access intereference (MAI) occur at the receiver. Interference

can be treated in the receiver in different ways. Irrespective

of the chosen receiver structure, resulting from the larger

user bandwidth of CDMA systems as compared to TDMA

systems, CDMA systems provide inherent frequency diversity

and, due to the larger number of cochannel interferers, inherentinterferer diversity [7]. Furthermore, CDMA systems provide

soft capacity, which means that the number of traffic channels

available in a cell is not strictly fixed by the number of time

slots and frequency bands available, but is, up to a certain

bound, a flexible parameter determined by the amount of

interference and the required service quality [ l] , [6]. However,

this feature is not relevant in a fully loaded system [l]. A

CDMA system can be designed to handle speech and data

services with different bit rates and also variable bit-rate

services like variable bit-rate speech and packet data since,

on the average, a service takes only as much of the capacity

of the system as is actually needed [l], [6], [34]. In general,

a cluster size of 1 can be realized, requiring no frequency

planning [6]. However, even for a cluster size of 1, planning of

a system with and adjacentOr

overlapping cells of different sizes is necessary [11. Basically,

two types of receivers may be distinguished: one treating

IS1 and MA1 as noise, and the other exploiting same or all

Manuscript received June 28 , 1993; revised Nov. 25, 1993. This work wassupported by a research contract granted by Siemens AG, Munich, Germany.This paper was presented in part at the International Zurich Seminar, Zurich,

March 8-11, 1994.The authors are with the University of Kaiserlautem, D-67653 Kaiser-

lautem. Germanv.IEEE Log Nu h e r 9215408. knowledge about IS1 and MAL The first type of receiver can

0733-8716/94$04.00 0 1994 IEEE



BLANZ et al.: CELLULAR HYBRID CmDMA MOBILE RADIO SYSTEM 569

be realized, e.g., by a conventional matched filter or RAKE

receiver [8]. A RAKE receiver is able to resolve and combine

signals of one user received over different propagation paths.

However, both IS1 and MA1 still occur at its output. This

first type of receiver allows a flexible multiple access needing

only little effort for multiple access management, since it

does not exploit any information about interfering signals.

It allows the exploitation of the benefits of voice activity

monitoring [6] and cell sectorization [6] practically without

additional expense and organization overhead. On the other

hand, it requires a very tight power control [6] and leads

to an interference-limited system requiring soft handover [6]

in order to be able to provide service to users, e.g., at the

cell boundaries experiencing much interference. Soft handover

entails a large data exchange between different base stations

(BS's). The second type of receiver exploits knowledge about

IS1 and MA1 either by interference cancellation (IC) or by joint

detection (JD). IC means the subtraction of interference either

by serial cancellation, i.e., subtraction of the users' signals

in the order of decreasing signal strengths [2], 191-11 11, or by

parallel cancellation, i.e., creation and subtraction of cochannel

interference replica to produce a "cleaned" signal of each user

[9], [IZ], [13]. JD means the simultaneous detection of allusers' signals, which has to be done anyhow by the BS for

uplink transmission, using the knowledge about IS1 and MAI.

The optimum JD receiver derived in [35], [14], unfortunately,

is too complex to be implemented at present. Suboptimum JD

algorithms are proposed in [15]-[17], [4], [36], [37]. Going

from conventional-type CDMA receivers to IC and JD, the

performance of the receiver improves and thus capacity may

be increased because successively more knowledge about the

received signal is exploited. However, this improvement has

to be paid for by an increasing demand for multiple access

management.

In this paper, a hybrid system termed JD-CRDMA system

using a combination of TDMA and CDMA with JD is pre-

sented, combining the characteristics of both. Although only

the uplink is considered, the term system is used throughout.Cellular spectrum efficiency of the uplink of the JD-CRDMA

system will be investigated in detail. A poor cellular spec-

trum efficiency could, of course, be compensated by smaller

cells [l]. However, future mobile radio systems should be

able to handle high traffic densities with the restriction of

a limited system bandwidth without increasing infrastructure

costs. Therefore, the cellular spectrum efficiency is one of

the most important parameters of mobile radio systems. Both

the TDMA and CDMA components of the JD-C/TDMA

system enable voice and data services with different bit

rates, leading to a very flexible system. By introducing the

CDMA component, the advantages of inherent frequency and

interferer diversity and soft capacity can be exploited. Clearly,

the TDMA component entails the need of multiple access

management leading to overheads. However, this multiple

access management makes it possible to use JD in the CDMA

receiver, resulting in increased system capacity compared to

the conventional CDMA receivers including those using IC.

Furthermore, due to JD, a cluster size equal to 1 can be realized

without needing soft handover. Slow frequency hopping (SFH)

Fig. 1.

with CRAD.System model of the uplink of the JD-C/TDMA mobile radio system

can be introduced to further increase interferer diversity.

The paper is organized as follows. In Section 11, the model

of the uplink of the considered JD-C/TDMA mobile radio

system is presented. In this system, coherent receiver antenna

diversity (CRAD) is used at the BS receiver, leading to both

energy and diversity gain. A JD data estimation algorithm,

with CRAD according to [18], is described in Section 111.

The single cell E b / N o performance of the uplink of the JD-

CRDMA mobile radio system with CRAD is investigated in

Section IV for typical mobile radio situations. In Section V, the

interference situation in the uplink of the JD-C/TDMA mobile

radio system with CRAD in a cellular environment is treated.

From the results presented in Section IV and V, in Section VI

the cellular spectrum efficiency in terms of bit/s/Hz/BS of the

uplink of the cellular JD-C/TDMA mobile radio system with

CRAD is determined. The method for determining the cellular

spectrum efficiency is a general approach, and can also be

applied to other mobile radio systems.

11. SYSTEMMODEL

In this section, the equivalent lowpass model of the uplink

of a JD-C/TDMA mobile radio system with CRAD including

forward error correction (FEC) coding will be described. The

model of the uplink of a JD-C/TDMA mobile radio system

without CRAD and without FEC coding has been presented

in [19]. The basic structure of the uplink is depicted inFig. 1. Within the same cell in the same frequency band

of width B , a number of K users, i.e., mobile stations

(MS's), are simultaneously active, which are only separable by

their different user-specific signature sequences. Each MS is

assumed to have a single transmitter antenna. The transmitted

signals are received at the uplink receiver, i.e., the BS, over K,receiver antennas. Therefore, the transmission of the K user

signals takes place over K .K, different radio channels with

the time-variant complex impulse responses /-z('"'"")T, t ) , C =

1...IT,C, = 1...Ka,here the radio channel with the

impulse response b('"IkQ)(~,) refers to the connection of

mobile IC with receiver antenna IC,. The parameter T denotes

the delay time referring to time spreading of the transmitted

signals due to multipath reception and t denotes the real time

referring to the time variance of each radio channel.

Each of the K users transmits in bursts. The duration T b u

of each burst is chosen such that during the transmission

of one burst, the radio channel can be assumed to be time-

invariant. The burst structure is shown in Fig. 2. Like in the

GSM, each burst consists of two data blocks, a user-specific



~

570

L T,

Fig. 2. Burst structure.

IEEE JOURNAL ON SELECTED AREAS IN COMMUNICA TIONS, VOL. 12, NO. 4, MAY 1994

Fig. 3. Block structure of transmitter k .

midamble, i.e., a training sequence, which is used for chanel

estimation, and a guard interval to prevent subsequent bursts

from overlapping at the receiver. Each data block contains

a number N of data symbols of duration T, and each datasymbol consists of Q chips of duration T, =T,/Q denoting

the user-specific signature sequence. The midamble consists

of Lmid chips and the duration of the guard interval is Tg.

The bursts of the K users, which are simultaneously active

in the same frequency band, are assumed to be synchronized

at the receiver except for a timing error in the order of a

fraction of the symbol duration T,. By allocating a time

slot to each K-tuple of bursts, the TDMA component is

introduced in the JD-C/TDMA system. The JD-C/TDMA

system is also capable of providing an FDMA component.

In the JD-C/TDMA system, services with different bit rates

can be provided without changing the chip duration T , and

the spreading factor Q.

The transmitter and receiver structures (see Fig. 1) shall

be discussed in more detail. In Fig. 3, the block structure of

transmitter k is shown. A bit stream representing voice or

data information is convolutionally encoded and interleaved

to avoid burst errors. More significant bits may be protected

by stronger coding than less significant bits, similar to the

coding scheme for speech transmission of the GSM [20]. The

encoded binary data stream is mapped onto a symbol stream

consisting, in general, of complex data symbols. Each data

symbol is spread by a user-specific signature sequence. Bursts

as shown in Fig. 2 are generated by linking together two data

blocks and the user-specific midamble. After linear modulation

and D/A-conversion, the signal is passed through a transmitter

filter for lowpass filtering and an amplifier.

The block structure of the uplink receiver is illustrated

in Fig. 4. The amplified signal received at receiver antenna

k, is the sum of K user signals. This sum is filtered for

band limitation and noise suppression, and AD-converted.

The sampling frequency is the inverse of the chip duration

T,. A synchronization unit in the receiver has to compensate

for the slightly different delay times of the K, received

signals resulting from the different propagation paths to the

K, antennas. The set of samples resulting from K user

bursts is separated into a subset of samples corresponding

to the K midambles of the K users and two subsets of

samples corresponding to the K .N data symbols of the

K user bursts transmitted before and after the midamble,

respectively. A subset of samples corresponding to the K

midambles of the K users exists if the K, receiver antennas

are not too far apart from each other since the K bursts are

assumed to arrive synchronously at the receiver. Distances ofthe K, receiver antennas of a few wavelengths are sufficient

to generate independent fading processes at the K, antennas

[21]. From the subset of samples corresponding to the K

midambles, estimates of the K channel impulse responses

- ( k , k a ) ( ~ ,) ,k = 1 ..K, bandlimited to the user bandwidth

B,are determined by channel estimator k,. Channel estimation

can be performed by applying the algorithm described in [22].

A data estimator applying a JD algorithm with CRAD deter-

mines continuous valued estimates of the K .N data symbols

transmitted before and after the midamble, respectively, using

the knowledge about the estimates of the K .K, channel

impulse responses h ( k 3 k a ) ( T , ) , = I . . K ,IC , 1...K,,

the K user-specific signature sequences and the K, subsets of

samples corresponding to the K .N data symbols transmitted

before or after the midamble, respectively. The user-specific

signature sequences are known at the uplink receiver, as they

are allocated to each mobile by the base station during first

access or handover. The 2 .N complex, continuous valued

estimates of the data symbols transmitted by user k ,k =

l . - . K , are mapped onto a real-valued data stream. This

data stream is deinterleaved and convolutionally decoded in

a soft input decoder. Furthermore, a reliability measure may

be processed in the convolutional decoder as the sum of theK,

energies of the K , estimates of the channel impulse responses

-( '>")(~, t ) , lc ,= l . . .K, , for fixed k, which differs from

burst to burst. More sophisticated reliability measures could

also take into account the distance of the data symbols of

one burst from the midamble and the velocity of the mobile

since, with increasing distance and increasing velocity, thedifference between the true channel impulse response and the

corresponding estimate increases.

111. JD DATAESTIMATIONALGORITHMITH CRAD

A JD data estimation algorithm with CRAD based on zero

forcing (ZF) equalization [18] is described. In [41, [15]-[17],

[23], JD data estimation algorithms have been proposed for

the single receiver antenna case. In [18], JD data estimation

algorithms with CRAD based on ZF and minimum mean

square error (MMSE) equalization with and without descision

feedback are introduced. The description of the JD data

estimation algorithm with CRAD based on ZF equalization

considered in this paper emerges from a discrete-time lowpass

representation of the uplink of the JD-C/TDMA mobile radio

system with CRAD, (see Fig. 9, epresenting the blocks be-

tween the data-to-symbol mapper in Fig. 3 and the symbol-to-

data mapper in Fig. 4. The sample interval for the discrete-time

modeling is the chip duration T,. In the following, sequences,

vectors and matrices are in boldface, complex values are



BLANZ et a l . : CELLULAR HYBRID CITDMA MOBILE RADIO SYSTEM

channel

sample symbol to convol-stimator -utiond

decoder sink 1 *

- 1 to dataata deinter-

mapping- eaver -1 1 1

-I

JD

symbol to convol-data deinter- utiond

mapping- eaver - ecoder sink 1 *1 1 1

1 to data-I

JD

..

antenna receiver

-J ip: 1

I

synchronization :

channel

separator

1 - 1 tsynchrkization

Fig. 4. Block structure of the uplink receiver.

underlined and the symbols (.)* and ( . )T designate complex

conjugation and transposition, respectively. The discrete-time

modeling is derived analogously to the single antenna receiver

case described in [4]. The transmission of one single data block

before or after the midamble, (see Fig. 2) is considered, and theinfluence of the midamble on the data blocks due to multipath

reception is assumed to be perfectly eliminated. The K users,

which are simultaneously active in the same frequency band,

are transmitting finite data symbol sequences:

-k) &I, &) . .&')T, &) E y ,

k = l . . . K , n = l . . . N , K , N € I N (1)

of N M-ary complex data symbolsgLk)with durationT, which

are taken from the complex set:

V={v, ,v , . . . IM),vp E e,

p = l . . . M , MEIN. (2 )

The size M of the set and, thus, the actual data rate may

be varied due to the service provided or due to the desiredtransmission quality. Each of the data symbols c lLk) , n =

1. . N , of mobile IC is spread by the user-specific signature

sequence:

= . . ( k ) T ( k ) E c,- CQ 'CQ

k = l . . . K , q = l . . . Q , K , Q E W (3 )

consisting of Q complex chips of duration T,. The

influence of the modulator in Fig. 3, the analog components

in Figs. 3 and 4, and the mobile radio channel with the

impulse response b(k>ka)(~,) s represented by the discrete-

time channel impulse response:

h ( k , k a ) = (k,ka) ( k & a ) , .. (k W ) T- (h , rb3 hw' >

h c ' k a ) E C ,k = l . . . K , IC,=l...K,,

w =1. .W, K ,K,, W E IN (4)

consisting of W complex samplesh c , k Q )aken at the chip rate

l/Tc.The combined channel impulse responses are defined as:

b(k,ka) = b("'"a) b ( k , ' " a ) . . . ( k , k a ) T- -1 7-2 bQ+W-,)

estimator

withata I...

CRAD ~ ~ H ~tional to data

mapp ing leaver decoder sink K

I

1'I . I

&)

JD

data

estimator7-ith

Discrete-time lowpass representation of the uplink of theJ6 Q T D M A mobile radio sytem with CRAD.

--(k>ka)c ( k ) ,

tp)c, IC =1 . . .K , IC, =1 . . K,,

1 =1.. Q

+W - 1,

K ,K,, Q,W E IN( 5 )

i.e., as the discrete-time convolution of h("'"-)ntroduced in

(4) with the signature sequences g ( k )defined by (3).

First, the well-known case of a single receiver antenna with

label k a is considered [4]. According to Fig. 5, the received

sequence of length ( N .Q+W - 1) prevails at antenna



512 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 12, NO . 4, MA Y 1994

withwhere the components of d are given by:

%fe(k.), k , =1 . . Ka,G(N .Q+W - l ) . ( k , - l ) +n --n

n =l . . . N . Q +W - 1,N . ( k - l ) + - n %fd-nk ) k = 1 . . . K ,

(9) Kay N , Ql W E N. (17)= l - - . N , K ,N E N

ani defining the (N .Q +W - ) x K .N matrix The combined received factor e, according to (16), is pro-

cessed in a JD data estimator with CRAD in order to determineA ( k a ) (A!'U)), i = 1 . . N . Q + W - 1 ,--2J continuous valued estimates:

( oa) -(lIT -(2)T k K l T ) Tj = l - . - K . N , k ,= l. .. K, ,

-= ( d ,d ...-=(41,4 2 . . . N . K ) (18)

of d defined by (8). The basic concept of the JD technique

with CRAD considered in this paper is given by the set of

wherelse

the received sequence can be represented by:M=(A&), i = l . . . K . N ,

j = 1 ..K a . ( N . Q +W - 1) (20)

is a K .N x K, . ( N .Q +W - 1) matrix. The choice of

the matrixM determines the equalizer type. In the following,

,(ka) = .. ( 'a ) T%"Q+W-l)

- (Ica)d+n ( k a )- - -

(11)a -I . . . Ka7 Ka,N ,Q ,W E N.

the representation of M is introduced for the zero forcing

block linear equalizer (ZF-BLE) for JD with CRAD [18].

The representation ofM or the minimum mean square emor

block linear equalizer (MMSE-BLE) can be found in [18].

Now, the unified mathematical representation for the case of

Ka receiver antennas is presented. With the Ka . ( N .Q +W - 1) x K .N matrix:

-= ( ~ ( 1 1 T-A ( ~ ) T ..-( K ~ ) T ) T , ya E IN (12)For the derivation, it is assumed that the combined channel

impulse responses b(k7ka)see (5)] are perfectly known at the

receiver. The ZF-BLE [24], [4], [23], which is based on thend, with the combined noise vector,



BLANZ et al. : CELLULAR HYBRID CmDMA MOBILE RADIO SYSTEM 51 3

of defined in (8). The estimate d is free of IS1 and MA1 but

still contains a perturbation term represented by filtered noise.

The computational effort to calculate d according to (22) is

significantly higher compared to the effort in the case of a

conventional matched filter or RAKE receiver, but much lower

than the effort needed for the optimum JD receiver proposed

in [141. For a deeper insight and for the sake of completeness,

a decomposition of the matrix M will be explained and the

SNR per symbol

k =1 . .K , n = l . . - N (23)

a,f the output of ZF-BLE associated with the estimate

&,.(k- l )+n of the data symbol cifk) transmitted by user IC

is determined. With the Cholesky decompositions [27],

-*T€&lA =H*T.E.EH (24)

where H is a normalized upper triangular matrix, E s a real

valued diagonal matrix, and

.I ; (25)

- 1 L*T

-n - -

where L is an upper triangular matrix, the matrix M can be

displayed as-=(A*%,- 'A*%,-

( IJ*T.E) 1

(L )

- - -S1 and MA1 canceler whitening filter WF2

v v . (26)matched filter MF whitening filter WF1

Therefore, the JD data estimator with CRAD of Fig. 5 contains

a whitening or decorrelating filter WFI for noise prewhitening,

a matched filter MF matched to the concatenation of the

combined channel impulse responses b("'""),nd the impulse

response of the whitening filterW F I , a second whitening filter

WF2, and an IS1 and MA1 canceler, cf. [18], [23]. With (23),

the SNR y('>,) per symbol at the output of the ZF-BLE is

equal to:

k = l . * . K , n = l . . - N , K ,NE IN (27)

with the element in the ith row and i t h column of the

matrix X.After symbol-to;data mapping-cf. Fig. 4, the continuous

valued estimates d are taken as soft inputs to the con-

volutional decoder. In [18], [23], versions of the ZF and

MMSE equalizers with decision feedback are presented, which

perform better than the corresponding linear equalizer types.

In equalizers using decision feedback, as a first step a subset of

the continuous valued estimates df), n =1. . .N , k =1 . Kis quantized, and the influence of the quantized, i.e., discrete-

valued estimates, is subtracted from the received signal. In a

second step, new continuous-valued estimates for the rest of

the data symbols are determined starting from the adjusted

received signal. Steps one and two are repeatedly performed.

Nevertheless, for the decision feedback versions, it would be

possible to generate soft inputs for the convolutional decoder

by using the quantized estimates for decision feedback and the

continuous-valued estimates before quantization for decoding.

Decision feedback versions will not be considered in this

paper.

IV. E b / N o PERFORMANCE OF TH E UPLINK OF A

JD-C/TDMA MOBILERADIO YSTEM WITH CRAD

In this section, the advantage bit error rate (BER) pb in

the uplink of a JD-CRDMA mobile radio system with CRAD

is investigated for a single cell as a function of the average

signal-to-noise ratio (SNR) & / N o per net information bit

and per antenna at the receiver input based on a continuous-

time system model. The additive stationary noise sequence

in (13), which could be caused by intercell interference in the

interference-limited case, is assumed to be white and Gaussian

distributed with zero mean and covariance matrix

& = u 2 . 1 , a 2 E R , u 2 > o (28)

with I the identity matrix and u2 the variance of the noise.

Variations of the received power due to path loss and shad-

owing, i.e., lognormal fading, are assumed to be perfectly

eliminated by power control. However, the variations of the

received power due to Rayleigh fading are present in the

received signals. The time-variant Rayleigh fading channels

are simulated according to the channel models specified by

COST207 [28], [29] for bad urban area (BU) and rural area

(RA). Channel estimation is performed by the algorithm de-

scribed in [22] with channel estimate postprocessing according

to [30]. The parameters of the JD-C/TDMA mobile radio

system with CRAD used for the simulations are given in

Table I. The user bandwidth B of 1.6 MHz is smaller than the

inverse of the chip duration T,. This results from the choice

of a digital chip impulse filter, i.e., linear modulator with

impulse response equal to the GMSK basic impulse CO(.)

of time bandwidth product 0.3 [31], leading to a compact

spectrum. Furthermore, the digital chip impulse filter and the

user-specific signature sequences have been designed in such

a way that the magnitude of the complex envelope of the

transmitted signals is approximately constant except at the

transition between two consecutive data symbols. The user-

specific signature sequences are nonorthogonal. Two antennas

with uncorrelated channel impulse responses, i.e., independent

fading processes, are assumed. The data symbols are 4PSK

modulated. In the symbol-to-data mapper (see Fig. 4), the

in-phase and quadrature components are separated. The con-

volutional decoder processes the soft input information and a

reliability information which is the sum of the K, energiesof the K, estimates of the channel impulse responses, (see

Section 11). A rate 1/2 convolutional code with constraint

length K, equal to 5 is assumed, which is also used in the

GSM for the most significant bits [20]. Block interleaving over

I o equal to four bursts is underlying the simulations with a

time gap Dbu equal to 6 ms between the beginning of two



574 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 12, NO . 4, MA Y 1994

user bandwidth Bantennas h',burst structure:

burst duration Tbu

midamble chips L m i d

guard interval T R

data sym bols per data block Nsymbol duration T.

chips per symbol Qchip duration TC

TABLE IPARAMETERSF THE JD-CRDMAMOBILE ADIO

SYSTEM WITH CRAD FOR THE SIMULATIONS

1 . 6 M H z

2, independent

0.5 ms

26 8

30 ps

247P14

0. 5 i s

constraint length

rate

interleaver

interleaving depth

t imeeginningap betweenf tw ohe

consecutive bursts

of the same userresulting net data rate

per user

filterstransmitter filter

receiver filter

size of data symbol alphabet I M I 4 (4PSK)convolutional encoder: I I

K , 5 (a )

R,

ID 4 bursts

11 2 %'[ I 1 I I I 1

D b u 6 ms

0 1 : w:::K = 8

R 8 kbit/s 0 0 1 :

Butterworth filter of order 4

Butterworth filter of order 10

0001 7

by the interleaving depth ID. Due to the TDMA component,

the data rate per user can easily be varied. If one user-specific

signature sequence is assigned to a user in every time slot, the

resulting maximum net data rate per user achievable is equal

to 96 kb/s. The net data rate per user can further be increased

by assigning more than one user-specific signature sequence.

However, these assignments have impacts on the distributionof intercell interference, which is discussed in Section V.

The simulation results given in the following show the

performance of the JD-CRDMA mobile radio system with

CRAD with the parameters of Table I in two typical mobile

radio situationsfor different numbers K of active users and for

the ZF-BLE for JD-see Section 111. In Fig. 6(a), the average

BER Pb is depicted versus the average SNR E b / N o per

antenna and per net information bit at the receiver input for the

BU channel model, a velocity w of the mobiles of 30 km/h with

respect to a carrier frequency fo of 1800 MHz for numbers

K of active users equal to 2, 4, 6, and 8. With increasing K,

the & , / N o equired for a given BER Pb increases. This well-

known effect in CDMA systems is called soft degradation or

soft capacity. It gives the possibility to overload the system

occasionally at the price of a reduced service quality for all

users. By applying voice activity monitoring [6] for voice

services, the performance of the JD-CRDMA mobile radio

system with CRAD can be further enhanced, on the one hand

The performance enhancement by voice activity monitoring

will not be covered in the following.

In Fig. 6(b), the average BER Pb is shown versus the

average & / N o at the receiver input for the RA channel

model, a velocity w of the mobiles of 150 km/h with respect

to a carrier frequency fo of 1800 MHz for a number K of

active users equal to 2, 4, 6, and 8. For simplicity, whensimulating the RA channel model, the amplitude of the line-

of-sight path was assumed to be Rayleigh instead of Ricean

distributed, representing a worst-case situation. Compared to

the BU channel model [see Fig. 6(a)], the inherent frequency

diversity is much lower for the RA channel model. Thus, the

gain achievable by interleaving and CRAD is larger for the RA

channel model than for the BU channel model. With increasing

velocity w of the mobiles, on the one hand, the interleaving

gain increases further but, on the other hand, errors due to

the fact that the channel estimates differ from the true channel

impulse responses, in particular towards the beginning and end

of the burst, lead to performance degradation. From Fig. 6(a)

and (b), it turns out that the performance of the JD-CRDMA

system is better for the BU channel model and w equal to 30

km/h than for the RA channel model and w equal to 150 km/h.

v. INTERFERENCE SITUATION IN THE UPLINK OF A CELLULAR

JD-CRDMA MOBILERADIOSYSTEM WITH CRAD

because of the decreasing requirements for & / N o and on the

other hand because of the decreasing intercell interference.

The & / N o performance discussed in the previous section

gives the performance of the uplink of a JD-CRDMA mobile



BLANZ et al.: CELLULAR HYBRID CF DM A MOBILE RADIO SYSTEM 575

radio system with CRAD if a single cell is considered. To

analyze the performance of the uplink of the JD-CRDMA

mobile radio system with CRAD in a cellular environment, the

interference situation in the interference-limited case shall be

investigated. The statistics of the effective interference power

are derived from the cochannel interference when neglecting

adjacent channel interference. In this section, the cumulative

distribution function (cdf) [24] of the effective carrier-to-

interference ratio C/I for the JD-CRDMA system with CRAD

obtained by simulation is presented for different cluster sizes

r and different grades of interferer diversity. The knowledge

of the cdf of the effective C/I will be used to determine

the cellular spectrum efficiency later on. In general, the total

interference power Itotxperienced by a BS can be expressed

by:

..

-’

._

.’

Itot =I in t r a + i n t e r -k N t h (29)

with Iintra denoting the intracell interference power produced

by MS’s inside the BS’s cell, Iinter the intercell interference

power produced by MS’s outside the BS’s cell, and N t h

thermal noise power. For the JD-CRDMA system under

consideration in this paper, the signals originating from MS’sinside the BS’s cell are perfectly separated due to the appli-

cation of the ZF-BLE discussed in Section 111. Thus, I in t r a in

(29) does not contribute to the effective interference power.

Furthermore, since the interference-limited case is considered,

N t h is set to zero. Therefore, the effective interference power

I is solely given by the intercell interference l i n t e r .

In order to derive the cdf of C/I by simulation, the following

assumptions are made: The attenuation of a link BS-MS is

the same for all K, antennas and is given by path loss and

shadowing. The path loss is proportional to d - ” , where d is

the distance between BS and MS . The attenuation exponent

a , which depends on the environment and the antenna height,

is assumed to be equal to 4 [6], [32]. The attenuation due to

shadowing is lognormally distributed with a standard deviation

qog f 8 dB. Local correlations of the attenuations due to

shadowing are not taken into account. An idealized regular

cellular structure of equally sized hexagons is considered, (see

Fig. 7). In the middle of each hexagonal cell, a BS is located.

The MS’s are uniformly distributed over the whole cellular

structure. Frequency reuse patterns with cluster sizes r equal to

1, 3, and 4 are regarded. For a cluster size r , the total available

system bandwidth is divided up into r disjoint subsets, each of

them containing several disjoint frequency bands of width B.

Each particular subset is reused in each r t h cell. Since adjacent

channel interference is neglected, without loss of generality,

only one particular frequency band of widthB of each subset is

considered. Thus, a total amount of r frequency bands of width

B is used to calculate the interference. These r frequency

bands are distinguished by an index p , p =1. T (see Fig. 7) .Power control is ideal, i.e., the variations of the power received

at a BS from a MS affiliated to this BS due to path loss

and shadowing shall be perfectly eliminated by power control.

Handover is perfectly power controlled, which implies that a

MS is always connected to the BS with minimum attenuation.

By doing so, a lower interference margin is obtained.

reference

cluster

Fig. 7. Regular cellular structure for T =3 .

To determine the effective interference power I , a reference

cluster is introduced, (see Fig. 7). Each BS of the reference

cluster experiences an interference power I ( P ) , = 1 . .r ,which is the sum over all received powers I k ) ,where 12) s

the power received from the particular MS m not located in

the reference cluster but using the same frequency band p , i.e.,

d l m outside

reference c luster

The interference power 12) roduced by MS m served by BS

b outside the reference cluster at BS 0 of the reference cluster

using the same frequency band p is given by the distance 6

between MS m and BS b and the lognormal variable

(22,defining path loss and shadowing, respectively, of the link

between MS m and BS b (cf. Fig. 7) and the quantities 6giand < characterizing the attenuation of the link between

MS m and BS 0 according to:

The constant C is the power received at BS b originating from

MS rn affiliated to it. Due to ideal power control, the received

carrier power C is the same for all links. Now, with (30)

and (31), the carrier-to-interference ratio for each BS of the

reference cluster is:

reference c luster

(32)

The cdf of the random variable C/I(P) s the same for all

p , p = 1 . . r. Therefore, the index p is omitted in what

follows. The cdf of C/I for the uplink of the JD-CRDMA



516 IEEE JOURNAL ON SELECTEDAREAS IN COMMUNICATIONS, VOL. 12, NO. 4, MA Y 1994

mobile radio system with CRAD depends on the cluster size T

and the average numberr of MS's which are simultaneously

active within the same cell in the same frequency band. In

Fig. S(a)-(c), the cdf of C/I for the uplink of the JD-CDDMA

mobile radio system with C U D s depicted for cluster sizes

T equal to 1, 3 , and 4 with as a parameter (dashed lines).

For a given r, /I increases with increasing cluster size T

because of increasing distance, i.e., path loss, between cells

using the same frequency band. For a given T , C/I decreaseswith increasing K, .e., average number of interferers per

cell, but the slope of the curves gets steeper. The increasing

steepness of the curves results from an increasing grade of

interferer diversity [l], [6]. A steeper curve results in a more

constant C/I, which is desirable in order to maintain a constant

service quality.

Another measure to reduce the fluctuations of C/I and, thus,

to increase interferer diversity is to apply slow frequency

hopping (SFH) in combination with interleaving and coding,

(see Section IV). When interleaving over U bursts with one

hop per burst is performed, different MS's are interfering from

burst to burst and the effective carrier-to-interference ratio is

given by

(7)1. 2(4)u'=l U'

(33)

with ( C / I ) v ~he carrier-to-interference ratio according to (32)

measured in the time slot used for transmission of burst

U'. For the simulations, it is assumed that the interference

situations at the BS's of the reference cluster are statistically

independent from hop to hop. In a real system in the uplink,

this can approximately be achieved by using different hopping

sequences in cells with the same frequeEy band as in the

GS M [20]. In Fig. S(a)-(c), the cdf of (C/ I ) of the uplink of

the JD-C/TDMA mobile radio system with CRAD and SFH

over U equal to four frequency bands is shown for cluster

sizes T equal to 1, 3, and 4 with K as a parameter (solid

lines). As expected, the curves with SFH are steeper than thecurves without SFH. Note that the expectation of the carrier-

to-interference ratio with SFH is the same as without SFH, cf.

(33). A similar increase in interferer diversity can be achieved

by applying time hopping (TH) instead of SFH. The steepness

of the curves can further be increased by applying C/I control

instead of power control, which will not be considered in this

paper. The steeper the curves, the faster and more reliable the

handover procedure has to be in a real system [ 5 ] .

VI . CELLULAR SPECTRUM EFFICIENCYF TH E UPLINK OF A

CELLULARD-CR DMA MOBILE ADIOSYSTEM WITH CRAD

The cellular spectrum efficiency Q in terms of bit/s/Hz/BS

[7], [33] is determined in the interference-limited case for

the uplink of the cellular JD-CDDMA mobile radio system

with CRAD. First, the exact method to determine the cellular

spectrum efficiency Q is explained, and second, a simplified

method applied in this paper is described. Both methods

originate from the assumption that a BER Pb equal to

is necessary to obtain the desired service quality, which is a

common value for mobile radio transmission. The situation of

Prob (10 . og,, (F) <z)

1

0.1

0.01

0.0oj-15 -10 -5 0 5 10 15 0

X

-10 -5 0 5 10 15 20

X

(b)

Prob (10' log,, ( y ) <z)

-10 -5 0 5 10 15 20

X

(C)

Fig. 8. cd f of C/l of the uplink o f the J D - C W M A system with I; s aparameter without SFH (- - ) and with SFH (-) over four frequency bands.(a) T = 1. (b) r = 3 . ( c ) T =4.

interest is an evenly loaded system, i.e., a system with the

same average number of users in every cell. The outage

probability Pout ives the probability that Pb equal to l o p 3cannot be achieved in a cellular environment for a certain

cluster size T and a certain average number K of users in

every cell, i.e.,



~

578 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 12, NO. 4, MAY 1994

q / b i t / s /H z /WS )

(a)

.05 . I .15 .2 .25 . 3

7 / b i t / s /H z /WS )

(b)

Fig. 9. Pout ersus 9with T as a parameter of the uplink of the JD-C/TDM Asystem with CR AD according to Table I, fo =1800 MHz, without SFH-) and with SFH (-). (a) BU channel model, ti =30 kmh. (b) RA channelmodel, t i = 150 kmh.

spectrum efficiency 7.Of course, cellular spectrum efficiency

could be further increased by applying soft handover.

The capacity of the digital cellular TDMA systems ADC,

GSM, and JDC is discussed in detail in [ 5 ] . For the sake

of comparison, a very rough estimate of cellular spectrum

efficiency 7 of the GSM yields 8 . 1 3 .103) / [ (200 . lo3) . 3 ]bit/s/Hz/BS =0.173 bit/s/Hz/BS, since a maximum number of

eight users with a net data rate of 13 kb/s can be active in

a frequency band of 200 kHz. The cluster size T for which

this number of users can be achieved is 3, which is a rather

optimistic assumption [ 5 ] . For a cluster size T equal to 4,

the cellular spectrum efficiency 7 of the GSM roughly equals

0.13 bit/s/Hz/BS. Although cellular spectrum efficiency is a

major issue, for detailed system comparisons other important

points have to be considered like additional features, e.g.,

system flexibility concerning the introduction of new cell

sites, new frequencies and umbrella cells, traffic planning,

handling of services with different bit rates, sensitivity to

imperfections of, e.g., power control and handover, irregular

cellular structures and planning constraints, expense in terms

of need for frequency planning, receiver complexity, cost and

power consumption, and additional gains achievable.

VII. CONCLUSION

In the present paper, the uplink of a hybrid JD-CRDMA

mobile radio system applying a combination of TDMA and

CDMA with JD has been presented and its performance

has been investigated. In this system, it is easily possible

to offer voice and data services with different bit rates.

Coherent receiver antenna diversity (CRAD) is used at the BS

receiver, leading to both an energy and diversity gain. With

an increasing number of active users, the & / N O performance

of the system gradually degrades, which is the soft capacity

feature. The cdf of C/I characterizing the interference situation

in a cellular environment is rather steep due to inherentinterferer diversity, and its steepness can be further enhanced

by providing additional interferer diversity by introducing

SFH. The cellular spectrum efficiency, which is one of the

most important parameters of mobile radio systems, takes

values up to 0.2 bit/s/Hz/BS in the uplink, depending on the

actual transmission conditions. This is achieved for a cluster

size of 1 without application of soft handover. For a balanced

statement, certainly other important points besides cellular

spectrum efficiency need more detailed investigations than

could be given in this paper, e.g., radio network planning,

flexibility with respect to future system evolution, performance

improvements achievable by cell sectorization and voice ac-

tivity monitoring, sensitivity to imperfections, and expense in

terms of infrastructure and handheld cost. At a first glance,

most of these points seem to be in favor of the proposed

JD-C/TDMA system with CRAD.

ACKNOWLEDGMENT

This paper is devoted to Prof. Dr.-Ing. habil. Paul Walter

Baier on the occasion of the 20th anniversary of the Research

Group for RF Communications. The authors wish to thank the

staff of the computer center “Regionales Hochschulrechenzen-

trum Kaiserslautern (RHRK)” and acknowledge the usage of

the SiemensFujitsu VPlOO supercomputer. The authors are

grateful to Dip1.-Phys. Dr.-Ing. P. Jung for major contributions

to the theory of joint detection with coherent receiver antenna

diversity presented in Sections I1 and 111. Furthermore, the au-

thors wish to thank Dip1.-Ing. B. Steiner for numerous helpfuldiscussions. The authors are also grateful to the anonymous

reviewers for their helpful comments.

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ests are in the field of

Josef Blanz was bom in Kaiserslautem, Germany,in 1967. He received the Dipl-Ing. degree in elec-trical engineering from the University of Kaiser-slautem in 1993.

During 1990-1992, he freelanced with the Mi-croelectronics Centre of the University of Kaiser-slautem, where he was engaged in the design of

digital circuits with field programmable gate arrays.In March 1993, he joined the Research Group forR FCommunications of the University of Kaiserslauternas a Research Assistant. His present research inter-

antenna diversity techniques for mobile radio systems.

Anja Klein (S’93) was bom in Kaiserslautem, Ger-many, in 1967. She received the Dip1.-Ing. degreein electrical engineering from the University ofKaiserslautern in 1991.

Since then, she has been a Research Assistantwith the Research Group for RF Communicationsof the University of Kaiserslautem. Her researchinterests are mainly concentrated on mobile radiocommunications and estimation theory.

Markus NaRh an was born in Zweibriicken, Ger-

many, in 1965. He received the Dip1.-Ing. degreein electrical engineering from the University ofKaiserslautem in 1992.

Since then, he has been a Research Assistant withthe Research Group for RF Communications of theUniversity of Kaiserslautem. His research interestsare mainly concentrated on simulation techniquesand multiple access techniques for mobile radio

systems.

Andreas Steil was bom in Cochem, Germany, in1964. He received the Dipl.-Ing. degre e in electricalengineering from the University of Kaiserslautemin 1991.

Since then, he has been a Research Assistant

with the Research Group for RF Communicationsof the University of Kaiserslautem. His present re-search interests are mobile radio systems, especiallycellular aspects and spectral efficiency.

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