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A scheme of Multi-User Reusing One Slot

on Enhancing Capacity of GSM/EDGE Networks

Xiang Chen,Zesong Fei,Jingming Kuang,Linnan LiuRCDCT, Modern Communication Lab, Dept. of E. E.

Beijing Institute of Technology (BIT)Beijing, P. R. China

xiaxiang@bit.edu.cn, feizesong@bit.edu.cn

Guang YangWireless Technologies Division, Research Institute

China Mobile Communication Corporation (CMCC)Beijing, P. R. China

yangguangyj@chinamobile.com

AbstractGSM network is seeing its greatest expansion because

of the growing demand for mobile voice services in emerging

markets recently. A newly proposed technology, multi-user

reusing one slot (MUROS), would help operators in densely

populated cities to alleviate the strain on their networks. The

concept of MUROS is based on multiplexing two or more users

onto one time slot without degrading the speech quality. We

improved one solution of MUROS, orthogonal sub channel

(OSC) in this contribution. For the downlink (DL) OSC, we

designed new training sequence codes (TSCs) which are low

cross-correlated with legacy TSCs. For the uplink (UL) OSC, we

adopted successive interference cancellation based on minimum

mean square error (MMSE-SIC) algorithm. Theoretical analysis

and simulation results shows that with proper TSC design, OSC

is a promising scheme of MUROS due to its ability to double the

capacity of GSM/EDGE networks without degrading the speech

quality very much.

Keywords- Multi-User Reusing One Slot (MUROS);

Orthogonal Sub-channel (OSC); Training Sequence Code (TSC);

GSM/EDGE;

I. INTRODUCTIONBeyond dispute, GSM network is the most successful

commercial cellular mobile communication system hitherto. Itis reported that the number of GSM users all over the worldhad reached 2.5 billion by 2007. The increase in user amountand voice traffic puts a huge pressure on operators especiallywithin populous countries. Furthermore, as voice and dataservice price gets cheaper, most operators face the challenge toobtain efficient utilization of hardware and spectrum resource.

According to the demand of operators, many vendors haveprovide their solutions, such as adaptive multi-rate half-rate(AMR-HR) with interference canceling technologies on bothBTS and MS sides, enhanced full rate (EFR), carrier frequency

pool (CFP) and intelligent underlay overlay (IUO) of dual-bandnetwork. These solutions are based on the traditional conceptthat one user occupies one time slot. The capacity gain of EFRor CFP is limited. IUO simply adds new carrier frequencies in1800 MHz band. There is no gain in spectrum efficiency.AMR-HR provides significant voice capacity gain as a speechcodec, but contributes little to the data service. The multi-userreusing-one-slot (MUROS) technique originates a new idea toenhance the capacity of both voice and data service [1].

Just as its name implies, MUROS is a concept of increasingvoice capacity by multiplexing more than one user on a singletime slot and ARFCN, thus the capacity can be considerablyimproved for a certain TRX hardware and possibly spectrumresource. Specifically, the MUROS technique constructs amultiple-input and multiple-output (MIMO) system in uplink

that different handsets transmit signals through their ownantenna and base station (BS) receives signal with two or moreRx antennas. In downlink, multiple users data are transmittedwith higher-order modulation mode, and each mobile station(MS) receives its own signal in specific way. Note that thehigh-order modulation scheme should be selected carefully tomake sure that it is available at the edge of a cell.

In theory, MUROS can double voice capacity or even morewith negligible impact to handsets as well as to networks.MUROS is unlike the speech codec approach to increase voicecapacity, e.g. packing two GSM-HR mobiles onto one time slot

but rather to multiplex four GSM-HR mobiles onto one timeslot. Namely, the MUROS feature is expected to provide voice

capacity improvement both for full rate and half rate channels.Its capability of increasing voice capacity is very attractive.

Another important motivation for MUROS is that it cantake advantage of widely available DARP phase I capable MS,i.e. handsets supporting single antenna interferencecancellation (SAIC) technology [2, 3]. This implies that higher-order modulation scheme may not be necessary in downlink.SAIC supported MS is able to detect its own signal when itworks at MUROS mode [4].

We improved one solution of MUROS, orthogonal subchannel (OSC) [5, 6] in this contribution, which concentrateson the two user reusing one slot case. For the downlink (DL)OSC, we designed new training sequence codes (TSCs) which

are low correlated with legacy TSCs. For the uplink (UL) OSC,we employed successive interference cancellation based onminimum mean square error (MMSE-SIC) algorithm.

The rest of the paper is organized as follows. Section IIintroduces the concept of MUROS, as well as the OSC schemeto implement the concept. The detail of OSC technology isdescribed in section III. We proposed low cross-correlationtraining sequence code in section IV. Then we present andanalyze the simulation results in section V. The conclusions aregiven in Section VI.This work is sponsored by Research Institute, China Mobile Communication

Corporation. Contact author: Zesong Fei, feizesong@bit.edu.cn

1-4244-2424-5/08/$20.00 2008 IEEE ICCS 20081574

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Figure 1. MUROS concept description.

II. THE CONCEPT OF MULTI-USERREUSING ONE SLOT

The GSM network is a TDMA system. Each carrierfrequency is divided into eight time slots. Each slot is allocatedto only one user to transmit its burst. As its name suggested,MUROS concept provides two or more users allocated on thesame radio resource, i.e. the same time slot on the same carrier

frequency, as shown in fig. 1. Each user occupies oneorthogonal sub-channel, or in another way, the MUROS userssignals are mixed together, receivers distinguish them by lowcross correlation training sequence codes (TSCs).

The GSM network has eight legacy TSCs, as table II given.Each training sequence code (TSC) is assigned to a cell, andthe adjacent co-channel cells must be assigned with differentTSCs, so that a BSS can tell which MS is in its cell bychecking TSC in the middle of each burst. MUROS conceptintroduces eight or more new TSCs in order that each cell can

be assigned two or even more TSCs without changing currentfrequency planning. Each BSS uses the TSCs to pack two oreven more users onto one slot. The TSCs assigned to the same

BSS must be low cross-correlated so that receivers candifferentiate its own signal from MUROS partners in the sameslot, with interference cancelling (IC) technology, such asspace-time interference rejection combining (STIRC) [7],successive interference cancellation (SIC) [8] or jointdetection (JD) [9] in uplink, or single antenna interferencecancellation (SAIC) in downlink.

In theory, MUROS can double voice capacity or even morewith negligible impact to handsets as well as to networks.Combining with GSM-HR speech codec approach, the voicecapacity can be largely increased.

Figure 2. Diagram of downlink OSC.

Figure 3. QPSK mapping on 8PSK constellation

OSC based on legacy GMSK handset is one solution of twouser MUROS, namely, multiplexing two users onto the sameslot, the two users are named MUROS pair. The key change isintroduction of eight new TSCs paired with the eight legacyTSCs for lowest cross-correlation to enable separation of sub-channels. First sub-channel can use an existing TSC andsecond sub-channel should use the corresponding new one for

both downlink and uplink. Thus OSC concept relying onGMSK only handsets could offer double voice capacity. DLOSC and UL OSC are described in detail in the next twosections.

III. ORTHOGONAL SUB-CHANNEL

A. Principle of Downlink OSC

Figure 2 presents the diagram of DL OSC. A BSS isassigned two low cross-correlation TSCs, one legacy TSC andone new TSC. Each MUROS MS inserts one TSC in its burst.Bursts of MUROS pair are sent respectively by BTStransmitter in I- or Q- sub-channels of QPSK modulation. MSreceiver tells its sub-channel by TSC to demodulate its own

signal.

Two modulation schemes are supported by GSM/EDGEsystem, i.e. GMSK and 8PSK [10]. To introduce OSC solutionin downlink, four points on the 8PSK constellation are selected

by a BTS transmitter to form the QPSK constellation. MUROSpair's symbols are mapped respectively onto I and Q channelsof QPSK constellation, as illustrated in table I and fig. 3. Thefirst sub channel (OSC-0) is mapped to MSB and the secondsub channel (OSC-1) is mapped to LSB.

TABLE I. QPSK SYMBOL MAPPING ON 8PSKCONSTELLATION

8PSK Gray

mapping(d3 i, d3 i+1, d3 i+2)

DL OSC mapping

(OSC0, OSC1)

Symbol parameter

8/2 lji es

=

(1,1,1) - j=0

(0,1,1) (1,1) j=1

(0,1,0) - j=2

(0,0,0) (0,1) j=3

(0,0,1) - j=4

(1,0,1) (0,0) j=5

(1,0,0) - j=6

(1,1,0) (1,0) j=7

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Figure 4. Diagram of uplink OSC.

B. Principle of Uplink OSC

For compatibility reason, UL OSC allows MS to usenormal GMSK transmitter with good cross-correlation property.IC technology is necessary at BTS receiver to detect signals ofMUROS pair simultaneously. It is assumed that BTS uses e.g.STIRC or SIC receiver to receive orthogonal sub-channels used

by different MSes.

In this paper, we focus on the SIC receiver. SIC is a processof multi-user detection (MUD), when SIC receiver detectssignals of several users, it will demodulate them in decreasingorder of signal power. The strongest signal is detected anddemodulated first, and then removed from the mixed signals,then is the second strongest signal and so on. The detectionalgorithms for SIC receiver are zero forcing (ZF), minimummean square error (MMSE) and so on. MMSE method could

be adopted by BTS receiver with two or more receive antennas.

Let x be the transmitting signal, and x be the estimated signal,and rbe the received signal, the criterion of MMSE algorithm

is illustrated as follow,

{ } { }2 22 min( ) min min = = x x x W r (1)

Where2

is the mean square value, and W is the weightedmatrix.

H 1 H1( )tN

= + W H H I H (2)

Where is the signal-to-noise ratio (SNR),tN

I is identity

matrix,tN

is the number of transmit antennas.

TABLE II. THE EIGHT LEGACY TSCS SPECIFIED IN 3GPP STANDARD

TSC

indexLegacy TSC

0 0 0 1 0 0 1 0 1 1 1 0 0 0 0 1 0 0 0 1 0 0 1 0 1 1 11 0 0 1 0 1 1 0 1 1 1 0 1 1 1 1 0 0 0 1 0 1 1 0 1 1 12 0 1 0 0 0 0 1 1 1 0 1 1 1 0 1 0 0 1 0 0 0 0 1 1 1 0

3 0 1 0 0 0 1 1 1 1 0 1 1 0 1 0 0 0 1 0 0 0 1 1 1 1 0

4 0 0 0 1 1 0 1 0 1 1 1 0 0 1 0 0 0 0 0 1 1 0 1 0 1 15 0 1 0 0 1 1 1 0 1 0 1 1 0 0 0 0 0 1 0 0 1 1 1 0 1 0

6 1 0 1 0 0 1 1 1 1 1 0 1 1 0 0 0 1 0 1 0 0 1 1 1 1 1

7 1 1 1 0 1 1 1 1 0 0 0 1 0 0 1 0 1 1 1 0 1 1 1 1 0 0

TABLE III. THE DESIGNED EIGHTNEW TSCS ACCORDING TO THE RULES

TSC

indexNew TSC

0 0 0 0 1 1 1 0 1 1 0 1 0 1 1 0 1 0 0 0 1 1 1 0 1 1 0

1 0 1 1 1 1 1 1 1 0 1 0 0 0 1 1 0 0 1 1 1 1 1 1 1 0 1

2 0 0 0 1 1 1 0 1 1 0 1 1 0 1 0 1 0 0 0 1 1 1 0 1 1 0

3 1 1 1 0 1 1 0 1 1 1 0 0 0 1 0 1 1 1 1 0 1 1 0 1 1 14 1 1 0 1 1 1 0 1 1 0 1 0 0 0 0 1 1 1 0 1 1 1 0 1 1 05 0 1 1 1 0 1 0 0 0 0 1 0 0 1 0 0 0 1 1 1 0 1 0 0 0 0

6 1 1 1 1 1 1 0 1 1 0 1 0 1 0 0 0 1 1 1 1 1 1 0 1 1 07 0 1 1 1 1 1 1 0 1 1 1 0 0 0 1 0 0 1 1 1 1 1 1 0 1 1

IV. NEW TSCS DESIGN FORMUROS

New TSCs design is of utmost importance to guarantee theperformance of both DL OSC and UL OSC. Because channelestimation and synchronization are based on the detection ofTSC. Orthogonal TSCs design will eliminate the intra-cell andinter-cell co-channel Interference (CCI). Though it is difficultto obtain absolutely orthogonal TSCs, the lower the cross-correlation coefficients between TSCs are, the lower the intra-cell and inter-cell CCI are.

Assume that tk and ck are two TSCs of the two MUROS

sub-channels. The two sequences are in-phase with thecorresponding binary sub-channel symbols. The discrete signalof the TSC part at the receiver side is

k l k l l k l k

l l

r h t j hc n

= + + (3)

Correlation is the general operation at the receiver side forchannel estimation. The correlation of received rk with TSC tkyields

2* * * *

0

m k m m k k l m m l k l m m l m k m

m m l m l m m

t r t h h t t j h t c t n+ + + +

= + + + (4)

The first item of the RHS is the desired channel response.

The second item relates the autocorrelation of TSC tk withdelay. The third item of the RHS is related with the cross-correlation between the two sub-channel TSCs. Usually goodTSC will ensure the second and third item to be zero.

According to this, we design the eight new TSCs as listedin table III, the legacy TSCs specified in 3GPP standard arelisted in table II for comparison. Note that the nearer to zerothat the cross-correlation coefficients are, the lower the intra-cell and inter-cell CCI are. However, there are no eightdifferent new TSCs which meet the condition that all cross-correlation coefficients are zero. So we set lower bounds forcoefficients. Table IV lists the maximum value of the absolutevalues of new TSCs autocorrelation coefficients and cross-correlation coefficients between new TSC and correspondinglegacy TSC with multi-path delay.

TABLE IV. MAXIMUM CORRELATION COEFFICIENTS OFNEW TSCS

Correlation with multi-path delay

Maximum Correlation

Coefficient

(absolute value)

Autocorrelation with delay = 1, 2 symbol(s) 0.145

Autocorrelation with delay = 3, 4, 5 symbol(s) 0.204

cross-correlation with delay = 0, 1, 2 symbol(s) 0.177cross-correlation with delay = 3, 4, 5 symbol(s) 0.598

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02

46

8

-5

0

5-1

-0.5

0

0.5

1

TSC indexdelay (symbol)

cross-correlatio

n

co

efficien

t

Figure 5. The cross-correlation coefficients between new TSC andcorresponding legacy TSC with multi-path delay.

Figure 5 shows the detail of the cross-correlationcoefficients of new TSC and corresponding legacy TSC withmulti-path delay, which are no more than 0.5979. Simulationresult in fig.10 verifies that coefficients less than 0.62 keep thedegradation within 1dB compared with orthogonal TSC pair.

V. SIMULATION RESULTS

To evaluation the link level performance of DL OSC andUL OSC, we developed the link level simulation platform ofGSM. We observed an interesting phenomenon when the ULOSC receiver uses MMSE-SIC algorithm. Then we verified theimportance of the cross-correlation properties of TSCs tominimize CCI. The simulation parameters are listed in table V.

A. The BER/BLER performance of DL OSC

Figure 6 illustrates the burst BER and FER performance of

DL OSC. It can be observed that at FER=0.01, the degradationis about 3.5dB, and that at BER=0.01, the degradation is about4dB, which is acceptable performance degradation, asconfirmed in [5, 6]. Notice that the degradation is reasonable,since we adopt higher-order modulation scheme, namely, weuse QPSK instead of GMSK. Further more, due to cross-correlation of TSC pair and multipath fading channel, varyingchannel phase makes I-phase and Q-phase sub-channelsinterfere with each other, the degradation is more than 3dB.

TABLE V. SIMULATION PARAMETERS

Aspect Working Assumption

Speech codec GSM AFS 12.2 or AFS 5.9

PropagationEnvironment

Typical Urban (TU

Mobility 3 km/h

FrequencyHopping

ideal Frequency Hopping (iFH

TrainingSequences

Optimization

Usage of combination oflegacy TSCs and new TSCs

with improved cross correlation properties

TransmitPulse Shapes

legacy linearized GMSK pulse shape

Test Scenario(TS)

TS-0: MS-1 and MS-2 move randomlyTS-1: MS-2 hold still, MS-1 moves towards BTS

TS-2: MS-2 hold still, MS-1 moves away from BTS

-10 -8 -6 -4 -2 0 2 410

-2

10-1

100

FER

/BurstBER

C/I (dB)

FER, GMSK

FER, DL OSC

burst BER, GMSK

burst BER, DL OSC

Figure 6. Link level performance of DL OSC for TS-0, Burst BER/FERversus C/I for AFS 5.9, TU3iFH, the cross-correlation coefficientp between

legacy TSC3 and legacy TSC2 is 0.617.

B. The BER/BLER performance of UL OSC

Figure 7 shows the BER and BLER performance of UL

OSC. Note that user 1 represents the user whose signal isstronger at the BTS receiver, and user 2 denotes the weaker one.We can observe that at BER=0.01, the degradation of user 1 isabout 4dB, the degradation of user 2 is about 8dB. Notice thatuser 1 uses legacy TSC0, user 2 adopts legacy TSC1, the cross-correlation coefficient is 0.677, so the performance is withoutTSC cross-correlation property optimization. The next sub-clause will show the gain of optimal TSC design.

We did further study of UL OSC with MMSE-SICalgorithm in TS-2 and TS-3 scenarios. The BLER performanceis illustrated in fig. 8 and fig.9 respectively. When user 1moves towards the BTS, its link performance improves

because the SNR condition gets better, at the same time, it can

be seen that the link performance of user 2 becomes better,while its SNR condition remains the same, for user 2 doesntmove. That is a feature of SIC, the more veracious detection ofuser 1s signal means the better interference cancellation have

been done for the detection of user 2s signal.

-10 -5 0 5 10 1510

-3

10-2

10-1

100

Es/No(dB)

BER/BLER

MMSE user1 ber

MMSE user1 bler

MMSE user2 ber

MMSE user2 bler

one user ber

one user bler

Figure 7. Link level performance of UL OSC for TS-0, BER/BLER versusSNR for AFS 12.2, TU3iFH. User 1 uses legacy TSC0, user 2 adopts legacy

TSC1, the cross-correlation coefficient is 0.677.

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-10 -8 -6 -4 -2 0 2 4 6 8 1010

-3

10-2

10-1

100

Es/No(dB)

BLER

user1 0dB

user1 1dBuser1 2dB

user1 3dB

user2 0dB

user2 1dB

user2 2dB

user2 3dB

Figure 8. Link level performance of UL OSC for TS-1, BLER versus SNRfor AFS 12.2, TU3iFH, the cross-correlation coefficient is 0.677.

The conclusion in TS-3 scenario is different. When user 1moves away from the BTS, its link performance decreases

because the SNR condition gets worse, in the meantime, thelink performance of user 2 doesnt change much. That is

because user 2s signal becomes the stronger one. It is detectedfirst. As user 2 doesnt move, its channel conditions remain thesame, so its link performance almost doesnt change, as shownin fig. 9.

C. The Impact of the cross-correlation property

We chose 3 TSC pairs to introduce OSC, legacy TSC3 withlegacy TSC2, legacy TSC3 with new TSC3, and legacy TSC3with TSC=[0 1 0 0 0 1 0 1 1 0 1 1 0 1 0 0 0 1 0 0 0 1 0 1 1 0],the cross-correlation coefficientsp between them are 0.617, 0,0.856, respectively. The simulation results are illustrated in fig.10. At FER=0.01, the degradation of p=0.617 case is about1dB compared withp=0 case, i.e. the orthogonal TSC pair case,

while the degradation ofp=0.856 case is more than 5dB. So theoptimal TSCs design is crucial. The maximum cross-correlation coefficient is p=0.598 between designed new TSCwith corresponding legacy TSC, thus the degradation is within1dB for all TSC pairs, which is an acceptable degradation.

-10 -5 0 5 10 1510

-3

10-2

10-1

100

Es/No(dB)

BLER

user1 0dBuser1 -1dB

user1 -2dB

user1 -3dB

user2 0dB

user2 -1dB

user2 -2dB

user2 -3dB

Figure 9. Link level performance of UL OSC for TS-2, BLER versus SNRfor AFS 12.2, TU3iFH, the cross-correlation coefficient is 0.677.

-8 -6 -4 -2 0 2 4 6 810

-2

10-1

100

C/I (dB)

FER

p=0

p=0.617

p=0.856

Figure 10. Link level performance difference of 3 different TSC pairs for TS-0, FER versus C/I for AFS 5.9, TU3iFH. The the cross-correlation coefficientp between legacy TSC3 and legacy TSC2 is 0.617, between legacy TSC3 and

[0 1 0 0 0 1 0 1 1 0 1 1 0 1 0 0 0 1 0 0 0 1 0 1 1 0] is 0.856, between legacyTSC3 and new TSC3 is 0.

VI. CONCLUSION

MUROS is introduced in this contribution, which is a newconcept to enhance capacity of voice and data service of theGSM/EDGE networks. OSC is a specific solution to implementMUROS in current GSM network. We improved the

performance of DL OSC by optimizing the new TSCs design,and studied the performance of UL OSC with MMSE-SIC inthree scenarios. We conclude that OSC is a promisingtechnology to implement two-user-reusing-one-slot withoutdegrading the speech quality very much. Moreover, it is quitecompatible with current GSM network [5, 6].

REFERENCES

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[2] R. Meyer, W. H. Gerstacker, and R. Schober et al. "A Single AntennaInterference Cancellation Algorithm for Increased GSM Capacity,"IEEE Transctions on wireless communication, vol. 5, No. 7, July 2006

[3] 3GPP TR 45.903 v7.0.1, "Feasibility Study on Single AntennaInterference Cancellation (SAIC) for GSM networks," 3GPP GERAN,Aug. 2007

[4] GP-071738, "Speech capacity enhancements using DARP,"QUALCOMM Europe,3GPP GERAN#36,Vancouver,Canada,Nov.2007

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[6] GP-071792, "Voice Capacity Evolution with Orthogonal Sub Channels,"Nokia Siemens Networks, 3GPP GERAN#36, Vancouver, Canada, Nov.2007

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