physical layer design for packet data over is-136

8/7/2019 Physical Layer Design for Packet Data over IS-136

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Physical Layer Design for Packet Data over IS-136

Krishna R. Narayanan an d GordonL. StiiberSchool of Electrical an d C omp uter Engineering

Georgia Institute of TechnologyAtlanta, GA 30332-0250

Mark D. AustinBell South Cellular Corp.

1100 Peachtree st. N.E., suite 12D01Atlanta, GA 30309

ABSTRACT

Several Forward Error Correction (FEC) , interleaving andAuto matic Repeat reQuest (ARQ) schemes are evaluated fortransmitting packet data over IS-136 TDMA. The objective isto identify th e combination of FEC, interleaving an d ARQ th atprovides maximum data throughput at reasonable computa-tional complexity. The F EC schemes considered are rat e-n /n +lpunctured convolutional codes, long constraint length convolu-tional codes and punct ured Tu rbo codes. Th e ARQ schemesconsidered are ARQ with majority voting, ARQ with metriccombining and ARQ with code combining (or Type I11 ARQ).The performance of these techniques when used with one-slotand two-slot interleaving is studied. Results show tha t highestthroughput is achieved when a rate-516 convolutional code isused with one-slot interleaving and Type I11 ARQ scheme.

1. INTRODUCTION

Currently, a standardization effort for packet data over theIS-136 air interface is underway within the Global TDMA Fo-rum. In order to minimize the development effort, and t o uti-lize previous work, this stan dard intends t o use th e higher layerprotocols of th e Cellular Digital Packet Dat a (CDP D) stand ardalready developed for AMPS. The issues then open for stan-

dardization are confined to th e physical and MAC layers. Thi spaper compares several different physical layer designs in t ermsof their Frame Error Rate ( FER ), dat a throughput and com-puta tion al complexity. Th e physical layer design options arelimited t o FE CI interleaving, an d ARQ protocols, so th at th eframe size, time slot, and modulation are kept the same as in

The paper is organized as follows - Section I1 discusses thesystem model. Sections 111, IV and V treat the different FECtechniques, ARQ techniques and interleaving schemes respec-tively. Section VI presents th e resul ts an d compares the per-

IS-136.

40ms- 26.67ms

I I DataFigure 1: Slot s t ruc ture of IS-136

Encixler Puncturer Interleaver

Channel+ifferentialDeinterleaver )-I emodu,ator

Figure 2: System model

is th en encoded using a convolutional encoder. T he encoded bi-nary ou tpu t is 260 bits in length including trail bits. The 260

encoded o utp ut b its are interleaved using a 26 x 10 block inter-leaver and then xI4-DQFSK modu lated into 130 symbols. Twotype s of channels are considered, the Additive Wh ite GaussianNoise (AWGN) limited channel a nd t he CoChannel Interference(CCI) limited channel. In eith er case, th e desired signal is as-sumed to be affected by frequency non-selective (flat) Rayleighfading. i.e., th e envelope of the desired received signal has th eprobability density

p ( x ) = Xexp{--}. 2a2 2U2

For th e AWGN limited channel, AWGN of power spectral den-sity N o / 2 is added to th e received signal. For the CCI limitedchannel, the interference is assumed to be from another userwhose signal is independently Rayleigh faded and with averagereceived carrier power P I . If Ps is the average received carrierpower of the desired signnf, then SNR and C/ I are defined as

formance of th e FEC , ARQ an d interleaving schemes. Finally,Section VI1 summarizes the results.

2. SYSTEM MODEL

Data is assumed to be transmitted in the form of packets,composed of 6.67 ms time sl ots (2 0 ms apart) as outlined in theIS-136 standa rd and shown in Fig. 1. The system simulationmodel is shown in Fig. 2 . A 16-bit CRC checksum is appendedt o th e data using the CRC polynomial z16 +z15 + z 2 + 1, o de-tect errors and request retransmissions at the receiver. T he dat a

SNR = lo log- PsNops

C/ I = lolog-.PI

(2 )

0-7803-3659-3/97 1 0.00 1 997 EE E 1029


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symbols

Figure 3: Differentially coherent detector for nI4-DQPSK

At th e receiver, th e received signal is demod ulat ed using a differ-entially coherent detector (shown in Fig. 3), deinterleaved, anddecoded using a soft-decision decoder [ l ] . The decoded packet isthen checked for errors using the CR C check. If the CR C check-sum is zero, the packet is accepted, otherwise a packet error isdeclared and a retransmission is requested. Th e performancecriteria considered are the frame error rate and data through-

pu t. The computatio nal complexity and storage requirementsat th e receiver ar e also compared. The performance of the dif-ferent schemes are compared both at low Mobile Station (MS)velocities (8 km/hr) and high MS velocities (100 km/hr).

3. FE C schemes

3.1 R a t e - n / n + 1 convolutional codes

It is well known that rate-n/n + 1 convolutional codes pro-vide good performance and can be efficiently decoded using asoft-decision Viterbi decoder. We consider rate-112, 2 /3, 415and 5/6 codes of constraint length U = 5. The rate-213, 41 5and 516 codes are obtained by puncturing a rate-1/2 code.The use of punctured codes permits the use of a soft-decisionViterbi decoder corresponding to the original rate-112 code with2 branches from each sta te, in stead of 2" branches for a genericnon-punctured ra te-n/ n + 1 code. Th e codes are chosen fromthe tables in [2]. A soft-decision Viterbi decoder is used with14-bit metric quantization. Since differential demo dulation isused, an estimate of the channel state information is not re-quired. Therefore, the branch metric used in the Viterbi de-coder is the Euclidean distance K l l r k - k 1 I 2 , where T k is thereceived signal and x k is the encoder output corresponding tothe branch transition. When th e code is punctured, the branchmetric corresponding to the punctured bits need not be com-puted. For the most part, rate-nln + 1 convolutional codes ofconstraint length U = 5 (32 sta tes) are considered. However,the performance of the best known rate-l0/11 code, obtainedby puncturing a rate-1/2 code of constraint length 8 (64 states ),is also considered.

3.2 Long Constraint length Codes

Th e performance of convolutional codes can be improved byusing longer constra int len gth codes. However, th e complexityof th e Viterbi decoder increases exponentially wit h const rain t

length and decoding becomes impractical beyond a certain con-str ain t length. However sequential decoding algori thms can beused to decode long constra int lengt h convolutional codes. Al-though the performance of sequential decoding algorithms issuboptimal, the decoding complexity is linear with the con-straint length and, hence, is suitable for long constraint lengthcodes. Th e performance of a v = 31 rate-516 convolutionalcode is studied when used with a soft-decision Fano sequentialdecoder.

3.3 Punctured Turbo Codes

Turbo codes are known to provide large coding gains overAWGN and flat Rayleigh fading channels [3]. Motivated by thetremendous coding gain, the performance of a rate-5/6 Turbocode obtained by puncturing rat e-l/ 2 component encoders isalso studied. Since th e complexity of a Turbo decoder is muchhigher t ha n th at of a Viterbi decoder for th e same constraintlength, the constraint length of the component codes in theTurbo code was chosen to be 4 (16 sta te code). The block lengthwas chosen to be 260, corresponding to two IS-136 packets.

4. A R Q schemes

This section discusses three different types of ARQ schemes.When the receiver decodes a packet and deems it erroneous,th e receiver requests retransmission of th e packet. The tr ans-mitter then transmits a copy of the packet, or other versionsof th e packet, as discussed below. A stop-and-wait protocolis assumed, i.e., the t ransmi tter waits until an acknowledgment(ACK) is received from th e receiver before tran smitti ng t he nextpacket. Th e ARQ protocol has to be slightly modified whentwo-slot interleaving is used.

4.1 A R Q w i t h M a j o r i t y Vo t i n g

When a decoded packet is deemed erroneous at the receiver,th e bit decisions of the Viterbi decoder are store d and a re-transmission is requested. If the ith transmission of a packet isdetected in error, the corresponding bit decisions &,G , ..., dare stored, where ij E [0,1] , corresponds t o t he decision onth e j t h bit during the it h transmission and N is the length ofthe data packet including the CRC bits. When the packet isdetected in error during th e third or subsequent transmissions(i 2 3) , a bit-by-bit majority voting is performed using the rule

-

1, if 2: 2d3 = 0 otherwise. (3

Then th e new decisions & , d ^ l , ..., (i, are checked for errors uing the CRC check. If the packet is still d e e m e d to be in error,a retransmission is requested. When a packet is retran smitte d

for the 5th time, majority voting is performed on all the 5 de-cisions, and the condition x : - , d j > 2 in (3 ) is now changedto ~ ~ - , d ~. If th e packet is not decoded correctly after5 attempts, a decoding failure is declared and the transmitterproceeds with the next packet.

This technique is very simple to implement and imposes onlysmall storage requirements on the receiver. A maxim um of 5 Nbinary numbers (decisions of the decoder) have to be stored.

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5. 2 Two-slot interleaving

Coderate

The major problem with one-slot interleaving is that inter-leaving is restricted to one time frame and, hence, the fadescannot be effectively uncorrelated at low speeds. Wi th two-slot interleaving, two adjacent packets are combined and inter-leaved. Th e receiver then deinterleaves two packets to ethe rand decodes th em separately. Specifically, if x r ) , ..., x Z s 98 andx ( ' + l ) , ...., x ( k + ' )59 represent the encoded outputs correspondingto packets at time IC an d k + l respectively, th en two new packetsare formed using the rule,

v 8 km/hr 100 km/hrFER I Throuahmt I FE R I ThrouahDut

*.z(le+l) - x i ' ) , if i mod 2 = 1- { x!'"), if i mod 2 = 0.

The two packets, Zr), ..., an d itr"), ..., 5gi1) re theninterleaved, modulated and transmitt ed over th e channel. Atthe receiver th e inverse op eration takes place.

1 / 25/ 610/11

6. Results and Discussion- - - -

5 8 % 5.98 kbps 2% 6.37 kbps5 29% 7.66 kbps 31% 7.45 kbps5 31% 8.14 kbps 45% 6.49 kbp she FEC schemes, ARQ schemes and interleaving schemes

were simulated and the performance studied. In terms of FERperformance, it was observed that the punctured Turbo codesdid not provide significant reduction in the FER compared torate-5/6 convolutional codes. This is because puncturing t hecomponent encoders to produce high-rate Turbo codes dras-tically reduces the effective free distance of the code (outputdistance corresponding to all input weight-2 sequences). To ob-tain an overall rate-5/6 code, requires rate-l0/11 componentencoders. Th e free distance for the rate-l0/ 11 component codeobtained by puncturing a rate-1/2, constraint length 4 compo-nent code is O! Therefore, the only improvement in performanceattain able is due t o t he interleaving gain, which is fairly low forsmall block lengths. Since th e Turb o decoder has significantlyhigher latency and complexity than the Viterbi decoder, it isnot a good choice. It should be noted th at th e original rate-1 / 2 Turbo code still offers a huge improvement in performanceover the best known rate-1/2 convolutional code of comparablecomplexity.

Similar results were obtained when a constraint length 31code was used with a soft-decision Fano sequential decoder.The performance of the rate-5/6, constraint length 31 codewhen used with sequential decoder is not significantly betterthan th at of a rate-5/6, constraint length 5 convolutional codewith Viterbi decoding. Thus , high-rate punctu red convolutionalcodes do not provide good performance when used with sequen-tial decoders. Similar resul ts were obt ained in [6]. Hence, th ebest FER performance from the coding options studied hereis obtained from convolutional codes when used with a soft-decision Viterbi decoder.

It is well known th at high-rate convolutional codes, obtainedby puncturing a low-rate convolutional code, have inferior FERperformance. However, for the same block length, more da tabits can be transmitted by using a high-rate convolutional code.Therefore, selection of th e code rat e involves a trade-off betweenthe increased FER an d th e increased actual number of dat a,b itsper packet, as result of pun cturi ng. Table 1 shows a sample

ARQ

Maj. VotingPkt. Comb

comparison of the achievable throughput in kbps when rate-1 / 2 , 5/6, and 10/11 codes of constraint length 5 are used withan AWGN-limited channel at MS velocities of 8 km/hr and 100km/ hr and SNR of 14dB. It can be seen th at t he performanceof th e rate 5 /6 code is good bot h a t low and high M S velocities.

We now discuss the performance of ARQ schemes and in-terleaving schemes, when used with a rate-5/6 code. A plot ofFER vs SNR for mobile velocities of 8 km/hr (slow speed) and10 0 km/ hr (high speed) is shown for the CCI imited channel in

Figs. 4 an d 5 respectively. Note th at th e performance of TypeI11 ARQ is uniformly better than the other two schemes andis significantly better than the ARQ scheme with majority vot-ing, especially at low C/ I. However, th e storage requirement isgreater for Type I11 ARQ. Table 2, provides a sample compari-son of F ER performance, complexity and st orage requirementsfor the three ARQ schemes when used with one-slot interleavingfor C/I of 14 dB.

FER Storage Extra8 km/hr 100 km/hr Adds

13.0% 23.2% N bit s12.68% 16.2% 2"+lN 2"+'N

Table 1: Throughput comparison for different rate codes

Although it seems intuitive th at two-slot interleaving is bet-ter than one slot interleaving, the results in Figs. 4 and 5 indi-cat e otherwise. T his can be explained as follows. At low speeds,errors tend to occur in bursts. The performance of the Viterbidecoder depend s largely on th e distri buti on of errors within eachpacket. Consider the situation when th e channel exhibits a pro-longed deep fade (equivalently low instantaneous C/I) duringth e transmission of a packet an d no deep fades (equivalentlyhigh instantaneous C/I) during the transmission of the nextpacket. Thi s situ atio n is typical at slow speeds. Wit h one-slot interleaving, the error bursts are confined to within onepacket, while with two-slot interleaving errors are distributedacross both the packets. Thus with one-slot interleaving onlyone packet is in error, while with two-slot interleaving bothpackets are likely to be in error. However, as the C/ I increases,th e performance of two-slot interleaving improves an d will be-come better th an one-slot interleaving beyond a particular C /I.This is because, at high C/I, after distributing the errors acrosstwo packets, the errors in both packets can be corrected. At highspeeds the channel does not exhibit prolonged fades. Therefore,th e performance of one-slot interleaving is bet ter at low C/I, a ndat moderate-to-high C/ I th e performance of two-slot interleav-ing is better than that of one-slot interleaving. It is interestingto note t ha t t he performance of a rate-1/2 code with two-slotinterleaving is better th an t ha t of one-slot interleaving even for

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moderate C/I. This is due to the fact that the error-correctingcapability (free distance) of the code is better and, hence, afterdistributing the errors, the errors in both the packets can becorrected. In conclusion, th e performance curves for one-slotand two-slot interleaving exhibit a crossover C/ I th at dependsnot only on the speed but also on the error-correction capabil-ity of th e code used. In part icula r, th e crossover C/ I decreaseswhen the error-correcting capability (free distance) of th e codeincreases or MS velocity increases. In th is case, one-slot int er-leaving is better than two slot interleaving up to 20 dB C/I atspeed of 8 km/h r and up to 14 dB a t speed of 10 0 km/hr.

7. Conclusions -fl 1 slot interleaving with majority voting+e 2 slot interleaving with majority voting

40.0

I

Uncoded

30.0 A 1 slot interleaving with packet combining4 2 slot interleaving with packet combiningt slot interleaving with Type 11 1AR Q+2 slot interleaving with Type Ill AR Q

We have stud ied th e performance of several FE CI interleav-ing and ARQ schemes for transmitting packet data using IS-136TDMA. Results show that short constraint length, punctured

2convolutional codes used with soft-decision Viterbi decodingprovide good performance at mode rate complexity. Th e per- 6formance of these codes can be improved by using a Type I11 EARQ scheme. Fur the r, it was shown th at one-slot interleavingis better than two-slot interleaving for low-to-moderate C/I, es-pecially at low MS velocities.

References

&

20,0

10.0

Data Services Task Group, RLP Protocol PerformanceWorking Paper, E.I.A/T.I.A TR45.3.2.5/93.07.06.

Y. Yasuda, K. Kashiki, and Y. Hirata, High Rate Punc-tured Convolutional Codes for Soft Decision Viterbi De-coding, IEEE !l!rans. Comm., vol. 32, pp . 315-319, Mar1984.

C. Berrou, A. Glavieux, and P. Thitimajshima, NearShan non limit Error-Correcting Coding and Decoding:Turbo codes, Proc. ICC 93, pp. 1064-1070, Geneva,Switzerland, May 1993.

J. Hagenauer, Rate Compatible Punct ured Convolu-tional (RCPC) codes and their Applications, IEEE Trans.Comm., vol. 36, pp . 389-400, Apr 1988.

S. Kallel, Complementary Punct ured Convolutional(CPC) codes and Their Applications, IEEE Trans.Comm., vol. 43, June 1995.

K . Muhammad, and K. B. Leataief, On the Performanceof Sequential and Viterbi Decoders for High-Rate Punc-tured Convolutional Codes, IEEE Trans. Comm., vol. 43,pp. 2687-2695, Nov 1995.

1

0.0 10.0-

2.0 14.0 16.0 18.0 20.0C/I (dB)

Figure 4: FER comparison at 8 km/hr100.0

M Jncodedw slot interleaving with majority votingM ! slot interleaving with majority votingDd 51 slot interleaving with packet combiningM slot interleaving with Type 111AR Q* 5 ! slot interleaving with Type 111AR Q

80.0

8

9 60.0a262 40.0E

v

L

U

20.0

10.0 12.0 14.0 16.0 18.0 20.0C/I (dB)

0.0

Figure 5: FER. comparison at 100 km/hr

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physical layer design for packet data over is-136

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