DS CDMA Scheme for WATM with Errors and Erasures Decoding

Size: px

Start display at page:

Download "DS CDMA Scheme for WATM with Errors and Erasures Decoding"

Pearl Shepherd
5 years ago
Views:

1 DS CDMA Scheme for WATM wih Errors and Erasures Decoding Beaa J. Wysocki*, Hans-Jürgen Zepernick*, and Tadeusz A. Wysocki** * Ausralian Telecommunicaions Research Insiue Curin Universiy of Technology GPO Box U 987, Perh WA 6845, Ausralia {beaa, hans}@ari.curin.edu.au ** Universiy of Wollongong School of Elecrical, Compuer and Telecommunicaions Engineering Norhfields Avenue, Wollongong NSW 5, Ausralia wysocki@uow.edu.au Absrac - In he paper, we presen simulaion resuls for he 3 channel DS CDMA WATM LAN uilising opimized complex spreading signaures based on Walsh funcions. The mehod o obain hose opimized spreading signaures, as well as he full se of he coefficiens giving he minimum level of cross-correlaion beween any pair of he channels is shown. The resulan sysem BER as well as he disribuion of errors wihin WATM cells is given. The obained resuls indicae ha wih he applicaion of a hybrid ARQ scheme wih errors and erasures decoding, he number of WATM cells which would require reransmission is in he order of.46%. I. INTRODUCTION During he 99s, direc sequence spread specrum code division muliple access (DS CDMA) echnology [] has maured as a echnique o provide muliple access o he radio channel for mobile communicaions. For example, i is used in mobile elephony complian wih IS-95 sandard []. Recenly, i has gained even more aenion wih ETSI approving i as a echnology for he European hird generaion mobile sysem (UMTS) [3]. I is expeced ha wireless LANs will complemen he hird generaion mobile elephony, providing a wireless vehicle for high rae mulimedia applicaions. ATM (Asynchronous Transfer Mode) is he echnique being he commonly acceped sandard for he broadband neworks, and is also becoming acceped as a common nominaor for all ypes of services and neworks. Also, here has been widespread use of wireless communicaions o suppor users requiremens for wireless access and erminal mobiliy in such scenarios as cellular elephony neworks and narrowband wireless LANs. As a furher advancemen, users are now beginning o require broadband services and erminal mobiliy o be provided ogeher hrough wireless access o ATM neworks. The major benefis of many DS CDMA is ha i can be effecive in combaing problems relaed o muli-pah propagaion, while providing good inerference from oher, narrowband devices operaing in he same frequency band. This is, however, dramaically reduced if only a small processing gain [] can be achieved, as in he case of WATM LANs where he raio of available bandwidh o he proposed daa rae is in he order of less han (e.g..4 GHz ISM band). Under such condiions, in-band jammers, like oher channels of he same WLAN acquired by means of CDMA cause severe muliple access inerference (MAI), which may block he communicaion. In heory, cancellaion of ha ype of inerference is possible if each of he users uilise orhogonal signals o ransmi he daa []. If delays beween ransmiers and receiver are anyhow differen, as is generally he case of erminal o base saion (BS) ransmission, he signals received by he BS canno be regarded as orhogonal. Wihin he 5m coverage area hose differences may be in he order of a few spreading code symbols (chips) depending on he daa rae. This effec is paricularly criical for very shor spreading sequences, like 6-bi Walsh funcions. In [4] we have proposed he mehod o reduce he ISI and MAI for DS CDMA wireless neworks, and in [5], we opimized he scheme for he sysem using 3 spreading signaures based on he 6-bi Walsh funcions. This paper deals wih furher design of DS CDMA WATM LAN where an error conrol mechanism is employed. The paper is organized as follows. In Secion II, we briefly discuss he mehod of designing he complex mulilevel spreading signaures based on 6-bi Walsh funcions. Secion III presens resuls of simulaed error performance for he 3 channel ATM WLAN uilizing he complex mulilevel spreading signaures. In Secion IV, we describe he error conrol scheme wih hard decision opimized for our DS CDMA sysem. Secion V hen exends he concep o errors and erasures decoding. Secion VI concludes he paper. II. DESIGN METHOD In [4], we have described he mehod o reduce ISI and MAI for non-synchronized CDMA signals by means of a modificaion o he carrier waveform. The modified carrier /$. (c) 998 IEEE

2 has been obained by a regular disorion o he frequency of he original carrier, resuling in he ih user line signal s i () expressed by: s i () = g i () cos ω c + w i ( τ) dτ + φ i (), () where w i () - frequency disoring funcion which can be opimized o minimize he cross-correlaions beween users, and o minimize he off-peak auo-correlaion of he ih line signal, φ i () - informaion carrying phase componen. A base-band approach [5] follows from he fac ha for he real spreading code g i (), equaion () can be rewrien in he exponenial form as: s i () g i () Re e jω c = exp j w i ( τ) dτ e φ i(). () The complex envelope [] σ() of such a signal is given by: σ () = g i () exp j w i ( τ) dτ e φ i () = g i ()e φ i(), (3) Because signal is usually processed using digial signal processing (DSP) echnology in he receiver, insead of he analogue spreading waveform: g i () = g i () exp j w i ( τ) dτ one can use he polyphase complex spreading signaure ĝ i (). The lengh of he signaure ĝ i () equals o he lengh of he original signaure g i () muliplied by he number of samples per chip used in he receiver, and is pulse levels are weighed by he facor W i () = exp j w i ( τ) dτ (4). (5) Such an approach allows for he baseband correlaional deecion of he signals, easier opimizaion of he funcions w i () for he given se of binary signaures, and faciliaes implemenaion of he receivers. In order o opimize he spreading sequences for he use in a 3 channel DS CDMA ATM WLAN, we have chosen a subse of 3 orhogonal Walsh funcions [] as a se of binary spreading codes g i (), i= 3,,,. These funcions are lised in Table. To minimize he cross-correlaion beween any pair of he spreading signaures, independenly of he relaive delay, we applied he described mehod wih he funcions w i (), i= 3,,,, being of he form: w i () = πα [ i ζ ( 6) + β i ζ( 8) + γ i ζ( 4) ], (6) where he riangular wave ζ() is defined as: ζ () = λ ( n), (7) and TABLE : SET OF 3 BINARY SPREADING SEQUENCES. Number λ() Binary spreading sequence = n =, < 4, < ,.5 <,. (8) The obained values of he parameers α i, β i, γ i, i =,, 3 are given in Table. The algorihm used o obain hem has been presened in [5]. III. SIMULATION OF DS CDMA WATM LAN In order o simulae he 3 channel DS CDMA WATM LAN we assumed he following: /$. (c) 998 IEEE

3 a WATM cell consised of 54 bis (44 bis of an ATM cell plus bis of an overhead), BPSK was used as a modulaion echnique, he informaion conens of each WATM cell was random, relaive delay beween he signals corresponding o differen channels was random wih he delay sep of.5 of a chip duraion, he received power was equal for all of he channels. The receiver anenna was simulaed as a sum of he signals arriving from differen channels, and he deecion was a correlaional one. The simulaion was repeaed for cells for each of he channels. The obained resuls show ha he qualiy of ransmission depends slighly on he signaure used o spread he signal, so he achieved BER is no uniform for all of he channels, which is refleced by he number of errors occurring in he received WATM cells. TABLE : SET OF THE OPTIMIZED COEFFICIENTS. Number α β γ For all 3 channels, he obained BER is equal o.6x -3, ranging from.3x -3 o 4.x -3, and he hisogram presening he disribuion of errors in he received WATM cells is presened in Figure x 4 NUMBER OF TRANSMITTED WATM CELLS = 3 x ERRONEOUS: 3549 ERROR FREE: NUMBER OF ERRORS IN A RECEIVED WATM CELL Figure : Hisogram of he numbers of ransmission errors in a received WATM cell for all 3 channels. IV. ERROR CONTROL WITH HARD DECISION DECODING For he proecion of ATM cells, we have designed an efficien coding scheme specifically for a WATM indoor environmen [6]. The coding scheme is based on shorened BCH codes. The BCH code used for FEC adds bis o each ATM cell. Resuls of hroughpu efficiency for hybrid Selecive Repea (SR) ARQ schemes on a channel wih average bi error rae (BER) of p b = % are shown in Figure. As can be seen from he plos, a hroughpu of slighly less han η = 8% is obained by inroducing m = pariy bis used o correc up o = errors, where he number of informaion byes can be chosen in he range of 5 o 7 byes wihou significan impac on hroughpu. THROUGHPUT (, m ) (,) (,) Selecive Repea (p_b=%) (,) NUMBER OF INFORMATION BYTES Figure : Throughpu of a hybrid SR ARQ vs number of informaion byes on a channel wih bi error rae of % (=errorcorrecing capabiliy, m=number of pariy bis) /$. (c) 998 IEEE

4 x 4 NUMBER OF TRANSMITTED WATM CELLS = 3 x.5 DECODED: 583 ERRONEOUS: CORRECT: 589 DETECTED: DETECTABLE BUT UNCORRECTABLE ERROR PATTERN NUMBER OF ERRORS IN AN UNCORRECTABLE WATM CELL NUMBER OF ERRORS IN A DECODED WATM CELL Figure 3: Hisogram of he numbers of errors in a WATM cell afer decoding for all 3 channels. Using an error-correcing capabiliy of =, we are able o correc he majoriy of hose error evens shown in Figure. The hisogram of errors in a WATM cell afer decoding and summaion over all 3 channels is depiced in Figure 3. Ou of he 6 ransmied WATM cells, he BCH decoder is able o decode 583 WATM cells whereas 7 WATM cells have been deeced as being corruped by an uncorrecable error paern. Furher on, a single WATM cell has been released erroneous wih 9 bis in error and his gives a pos-decoding BER of.4x -6. All oher released WATM cells are error free. For all 3 channels, he oal number of cells which require reransmission is abou.65%, ranging from % o.5%. V. ERRORS AND ERASURES DECODING To furher improve he performance of he proposed DS CDMA sysem wih error conrol, we now employ errors and erasures decoding insead of hard decision decoding. Because pos-decoding BER performance is already sufficien wih hard decision decoding, his approach is raher aken o reduce he number of reransmissions for real-ime and delay-sensiive services. Correcion of errors and erasures has been implemened according o he algorihm described in [7] and was chosen for our applicaion because of is simpliciy. I uses a sandard BCH decoding algorihm for a error-correcing BCH code and operaes as follows. Le he received word conain e erasures. Replace he e erased posiions wih zeros and decode his word wih a sandard BCH decoding algorihm. Replace he e erased posiions wih ones and decode his word wih a sandard BCH decoding algorihm. Selec ha obained codeword which relaes o he smalles number of errors correced ouside he erased posiions. In order o idenify wheher a posiion in a codeword has o be considered as being erased, we have o define a hreshold value for he correlaor oupu of he receiver. As an example, Figure 4 shows relaive frequency of normalized correlaor oupu for CDMA channel, i.e. when he corresponding signaure number is used. The qualiy of ransmission depends slighly on he signaure used o spread he signal. Accordingly, opimal hresholds are differen for differen signaures and he obained resuls are lised in Table 3. A hreshold of zero denoes simple hard decision decoding. Table 3 also shows he reducion of required reransmissions when hard decision is replaced by errors and erasures decoding. Acually, he oal number of reransmissions, i.e. uncorrecable WATM cells, for all 3 channels is reduced by 8.8% from 7 o WATM cells. In oher words, only.46% of WATM cells would require a reransmission. Figure 5 shows he hisogram of errors in a WATM cell afer correcion of errors and erasures and summaion over all 3 channels. As he sub-figure indicaes, he increase of correcly decoded WATM cells is achieved for hose which are corruped by errors. Noe also ha all released WATM cells are now error free and he even of an erroneous decoding did no occur. RELATIVE FREQUENCY NORMALIZED CORRELATOR OUTPUT Figure 4: Hisogram of he normalized correlaor oupu for channel and ransmission of WATM cells /$. (c) 998 IEEE

5 TABLE 3: NUMBER OF WATM CELLS WHICH REQUIRE RETRANSMISSION (ABSOLUTE AND IN %) CDMA Channel Hard Errors and Erasures VI. CONCLUSIONS Threshold 4 /.% 8 /.4%.7 / % 4 /.7% /.5% 37 /.85% /.% 4 /.% 5 /.% /.% 6 9 /.95% /.55%.54 7 /.% /.6% /.35% 4 /.% /.5% 3 /.5% 3 /.5% 3 /.5% / % / % /.% /.% 3 5 /.5% /.5%.37 TOTAL 7 /.65% /.46% In his paper we presened he simulaion resuls for he 3 channel DS CDMA WATM LAN uilising opimized complex spreading signaures based on Walsh funcions. The resuls are very promising, and indicae ha wih he applicaion of a hybrid ARQ scheme wih errors and erasures decoding, he number of WATM cells which would require reransmission is in he order of.46%. Furher research invesigaing he behaviour of he sysem wihou he perfec power conrol and incorporaing he realisic indoor channel model is required. x 4 NUMBER OF TRANSMITTED WATM CELLS = 3 x.5 DECODED: 5879 ERRONEOUS: CORRECT: 5879 DETECTED:.5.5 DETECTABLE BUT UNCORRECTABLE ERROR PATTERN NUMBER OF ERRORS IN AN UNCORRECTABLE WATM CELL NUMBER OF ERRORS IN A DECODED WATM CELL Figure 5: Hisogram of he numbers of errors in a WATM cell afer correcion of errors and erasures for all 3 channels. REFERENCES [] J.G.Proakis, Digial Communicaions. 3rd ed., New York: McGraw-Hill, 995. [] TIA/EIA IS-95, Mobile Saion-Base Saion Compaibiliy Sandard for Dual-Mode Wideband Spread-Specrum Cellular Sysems, Telecom. Indusry Assoc., July 993. [3] ETSI, Agreemen reached on radio inerface for hird generaion mobile sysem, UMTS (Universal Mobile Telecommunicaions Sysem), ETSI, Press Release, Tdoc 4/98, Paris, France, 9 Jan [4] B.Wysocki and T.Wysocki, A Mehod o Parially Suppress ISI and MAI for DS SS CDMA Wireless Neworks, in Proc. ICC 97, Monreal, Canada, June 997, pp [5] B.J.Wysocki, T.A.Wysocki, and H.-J.Zepernick, Error Performance of he 3-Channel DS CDMA WATM LAN, in Proc. ICUPC 98, Florence, Ialy, Oc 998, pp [6] H.-J.Zepernick, Reliabiliy, Throughpu and Delay of Hybrid ARQ Schemes in a Wireless ATM LAN, RSL-TR- 7, ATRI, Perh, Ausralia, Dec [7] G.C.Clark and J.B. Cain, Error-Correcion Coding for Digial Communicaions, New York: Plenum Press, /$. (c) 998 IEEE

TELE4652 Mobile and Satellite Communications

TELE4652 Mobile and Satellite Communications TELE465 Mobile and Saellie Communicaions Assignmen (Due: 4pm, Monday 7 h Ocober) To be submied o he lecurer before he beginning of he final lecure o be held a his ime.. This quesion considers Minimum Shif