Adjacent-channel, Co-channel and Intersymbol Interference Suppression for Carrier Interferometry based One-cell Reuse Single-carrier TDMA Systems

Transcription

1 6 th IST MOBILE & WIRELESS COMMUNICATIONS SUMMIT Adjacent-channel, Co-channel and Intersybol Intererence Suppresson or Carrer Intereroetry based One-cell Reuse Sngle-carrer TDMA Systes Chanta Srtapetch and Sech Sape Graduate School o Engneerng, Osaka Unversty Yaada-oka 2-, Suta-sh Osaka, Japan Eal: chanta@wreless.co.eng.osaka-u.ac.jp, sape@co.eng.osaka-u.ac.jp Abstract Ths paper proposes an ntererence suppresson technque usng edge-reoval lter, llng lter and turbo equalzer or carrer ntereroetry (CI) based one-cell reuse sngle-carrer te dvson ultple access (TDMA) systes. In the proposed schee, CI technque s eployed to generate CI sgnal whch s equvalent to sngle-carrer sgnal wth ts roll-o actor o zero. At the recever, receved CI sgnal s transored nto requency-doan sgnal va ast Fourer transor (). Adjacent-channel ntererence (ACI) ro the sae cell s rst suppressed by the edge-reoval lter and then co-channel ntererence (CCI) ro adjacent cell s suppressed by applyng requency-doan llng lter. Ater that, a sot canceller wth nu ean square error (SC/MMSE) based turbo equalzer s utlzed to copensate or channel nduced ntersybol ntererence (ISI) as well as extra ISI produced by the edge-reoval lter and llng lter. Coputer sulaton conrs that ntererence can be suppressed wth the proposed schee. Index Ters ntererence suppresson; carrer ntereroetry; edge-reoval lter; llng lter; turbo equalzer I I. INTRODUCTION n downlnk broadband wreless councaton systes, ultcarrer transsson such as orthogonal requency dvson ultplexng (OFDM) has been wdely accepted due to ts pressve perorance under severe ultpath adng channel as well as ts lexble rado resource anageent capablty. However, t has a power back-o proble due to a hgh peak-to-average power rato (PAPR) whch s not preerable n the uplnk transsson systes. Thereore, carrer ntereroetry (CI) [, 2] based sngle-carrer transsson s consdered to be ore sutable or the uplnk transsson. However, when we would lke to apply a sngle-carrer transsson schee to one-cell reuse cellular systes, the receved sgnal s severely nluenced by ntererence ncludng co-channel ntererence (CCI) ro other users n the adjacent cells and adjacent-channel ntererence (ACI) cong ro the sae cell, as well as copensaton or ntersybol ntererence (ISI) due to requency selectve adng channel. Thereore, the recever has to be desgned to copensate or these ntererences. As the soluton to tgate ISI eects, turbo equalzaton [3] that works n an teratve anner based on turbo prncple has been proposed. However, due to eployent o trells dagra n the decodng process or turbo equalzer, ts coputatonal coplexty ncreases exponentally wth channel eory length. A ore coputatonally ecent teratve equalzaton algorth s the turbo equalzer cobned wth a sot canceller and nu ean square error lterng (SC/MMSE) schee [4, 5] proposed by Renolds and Wang n whch SC/MMSE and sot-nput sot-output (SISO) decoder s serally concatenated. In ton to cobat ISI due to ultpath adng, CCI ro users n the adjacent cells should also be sucently suppressed n one-cell reuse cellular envronents. In [6], the SC/MMSE concept s appled to ultple-nput ultple-output (MIMO) syste under requency selectve adng channel condtons consderng both ISI and ultple access ntererence (MAI). The work s extended then to suppresson o unknown ntererence whch s cong ro other cells n ton to the known ntererence cong ro users n the sae cell [7]. These works can be categorzed as the te-doan sgnal processng. Despte these te-doan sgnal processng, ths paper proposes the alternatve schee to suppress ACI, CCI and ISI utlzng requency-doan sgnal processng. The benet o requency-doan ntererence suppresson s that t has essentally the sae low coplexty as OFDM syste [8]. In the proposed algorth, the receved sgnals are rst transored to the requency-doan sgnal orat usng ast Fourer transor (), ACI and CCI are suppressed usng edge-reoval lter and llng lter, respectvely. Ater that, the sgnals are transored back to the te-doan sgnals va nverse (I). At ths stage, although ntererence can be suppressed, t would also cause extra ISI. Thereore, SC/MMSE turbo equalzer s eployed to copensate not only or channel nduced ISI but also extra ISI produced by the edge-reoval lter and llng lter.

2 6 th IST MOBILE & WIRELESS COMMUNICATIONS SUMMIT 2 II. CHANNEL MODEL The concept o one-cell reuse sngle-carrer CI/TDMA systes or the uplnk s shown n Fg.. In ths broadband syste, the desred sgnal n the reerence cell would be nterered by ACI, CCI and ISI. The sgnals ro other users n the sae cell would cause ACI because adjacent subcarrers are assgned n the sae cell, resultng n the utual overlaps n the spectra. Although the aount o ACI coponents s not so large, but derences n power levels between desred sgnals and ntererence sgnals can draatcally degrade the perorances. In ton to these ACI, reusng o the requency band n the adjacent cell would cause CCI and ultpath adng would lead to the ISI eect. As seen n Fg., at BS, the desred sgnal ro MS s nterered by sgnal ro MS2 whch locates n the sae cell, leadng to the ACI eect. Moreover, the sgnal ro MS3 locatng n the adjacent cell could also cause CCI because all the spectru s shared by all the cells n one-cell reuse systes. In ths paper we consder sngle-carrer CI/TDMA systes or the uplnk. CI s a very useul technque to lexbly anpulate spectru or a sngle-carrer transsson schee wth alost the sae sgnal processng coplexty wth OFDM schees. For exaple, t can equvalently generate a sngle-carrer sgnal wth ts roll-o actor o zero just by assgnng the sae apltude and phase to all the subcarrers o an OFDM sybol. Moreover, at the recever, the receved CI sgnal has eatures or both the OFDM and sngle-carrer waveors III. PROPOSED SYSTEM A. Conguraton o the Proposed Syste The transtter and recever conguraton o the proposed syste s depcted n Fg. 2. For each user, sgnal s transtted ro one antenna to M-eleent receve antennas. At the transtter, the data bt sequence s rst encoded, nterleaved and ed to a odulator to generate a BPSK sybol sequence. Ater odulaton, CI sgnal generaton process s operated. The sequence o odulated sybols s rst seral-to-parallel converted. Next, each sybol s apped onto N c subcarrers and lnear phase sht s appled to each subcarrer. In practce, CI sgnal s generated by applyng N c -pont nverse Fourer transor (I) as shown n Fg. 2 (a). Moreover, by adjustng lnear phase oset o each sybol, te sht o sgnal n the te doan can be controlled. Ths eans that CI sgnal can be regarded as sngle carrer baseband sgnal ltered by a root Nyqust lter wth ts roll-o actor o zero. Cyclc prex () s then ed at the begnnng o each block o CI sybols beore transsson. The recever structure s shown n Fg. 2 (b). s d ro the receved block to obtan only the sequence o N b data sybols. Then, we wll conduct a zero-sux png whch s the ost portant uncton or ntroducton o the requency-doan ntererence suppresson: When we soe requency coponents o the receved sgnal to ll out ntererence n the requency doan, the process would also produce extra ISI o the desred sgnal n te doan. Thereore, there wll be soe ntererence ro the last part o the receved sgnal block to ts head part due to ts cyclc nature o the operated sgnal n the te doan. To prevent such ntererence, s o length N s are appended at the end o the sequence o the receved data block. The receved blocks then conssts o N = N b + N s sybols as shown n Fg. 3. The resultng N sybol sequence s converted to a requency doan sgnal by the N -pont operaton. Then, the sequence o sgnals s ed to edge-reoval lter to suppress the ACI eect. At ths stage, SINR o each requency coponent s easured so that llng lter can utlze these SINR values to ll out CCI n soe requences havng low SINR. In ths process, requency coponents severely nterered by CCI are lled out. The llng lter output sgnal s transored back to the te doan sgnal by the I and s converted ro parallel to seral. N s zero-sux are then d to obtan the sgnal block o length N b. I the ber o receved antennas s two or ore, the above entoned process wll be done n each branch. Then, the zero-sux d sgnal or each dversty branch s ed to the te-doan SC/MMSE turbo equalzer to elnate ISI nduced by ultpath adng channel as well as extra ISI produced by the edge-reoval lter and llng lter. Rx # Rx #M Modulator θ CI Sgnal Generator S/P I P/S θ Nc (a) Transtter Flter Flter Nullng Flter Nullng Flter I I nserton desred sgnal ACI BS CCI BS 2 SISO Channel Te-Doan SC/MMSE Turbo Equalzer - SC/ MMSE Equalzer MS 2 MS MS 3 Fgure. One-cell reuse cellular syste or uplnk (b) Recever Fgure 2. Transtter and recever conguraton.

3 6 th IST MOBILE & WIRELESS COMMUNICATIONS SUMMIT 3 Data block 256 sybols 256 sybols 52 sybols All zero sux Fgure 3. Block o sgnals ater zero-sux nserton. B. Intererence Suppresson Schee Let the transtted sgnal sequence and ntererence sgnal sequence ro the outsde cell are denoted by { s ( t); t = ~ Nb } and { ( t); t = ~ Nb }, where t s sybol te ndex. At the recever, dscrete te receved sgnal o the th receve antenna s obtaned as L L s r ( t) = h ( l) s( t l) + h ( l) ( t l) + v ( t). () l= l= where h s (t) and h (t) are dscrete te representaton o the channel pulse responses at the th receve antenna or the transtted sgnal s(t), and that o the ntererence sgnal (t), respectvely. v(t) s tve whte Gaussan nose (AWGN). L denotes channel eory length whch s assued to be dentcal or both desred sgnal and ntererence sgnal. It s portant to note that the transtted sgnal s(t) n () ncludes the ACI coponents ro the users n the sae cell. Thereore, the receved sgnal r (t) conssts o the desred sgnal, ACI coponents, CCI coponents, and AWGN. In the proposed ntererence suppresson schee, the ntererence sgnal wll be suppressed n requency doan utlzng requency-doan edge-reoval lter and llng lter. Frequency-doan representaton o the receved sgnal n () can be expressed as s R = H S( k) + H I + V, (2) whch s obtaned by applyng N -pont operaton to r (t) n (), gven by N r t= t R = ( t)exp( j2π k). (3) N k = ~ N denotes requency coponent ndex. The requency-doan receved sgnal n (3) s then ed to edge-reoval lter. Frst, ACI coponents are suppressed by llng both edges o the receved sgnal. Ater that, llng lter wll suppress CCI coponents by ultplyng zero to soe specc requency coponents o the receved sgnal, whch s equvalent to ll out the coponents accordng to the easured sgnal to ntererence plus nose power rato (SINR), whereas the other requency coponents are kept the sae. Let us assue that the ber o requency coponents to be lled out be N, requency coponents havng the lowest N SINR are lled out, and requency ndex o the lled-out coponents be k ( =,2,..., N ). The output o the llng lter can then be expressed as ~ R = W R, (4) where k =,,..., N, and the tap weghts o the llng lter can be expressed as W, =, k = k k k. (5) SINR o each requency coponent k can be calculated by PS SINR = P I P, (6) + N where P S (k), P I (k) and P N (k) are the powers o desred sgnal, ntererence sgnal ro other user and nose, respectvely. Snce the ber o llng coponents s xed to N, (k) n (5) can be rewrtten as W W N = δ ( k k ), k ~ N. (7) = = Inverse Fourer transor o the lter tap weghts that corresponds to pulse response o the llng lter s gven by N w ( t) = δ ( t) exp( j2π t), (8) N = N or rewrtten n the other or as w ( t) = N N, N k exp( j2π N N = k t = t), t = ~ N. (9) As shown n (9), extra ISI s produced n the llng lter. Moreover, t s obvous ro (5) that the receved power s lost as the ber o llng coponents N s gettng hgh. Thus, the ber o N has to be optzed accordng to the tradeo between the receved power loss and the aount o CCI suppresson. C. Suppresson o ACI at the Transtter Sde In the prevous secton, we proposed the schee or ntererence suppresson that eploys edge-reoval lter to suppress ACI and llng lter to suppress CCI n whch all processes have been done n the recever sde. In ths secton, we propose another ethod to elnate the ACI eect by suppressng t at the transtter sde. Conguraton o ths algorth s llustrated n Fg. 4. The edge-reoval lter appeared n Fg. 2 s oved to the transtter sde. Ater CI sgnal s generated, the ACI sgnals wll be suppressed and then s ed beore transsson. Thereore, the transtted sgnals becoe ACI-ree.

4 6 th IST MOBILE & WIRELESS COMMUNICATIONS SUMMIT 4 Rx # Rx #M Modulator CI Sgnal Generator (a) Transtter Nullng Flter Nullng Flter Flter I I I nserton TABLE I. SIMULATION PARAMETERS Block length (N b ) 256 sybols Zero-sux (N s ) 256 sybols Modulaton BPSK convolutonal code ( R = ½, K =3) Nuber o antennas Tx :, Rx : 4 Channel odel 4-path Raylegh Fadng Nuber o subcarrers (N c ) 256 pont (N ) 52 Log-MAP Nu. o decodng teraton 4 Channel estaton Perect Te-Doan SC/MMSE Turbo Equalzer SISO Channel - SC/ MMSE Equalzer.E+.E- - no ntererence suppresson. E-2-2 (b) Recever Fgure 4. Suppresson o ACI at Transtter Sde..E-3-3. E-4-4 IV. SIMULATION RESULTS The perorance o the proposed schee has been evaluated by coputer sulaton. Sulaton paraeters are as suarzed n Table. For splcty o our analyss, we wll assue that the desred sgnal n the reerence cell (desred sgnal) s nterered by an ntererence sgnal (undesred sgnal) wth a certan average desred to undesred sgnal power rato. Fgure 6 shows the versus E b /N perorances n the case that desred sgnal power to CCI sgnal power rato (SCCIR) equals to db and desred sgnal power to ACI sgnal power rato (SACIR) equals to db. It s seen that the proposed algorth can eectvely suppress the eect o ACI and CCI. When coparng the perorances between suppressng ACI at recever sde and transtter sde, t s ound that suppressng ACI at transtter sde gve a slghtly better perorance. perorance when SCCIR decreases to db s depcted n Fg. 7. In ths case, the desred sgnals are strongly nterered by CCI sgnals. However, the proposed schees stll gve the good perorance. Moreover, when the syste s severely nterered as the SCCIR s db and SACIR s db whch ean that the desred sgnals have uch less power than the ntererence sgnals, as seen n Fg. 8, the syste wthout any ntererence suppresson aces the bad perorance, whle the proposed schees show the very good perorance. Ths ephaszes the eectveness o the proposed schee or ntererence suppresson.. E SCCIR = db, SACIR = db E b /N (db).e-6 Fgure 5..E+.E- -. E-2-2.E-3-3. E-4-4. E.E-6 perorace when SCCIR = db and SACIR = db. -6 SCCIR = db, SACIR = db no ntererence suppresson E b /N (db) Fgure 6. perorace when SCCIR = db and SACIR = db.

5 6 th IST MOBILE & WIRELESS COMMUNICATIONS SUMMIT 5.E+.E- - SCCIR = db, SACIR = db. E-2-2.E-3-3. E-4-4 no ntererence. E.E-6-6 suppresson E b /N (db) Fgure 7. perorace when SCCIR = db and SACIR = db. V. CONCLUSION Ths paper has proposed a novel ntererence suppresson algorth eployng requency-doan edge-reoval lter and llng lter cobned wth SC/MMSE turbo equalzer. ACI and CCI eects ro other users are suppressed by the edge-reoval lter and llng lter operated n requency doan, respectvely. Although the requency-doan lters produce extra ISI to the receved sgnals, the SC/MMSE turbo equalzer appled ater the lters can copensate or ths ISI as well as ISI nduced by the adng channel. Coputer sulaton conrs that the proposed syste can eectvely suppress the eect o ntererence sgnals especally when ntererence sgnal power s hgh. REFERENCES [] B. Natarajan, C. R. Nassar, S. Shatt, Throughput Enhanceent n TDMA through Carrer Intereroetry Pulse Shapng, Proc. IEEE, VTC2-Fall, vol. 4, pp , Sep. 2. [2] B. Natarajan, C. R. Nassar, S. Shatt, Innovatve Pulse Shapng or Hgh-Perorance Wreless TDMA, IEEE Councaton Letters, vol. 5, pp , Sep. 2. [3] C. Doullard, C. B. Mchel Jezequel, C. Berrou, A. Pcart, P. Dder, and A. Glaveux, Iteratve correcton o ntersybol ntererence : turbo-equalzaton, European Tran. Telecoun., vol. 6, no. 5, pp. 57, Sep [4] X. Wang and H. V. Poor, Iteratve (Turbo) sot ntererence cancellaton and decodng or coded CDMA, IEEE Trans. Coun., vol. 47, pp. 46-6, July 999. [5] D. Reynolds and X. Wang, Low-coplexty turbo- equalzaton or dversty channels, Sgnal Processng, vol. 8, no. 5, pp , May 2. [6] T. Abe and T. Matsuoto, Space-te turbo equalzaton n requency selectve MIMO channels, IEEE Trans. Veh. Technol., pp , May 23. [7] T. Abe S. Tosato and T. Matsuoto, A MIMO turbo equalzer or requency-selectve channels wth unknown ntererence, IEEE Trans. Veh., vol. 52 May 23. [8] D. Falconer, S. L. Aryavstakul, A. Benyan-Seeyar and B. Edso, Frequency doan equalzaton or sngle-carrer broadband wreless systes, IEEE Coun. Mag., vol. 4, pp , Aprl 22.