Figure 1: Multipath Channel Model Illustration.

Transcription

1 Te Baic: Multiple Input Multiple Output (MIMO) Radio Sytem by Bian Petted, CTO Ti aticle intoduce te ationale and tecnique involved wit MIMO (Multiple-Input, Multiple Output) baed communication ytem. Te communication impaiment due to multipat cannel ae dicued in ode to povide te bae motivation o te application o te MIMO tecnique and in paticula, te ue o OFDM (Otogonal Fequency Diviion Multiplexing) and patial multiplexing o equalizing boadband wiele cannel wit non-elective ading. Fading Cannel Te peence o multiple election and multiple communication pat between two adio teminal caue ignal ading impaiment to a wiele communication link. Tee i bot elective and non-elective ading. Non-elective ading i te cae wee te equency component ove te ignal bandwidt ae dynamically attenuated by te ame amount and do not ceate any ignal ditotion, but only tempoal ignal lo. Selective ading i te cae wee malle equency egment o te ignal pectum ae attenuated elative to te ote emaining equency egment. Wen ti occu, te ignal pectum i ditoted and ti in tun ceate a communication impaiment tat i independent o ignal level. Fo te cae o naow band ignal wit non-elective ading, te communication impaiment can be counteed by poviding moe ignal level magin in te communication link o uing election diveity tecnique to elect te bet antenna input baed on te elative ignal tengt. oweve, in te cae on non-elective ading, te ignal mut be equalized to etoe te ignal idelity and expected communication peomance. Ti ading i poduced by te aival o multiple eplica o te tanmitted ignal at te eceiving antenna. Te ignal eplica ae poduced by multiple andom election witin te communication medium, uc a an indoo communication cannel. Te multipat concept i own below in Figue 1 o eeence and i baed on te notion o an ellipoidal ading model [1]. Figue 1: Multipat Cannel Model Illutation. Popety o LS Reeac and not intended o ute publication witout te expe witten pemiion o te company. Page 1 o

2 I one contuct a imple two-ay ading model and i able to geneate te ignal attenuation veu communication ange, one ind te peence o cet and null due to te contuctive and detuctive inteeence caued by te aival o te ignal at dieent delay time and teeoe, dieence pae angle. In ome cae te pae angle can be at o nea 18 degee and can caue patial ignal cancelation, o conveely in te cae o ignal tat ae in-pae, patial ignal enancement. Te attenuation poile o a two-ay ading model i own in Figue 2 o eeence. Note te peiodic ignal null a te communication ange cange. Te peiod o te null pacing i one-al wavelengt. Figue 2: Multipat Eect on Attenuation veu Communication Range o a two-ay model. Antenna Diveity A mentioned peviouly, te eect o ti localized and udden attenuation can compenated by eceiving te ignal at two dieent eceive antenna placement location and electing te bet antenna baed on te ignal tengt o ome ote eceive peomance meaue. Te diagam Figue 3 illutate ti ituation by owing te ignal tengt at two dieent antenna placement ove communication ange and/o time. Te point o te illutation i tat te ignal tengt aociated wit te antenna ae geneally not coelated in time and/o pace and wen one ignal i in a null, te ote can be ound nea a maximum and elected o communication. Popety o LS Reeac and not intended o ute publication witout te expe witten pemiion o te company. Page 2 o

3 2 Figue 3: Relative Signal Stengt o two eceive antenna location witin a multipat cannel. Te election o one antenna ove te ote baed on ignal quality, o Selection Diveity i tecnique tat wok well o non-elective ading in tat te antenna election eentially i a ignal-tengt etoation tecnique. Te eect o antenna diveity o a Rayleig ading cannel ae peented below in Figue 4 owing a 17.5 db gain in peomance. Note tat te ame peomance can be acieved by intoducing additional ignal level magin o 27.5 db witout te additional antenna and election ceme. 1 BER veu S/N o S/M anging om -5 to +5 db in 2 db tep, Fade Magin (27.5 db) BER Diveity Gain (17.5 db) S/N (db) Figue 4: Bit Eo Ratio Cuve o Rician Cannel o vaiou Specula (Line o Site) powe elative to te um o all o te Multipat component, o S/M atio. Note eect o antenna election diveity on Rayleig Cannel, te wot cae Rician Cannel wit a S/M appoacing zeo (-5 db). Popety o LS Reeac and not intended o ute publication witout te expe witten pemiion o te company. Page 3 o

4 Selective Fading oweve, o boade band ignal moe commonly ued in moden wiele ytem uc a Wiele LAN, election diveity and ote ignal tengt etoation tecnique ae not enoug to equalize te impaiment impoed by te multipat cannel due to elective ading. Selective ading occu wen te Signal Bandwidt exceed te Coeence Bandwidt o te Cannel [2]. Te coeence bandwidt o te cannel i appoximately te ecipocal o te delay pead o te cannel [2]. Te delay pead i te m aveage o te delay time o te complex impule epone o te cannel. Te impule epone o te cannel i te eceived complex envelope at a paticula point in pace auming tat a caie ignal i modulated by a Diac Impule. Te aival o te vaiou ignal component can be modeled by a Finite Impule Repone Filte. Sown below in Figue 5 i a contuct o a elective ading cannel uing an abitay FIR model o illutation pupoe: Impule Repone Amplitude Sample Magnitude (db) and Pae Repone Magnitude (db) Pae (adian) Nomalized Fequency ( ad/ample) Figue 5: Contuct o Selective Fading Complex Cannel Impule Repone (dot tem=eal, ateik tem = imaginay) and Complex Low-Pa Tane Function (magnitude and pae). Ideally, te cannel tane unction only add a lat ading poce (contant attenuation veu equency) and a ingle ixed time delay (linea pae). oweve, te cae own above ceate a tane unction o te ignal tat povide aymmetic and equency dependent ading o Popety o LS Reeac and not intended o ute publication witout te expe witten pemiion o te company. Page 4 o

5 amplitude a well a a non-linea pae epone, auming tat igniicant ideband o te ignal ae ditibuted aco te entie tane unction own. I te ideband o a complex modulated ignal wee ditibuted aco te tane unction own, tee would be igniicant ignal ditotion and modulation conveion (AM to PM and AM to PM). Ti again, i te eect o a multi-pat cannel wit a coeence bandwidt tat i le tan te ignal bandwidt. In ode to compenate o tee eect, te tane unction encounteed via te communication cannel mut be equalized at te eceive. Ideally i te invee o te tane unction could be applied to ignal pio to detection to latten te amplitude epone and to lineaize te pae, te ignal impaiment would be eliminated. Tee ae eveal dieent type o tanveal equalize tat opeate on te time domain epeentation o te baeband ignal to acieve te equalization; oweve, many ely on taining equence and taining peiod (eceipt o non-inomation beaing taining inomation tat ceate communication oveead). OFDM and Cannel Equalization Te intoduction o Otogonal Fequency Diviion Multiplex (OFDM) ignaling not only povided a mean o pectally eicient communication, but alo povided o a mean o cannel equalization in te equency domain. Sown in Figue 6 i a diagam o an OFDM pectum, owing 48 data ubcaie and 4 pilot ubcaie. Te pilot ubcaie ae not inomation beaing but ae ued to elp maintain te caie and timing tacking in low-ignal to noie atio ince tey ae alway BPSK modulated. OFDM eentially povide a mean o ampling te magnitude and pae o te cannel at any o all o te ubcaie equencie, ince te caie pae tacked at te eceive and all o te ubcaie pae ae coeent to te main caie. oweve, in pactice, peoming continuou ubcaie pae tacking on eac o te ubcaie i not executed, ate, ot and long taining ymbol wit known patten ae tanmitted wit eac packet uc tat te eceive can detemine te cannel tane unction, invet it, and ten apply te invee to te eceived pectum in ode to equalize te cannel on a packet by packet bai. T G T FFT Powe X nc T in 2 nc T 2 FFT FFT R =.3125 Mz= 1/Tt -21 Pilot Caie: -21,-7,7,21 Powe Nd=48 Np= Mz Popety o LS Reeac and not intended o ute publication witout te expe witten pemiion o te company. Page 5 o

6 Figue 6: Contuct o entie OFDM pectum, 3 iolated ubcaie, Subcaie Spectum tat aie om amplitude making by FFT inteval and Guad Inteval. Figue 7 own below illutate ow te cannel tane unction due to elective ading weigt te eceived pectum. At te eceive, te magnitude and pae o eac o te ubcaie povide an etimate o te cannel tane unction, c (). An equalization tane unction, e (), can be applied to te cannel-weigted pectum to e-nomalize te pectum uc tat te eect o te cannel can be educed o eliminated. S RX c e c STX SRX S TX c e 1 c 1 1 c c -21 Pilot Caie: -21,-7,7,21 Powe N d =48 N p = Mz Figue 7: Cannel Tane unction eect on OFDM pectum. Eac ubcaie a it magnitude and pae alteed, but can ened and ued to geneate an equalization ilte tane unction. MIMO- Multiple Input Multiple Output Sytem In te peviou ection, multipat cannel, non-elective ading, elective ading and OFDM ignaling wee dicued. Fute, it wa own tat antenna election diveity i a tecnique o impoving te peomance in a non-elective ading cannel and OFDM povide o cannel equalization againt equency elective ading. In ti ection, te MIMO tecnique i dicued and eentially bing two element togete: OFDM and antenna diveity. Popety o LS Reeac and not intended o ute publication witout te expe witten pemiion o te company. Page 6 o

7 In geneal tee ae dieent pemutation o edundant tanmitte and eceive to impove te obutne o a communication link and te inomation caying capacity o te link. Sown below in Figue 8 ae te dieent, geneal edundancy coniguation, SISO, SIMO, MISO, and MIMO. SISO o Single-Input, Single-Output ytem do not povide any type o obutne o capacity impovement and epeent a baic wiele communication link. SIMO o Single-Input, Multiple- Output ytem ae coniguation tat povide eceive-ide diveity and povide additional obutne, but no capacity impovement. Example o SIMO ytem ae eceive wit election diveity ceme o Linea Maximal Ratio Combining (LMRC) ceme [2]. MISO o Multiple-Input, Single-Output povide tanmitte diveity in tat it couple to te cannel at a dieent point in pace uc tat te link will not ave te ame ading caacteitic toug to te eceive antenna and patial um o te ignal will be dominated by te tonge o te two ignal. O te tanmitte ignal deign can be uc tat te combining at te ingle eceive can made in an optimal aion a i done in Space-Time Coding Tecnique [2]. MIMO, Multiple-Input, Multiple-output ytem de-multiplex te ouce data team into multiple independent cannel team. Ti povide o bot edundancy and cannel capacity impovement [3]. Figue 8: Seveal Communication Sytem Coniguation Te MIMO tecnique i alo eeed to a Spatial Diviion Multiplexing, a ingle ouce data team i multiplexed between two patial team. A own in Figue 8, tee ae diect link and co link between te two tanmitte and two eceive. A a eult, tee ae now tee ae ou dieent communication cannel tat connect te two teminal and te caacteization o te cannel a a ige complexity. oweve, te ignaling tecnique wen Popety o LS Reeac and not intended o ute publication witout te expe witten pemiion o te company. Page 7 o

8 combined wit te popetie o OFDM modulation can lead to an eicient cannel etimation and equalization ceme. Sown below in Figue 9 i an illutation o te patial multiplexing ceme wee even ignal ae tanmitted on TX and odd ymbol ae tanmitted on TX1. Te two eceive, RX and RX1 eceive bot o te tanmitted team toug 4 poible cannel tane unction, 1, 1, and, wic om a cannel matix, []. 1 1 Figue 9: 2 X 2 MIMO a a patial multiplexe. In ode to equalize and extact te ouce ymbol team, an etimate o te cannel matix [] mut be made. Ti can be accomplied by tanmitting bot team uing a coodinated ytem o pilot ubcaie and null ubcaie a own in Figue 1. Peented in Figue i a epeentation o te eceived pectum owing tat elative level o eac ubcaie indicate elative level between te diect and co cannel tem in te cannel matix []. Popety o LS Reeac and not intended o ute publication witout te expe witten pemiion o te company. Page 8 o

9 Pilot Caie: -21,-7,7,21 TX Null Caie: -23,-9,9,23 Powe N d=48 N p =4 Pilot Caie: -21,-7,7,21 TX1 Null Caie: -23,-9,9,23 Powe N d =48 N p=4 Figue 1: 2 X 2 Coodinated Pilot Subcaie and Null Subcaie at eac tanmitte. RX Powe N d =48 N p =4 Figue : Received Spectum at RX owing te elative level o and 1 om te pilot ubcaie level. Te eceived ymbol at eac o te eceive input can be computed a ollow: o 1 1 R S Fo cae o pilot ubcaie elative to te null ubcaie: 1 o 1 Popety o LS Reeac and not intended o ute publication witout te expe witten pemiion o te company. Page 9 o

10 Since te magnitude and pae o pilot ubcaie i known, te -tem can be actoed out and te element o te cannel matix ae detemined. Given cannel matix, te eceived ignal can be equalized and etoed accoding to: S R adj 1 S R adj R 1 R 1 R o o R 1 1 T 1 1 B 1 1 o 1 Note tat te oiginally tanmitted ignal can be etoed peectly, i te cannel matix can be etimated peectly. One cae tat doe not wok due to a ingulaity in te matix inveion i te intance wee all o te cannel element ae equal o coelated: 1 o o o 1 S R B Te denominato o te leading tem aociated wit matix inveion deteminant will equal zeo, cauing te ingulaity. Teeoe, MIMO ytem opeate te bet wen te individual cannel element ae not coelated. Te patial multiplexing tecnique aociated wit MIMO i anote metod to minimize te eect o a multipat communication cannel toug it edundancy and cannel equalization popetie. Popety o LS Reeac and not intended o ute publication witout te expe witten pemiion o te company. Page 1 o

11 Reeence: [1] Diagam ate: MIMO Cannel Modeling and Emulation Tet Callenge, K.Betlin, Agilent Tecnologie, 28. [2] C. Oetge and B. Cleckx MIMO Wiele Communication, Academic Pe, 27. [3] B. olte, On te Capacity o te MIMO Cannel- A Tutoial Intoduction, Nowegian Univeity o Science and Tecnology, Depatment o Telecommunication, O.S.Bagtad pla 2B, N-7491 Tondeim, Noway. Popety o LS Reeac and not intended o ute publication witout te expe witten pemiion o te company. Page o