OnQuantized Multi-User Beamforming inSpatially Correlated Broadcast Channels
Vasanthan Raghavan, Venugopal V. Veeravalli · 2007
We consider aspatially correlated multi-user com- munication systemwithNtantennas atthebasestation (BS), U users(Ntofwhom areserviced), andeachuserwitha single antenna. Whiletheusersareassumedtoknowtheir channels perfectly, theBShasneither instantaneous channel state information (CSI) northespatial statistics corresponding toany oftheusers. We consider alimited feedback modelwhereeach userconveys B bitsofpartial channel information totheBS. Duetothelackofstatistical information attheBS,acodebook basedonrandomvectorquantization (RVQ)andtailored to uncorrelated Rayleigh fading isusedinsignaling. Ourfirst contribution istocharacterize theimpact ofspatial correlation (inparticular, theeigenvalues ofthetransmit covariance matrix) ontheperformance ofanRVQ scheme. We alsoshowthat spatial correlation hassignificant consequences on practical aspects oflimited feedback systems like storage andconstruction ofcodebooks. Whileindependent RVQ codebooks havetobe generated forthedifferent users inthei.i.d. case, undercertain relaxed assumptions onthecorrelation, thesamecodebook canbe reused across alltheusers. Thisisbecause thespatial signature induced bythecorrelation canbeexploited todistinguish the quantizers ofdifferent users. I.INTRODUCTION