Practicable Schemes for Multi-User Networks

In recent years multi-user networks have experienced a tremendousgrowth both in terms of the number of units deployed and in termsof their throughput requirements. There has been a lot of effort ondesigning transmission schemes for such networks, most of whichexploit spatial diversity to achieve improved throughputs. However,the schemes are often either too ad-hoc to be optimal or tooabstract to be implementable. This thesis derives practicable,near-optimal transmission schemes for multi-user networks, layingparticular emphasis on their ease of implementation.The first part of this thesis models the uplink of a genericmulti-user network and evaluates upper bounds on the capacity ofsuch a network. The thesis then refers to a practicabletransmission scheme that uses successive channel estimation andinterference cancelation to achieve throughputs close to the networkcapacity. Since such a scheme results in unequal user capacities,the thesis develops a rate-splitting algorithm to equalize the usercapacities. The thesis also analyzes the asymptotic behavior of thenetwork and proves that, even under the most optimistic conditions,expanding networks cannot sustain a constant per-user capacity.The second part of this thesis considers the downlink of multi-usernetworks and presents practicable techniques for channel statefeedback and user selection. All effective broadcasting schemesrequire channel state information at the transmitter, but most ofthe common channel feedback schemes have an exponentiallyincreasing complexity with an increasing feedback rate. The thesispresents a low-complexity rotate-and-quantize algorithm based on extended scalar quantization that has a tremendouslyreduced complexity when compared to traditional feedback schemeswhile achieving comparable throughputs. In networks where the numberof users far exceeds the number of transmitting antennas at thebasestation, the effectiveness of the broadcasting scheme is furtherimproved by identifying users with almost-orthogonal channelrealizations. The thesis presents practicable user selectionschemes for such networks based on a Grassmannian packings thatserve as good quantizers and allow the basestation to identifyorthogonal, and/or almost orthogonal receivers.