M.Sc Thesis

M.Sc StudentShaviv Dor
SubjectThe Multiple-Access Channel with Common Rate-Limited
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROF. Yossef Steinberg
Full Thesis textFull thesis text - English Version


The two-user multiple-access channel (MAC) consists of two transmitters who send information to a common receiver. The most common example for this channel is uplink communication from cellular phones to the base station. Normally the two transmitters cannot communicate with each other - being able to do so will improve the channel capacity.

    Feedback in the MAC can be used in a number of ways to increase capacity:
  1. To enable cooperation between the transmitters, where each user can decode the message of the other user by observing the output.
  2. To utilize some correlation between the channel inputs, without actually decoding the other user's message.

The assumption of perfect feedback, where the channel output is fully available at the transmitters, is unrealistic in many practical systems. Carleial suggested an achievable rate region for the MAC with generalized feedback. A special case of this channel is one where the feedback signal for each transmitter is a noisy version of the channel output. Noisy feedback is more realistic than perfect feedback; however, in many practical wireless systems, only a small percentage of the overall bandwidth is dedicated to the transmission of feedback and system information, and that portion of the bandwidth is usually coded. Therefore, the model of feedback under a rate limit better suits to describe such scenarios.

 This work studies the MAC with rate-limited feedback. The channel output is encoded into one stream of bits, which is provided causally to the two users at the channel input. An achievable rate region for this setup is derived, based on superposition of information, block Markov coding, and coding with various degrees of side information for the feedback link. The suggested region coincides with the Cover-Leung inner bound for large feedback rates. The result is then extended for cases where there is only a feedback link to one of the transmitters, and for a more general case where there are two separate feedback links to both transmitters. We compute achievable regions for the Gaussian MAC and for the binary erasure MAC. The Gaussian region is computed for the case of common rate-limited feedback, whereas the region for the binary erasure MAC is computed for one-sided feedback. It is known that for the latter, the Cover-Leung region is tight, and we obtain results that coincide with the feedback capacity region for high feedback rates.