A Class of Implicit Transmission Techniques for Throughput Enhancement
Throughput enhancing techniques are very valuable to keep up with the fast increasing data rates in communication systems. Implicit transmission is particularly attractive as it can transmit information without physically transmitting them over a channel. In this study, two separate throughput enhancing techniques using implicit transmission are investigated. First a multi-constellation signaling (MCS) technique that selects one out of N(> 1) constellations based on a set of implicit bits during every interval is introduced. The overall constellation used by a MCS scheme is a NM-ary constellation formed by replacing every point of a M-ary constellation by a cluster with N constellation points. Further, the size of clusters is reduced by employing multi-dimensional mapping. It is demonstrated that a properly designed MCS scheme can double and triple the throughput and also perform better than a scheme that employs a single constellation, and MCS schemes can perform better than turbo coded signals in the long term evolution (LTE). In contrast to MCS schemes, throughput enhancing concatenated codes (TECCs) schemes transmit bits implicitly without expanding the overall constellation. In a TECC, the coded sequence of a code C transmitted over a channel is altered according to the coded bits of a second coded sequence of an implicit code C’. In this study, TECCs select one bit in every segment of n coded bits of C based on ns ≤ log2n coded bits of C’, and flip that selected coded bit of C before transmission. It is shown that using iterative decoding between codes C and C’, the receiver can decode the coded bits of both the explicit code C and the implicit code C’ jointly. TECCs that can increase the throughput of C by 25% to 37.5% without sacrificing the performance of C are reported, however, at the expense of increased complexity. TECCs are extended to form a new class of TECC-2 schemes to include a second uncoded implicit stream. It is shown here that TECC-2 schemes can significantly increase the throughput enhancing capability of TECCs at high signal to noise ratio (SNR) with only a modest increase in complexity. The tradeoff between the low SNR performance and throughput expansion is also discussed.