Early detection using CRC precoding and polar codes for low latency communications

Abstract

One of the foreseen challenges of the fifth generation (5G) of wireless communications is to support ultra-low latencies for new real-time applications. Near future applications are based on machine-to-machine (M2M) communications that demand lower latencies than humancentric communications. The aim of this work is to reduce the detection latency of an orthogonal frequency-division multiplexing (OFDM) system while maintaining the error rate of its synchronous detection scheme. A scheme of sequential early detections or asynchronous detections, based on cyclic redundancy check (CRC) and polar codes, is used for this purpose. The sequential detection is carried out through a message-checking loop between the CRC decoder and a sampling buffer. Considering a communication system without any feedback of the channel status, we make sure that the Bhattacharyya construction method generates the best possible polar codes over AWGN channels. This is achieved by determining the best design signal-to-noise (SNR) ratio parameter of the polar code construction. Based on Vangala’s search method, block-error rates (BLER) of possible design-SNRs are compared to determine the best design parameter. Under the best design-SNRs, the Bhattacharrya construction method obtains similar block-error rates as the Tal&Vardy construction method for specific ranges of SNR. A multicarrier communication system is used considering that a block of symbols, constituting a message, can be transmitted in parallel and received simultaneously. Besides, the reception processing time of a short message in an OFDM system is much less than the duration of the OFDM symbol period. Taking advantage of this, the strategy of iterative early detections before the end of the symbol duration can reduce the detection latency of a message. However, a suitable setting of an early detection scheme based on concatenated CRC-polar codes is not yet known. We propose two selection processes to determine CRC polynomials and initial detection times (IDT) of the early detection scheme. Although the latency results do not achieve the optimal latency in the finite-blocklength regime, the CRC-based sequential early detection scheme using appropriate parameters achieves a detection latency improvement of 40% for a SNR of 4dB. The resulting error performance approaches to the BLER of a scheme without early detections as the block length of the polar code increases. Furthermore, three detection distributions are analyzed in terms of their statistical average latencies in the finite-blocklength regime and using CRC-polar codes. Results show that the average detection latency improves with short time intervals between asynchronous detections.

Date	5 Jun 2017
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	François Gagnon (Supervisor)