Code design for adaptive coding and modulation indicator in broadcast systems

Abstract

In this paper, we address the coding problem for adaptive coding and modulator indicators in communication systems where users are divided into several classes according to their channel quality. Two novel methods are described to construct codebooks with variable length codewords for such an application. The proposed constructions satisfy all constraints of the system model, showing considerable gain in both the maximal and average length of codebook with respect to the current state of the art. The methodology includes a systematic way for constructing variable length codebooks where codewords are not uniformly distributed in the space, and thus, some codewords are more protected than others. The proposed construction can be easily adapted, by zero padding, to obtain a fixed block-length code, with length equal to the maximal length of the designed variable-length code but still smaller than that of the best state-of-the-art code. KEY WORDS: adaptive coding and modulation; frame-header; unequal error protection 1. INTRODUCTION Real-time adaptation of transmission parameters according to the channel conditions is a highly desirable feature in communication systems where the channel parameters may change in time and/or according to the addressed receiver. Time and/or user varying channel condition is an important characteristics of most communication systems such as satellite, cellular networks, and broadcast systems in general. Adaptive coding and modulation (ACM) schemes are used in such systems to provide significant capacity gains by real-time adaptation of the forward error correction (FEC) code rate/length and modulation constellation [1].By employing an ACM scheme, the transmitter is able to switch between several constellations and codes choosing the largest available modulation and code rate, which ensures a target detection error rate (DER), thus providing the maximum reliable spectral efficiency to each user. To successfully decode the message, each user must be able to identify the parameters of the constellation and code, which has been used by the transmitter. To this purpose, each packet consists of two main parts: The first part is called the frame-header (or simply the header) and contains the information regarding the modulation and coding parameters (adaptive coding modulation indicator (ACMI)). The second part is the message, which is encoded using the corresponding ACM parameters. Therefore, it is crucial for each user to preliminary decode the information in the frame-header.In this paper, we concentrate on the coding design problem for the ACMI. Even though the ACMIs usually employ a maximum of 10 bits, the coding strategy is not trivial due to the wide dynamic range of the signal-to-noise ratio (SNR) of the system users. Coding design for frame-headers as a part of the physical layer strongly depends on the system architecture and must satisfy several constraints.

Short‐length rate‐compatible code design for noncoherent detection

“…These information, referred to as physical layer header (PLH), are encoded and sent before each FEC frame. The PLH headers in DVB‐S2X and DVB‐T2 have constant lengths; however, as it has been shown in the previous studies, [ 13–15 ] considerable gains on average code lengths may be obtained by employing variable length coding technique for PLH.…”

Decoding of Variable Length PLH Codes

“…Generally, communication standards foresee the use of constant length PLH headers. However, it has been proved that significant gains can be obtained by adopting variable length codes for the PLH [11]. This, however, makes the decoding of the PLH field especially challenging because the receiver needs to estimate also the length of the received PLH.…”