The 5G channel code contenders

Channel coding is a vital but complex component of cellular communication systems, which is used for correcting the communication errors that are caused by noise, interference and poor signal strength. The turbo code was selected to provide channel coding in 3G and 4G cellular systems, but the 3GPP standardization group is currently debating whether it should be replaced by the Low Density Parity Check (LDPC) or polar code in 5G. In this white paper, we dispel two myths that have been the genesis of this debate, namely that turbo codes cannot support high throughputs and that they have higher complexities than LDPC and polar codes. Indeed, AccelerComm has already demonstrated that turbo codes can achieve decoded throughputs exceeding the 5G target of 20 Gbps. Furthermore, this white paper shows that the overall implementation complexity of turbo, LDPC and polar codes depends on more than just their computation complexity. Since turbo codes have the benefits of low interconnect complexity and inherent flexibility, we show that they facilitate high-throughput flexible channel coding at lower implementation complexities than LDPC and polar codes. Furthermore, turbo codes offer the additional benefit of backwards compatibility to 3G and 4G, which offers the cellular communications industry some very significant cost savings. It is for these reasons that AccelerComm is promoting the turbo code for 5G. I. WHAT IS CHANNEL CODING? In cellular communication systems, wireless transmission is used to convey data between handsets and basestations, where the latter act as gatekeepers to the Internet and telephone networks. However, the received data typically differs to the transmitted data, owing to communication errors caused by noise, interference or poor signal strength. In order to correct these communication errors, cellular communication systems use channel codes. More specifically, a channel encoder is used in the transmitter (be it the handset or the basestation) to convert each so-called data block comprising K data bits into a longer encoded block comprising N > K encoded bits, which are transmitted. In the receiver, the additional (N −K) encoded bits provide the channel decoder with information that allows it to detect and correct communication errors within the original K data bits. If the noise, interference or poor signal strength is particularly severe, then a low coding rate R = K/N will be required for the channel decoder to successfully detect and correct all transmission errors. However, a low coding rate implies the transmission of a high number N of encoded bits, which consume transmission energy and bandwidth. Therefore, good channel codes are ones which allow the successful detection and correction of transmission errors at coding rates R that are as close as possible to the theoretical limit that is imposed by the channel capacity [1]. In the past couple of decades, several nearcapacity channel codes have emerged, including the turbo codes that are adopted in 3G and 4G mobile broadband standards, the Low Density Parity Check (LDPC) codes [2] that are adopted in WiFi standards [3], as well as the more recent polar codes [4]. Turbo and LDPC codes employ an iterative decoding process, in which each successive decoding attempt informs the next, until the process converges. By contrast, polar codes use a successive cancellation decoding process, in which the decoding of each successive bit informs the decoding of the next. Since the channel decoder must overcome the uncertainty introduced by noise, interference and poor signal strength, it typically has a much greater complexity than the channel encoder. Owing to this, it is the channel decoder that is typically the main concern when designing a channel code. ©AccelerComm 2016 www.accelercomm. com 1 ACCELERCOMM WHITE PAPER: THE 5G CHANNEL CODE CONTENDERS II. WHAT IS DRIVING THE DISCUSSION FOR 5G? While turbo codes have enabled high-performance communication in 3G and 4G cellular standards, a myth has emerged which questions their applicability to 5G. More specifically, the myth is that turbo codes have an inherently serial structure [5], which requires the steps of the turbo decoding process to be completed one at a time and in the right order. This is in contrast to LDPC codes, which are recognized as having an inherently parallel structure [5], allowing all steps of the decoding process to be completed simultaneously and within less time. If this myth were true, then it would be impossible to construct turbo decoders that achieve high decoded throughputs, measured in billions of bits per second (Gbps). Since all received bits must pass through the channel decoder, turbo decoders would therefore impose a severe bottleneck on 5G, which is targeting transmission throughputs of up to 20 Gbps [6]. However, at AccelerComm, we have dispelled this myth by demonstrating a turbo decoder that achieves a decoded throughput of 21.9 Gbps [7]. This has been achieved by redesigning the turbo decoder from the ground up, at both the algorithm and hardware level. In contrast to the conventional approach that has been adopted for more than two decades, AccelerComm’s turbo decoder enables the same fully-parallel degree of processing that can be achieved for LDPC decoders. We have therefore demonstrated that turbo codes can be applied in 5G, without imposing a bottleneck upon the achievable transmission throughput. Furthermore, another myth has emerged that turbo codes have greater complexity than LDPC or polar decoders. While it is true that turbo decoders require more computations than LDPC or polar decoders at most coding rates R, this white paper shows that it is the overall implementation complexity that really matters and that this depends on more than just a decoder’s computation complexity. More specifically, a decoder’s processing hardware resource requirement and energy consumption depends also on the complexity of the hardware interconnections that are required to meet the 5G requirement of flexibly supporting a wide variety of block lengths K and coding rates R. Owing to their low interconnect complexity and their inherent flexibility, we show that turbo codes facilitate high-throughput flexible channel coding at lower implementation complexities than both LDPC and polar codes. Owing to the persistence of the above-described myths, the 3GPP standardization group is currently considering the replacement of turbo codes with LDPC or polar codes in the 5G ‘new radio’. The rest of this white paper examines the 5G requirements for channel coding and discusses how they are met by each of turbo, LDPC and polar codes. We conclude that turbo codes are able to meet the 5G requirements to a greater degree than LDPC and polar codes, while offering the industry some very significant cost savings by offering backwards compatibility with 3G and 4G. It is for these reasons that AccelerComm is promoting the turbo code for 5G. III. WHAT ARE THE REQUIREMENTS FOR 5G? Like 3G and 4G, the aim of the 5G ‘new radio’ is to continue the trend of offering exponentially greater user experience and more diverse applications for cellular communications. However, this will impose greater requirements upon the channel code [8], as summarized in the following discussions.