Wireless Communications and Mobile Computing

Volume 2018, Article ID 5957320, 7 pages

https://doi.org/10.1155/2018/5957320

## A Novel Design of Downlink Control Information Encoding and Decoding Based on Polar Codes

^{1}Beijing Institute of Technology, Beijing 100081, China^{2}China Mobile Research Institute, Beijing 100081, China

Correspondence should be addressed to Zesong Fei; nc.ude.tib@gnosezief

Received 25 November 2017; Revised 1 March 2018; Accepted 1 April 2018; Published 13 May 2018

Academic Editor: Luca Reggiani

Copyright © 2018 Ce Sun et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

In legacy long term evolution (LTE) networks, multiple transmission modes are defined to cater to diverse wireless environment and improve the spectrum utilization. However, constrained by user equipment (UE) processing capability on blind detection of downlink control information (DCI), two transmission modes are allowed to be configured to UE simultaneously. In recent 5G standardization, the polar codes have supplanted the tail biting convolution codes (TBCC), becoming the channel coding scheme for downlink control information (DCI). Motivated by its successive decoding property, a novel design of DCI encoding and decoding is proposed in this paper. The proposed scheme could support dynamic configuration of transmission modes with decreasing the complexity of blind detection. Evaluation results from link level simulations show that the performance loss compared to conventional encoding/decoding scheme is generally negligible and the proposed scheme can comply with the false alarm rate (FAR) target of 5G standardization.

#### 1. Introduction

In the long term evolution (LTE) system, transmitter, and receiver communicate with each other by different transmission scheme. Each transmission scheme is corresponding to a transmission mode (TM) [1]. In the LTE, the multiple TMs are defined to cater to diverse wireless environment and improve the spectrum utilization. The LTE system supports nine TMs; the difference among those is the special structure of the antenna mapping, the reference signal of demodulation, and the feedback type [2].

The downlink control information (DCI) transited by base station to users can be used to schedule the downlink/uplink data transmission and convey essential configurations [3]. Specifically, the different DCI formats correspond to the different transmission modes. The length of DCI format will be adjusted with the different system configuration. Constrained by UE processing capability of blind detection of DCI [4], two transmission modes are allowed to be configured to a UE simultaneously.

The polar codes have been adopted as the channel coding scheme of the control channel in the next-generation communication networks [5]. It is based on the polarization theory and can achieve the capacity of arbitrary binary-input discrete memoryless channel (B-DMC) [6]. Furthermore, the complexity of polar codes is low with some optimized decoding algorithms, such as low-complexity list successive cancellation (LCLSC) decoding algorithm [7]. Blind detection of polar codes has been researched in [8]; that work focuses on fitting within the 5G parameters. A low-complexity blind-detection algorithm for polar-encoded frames is proposed in [9]. That scheme decreased the complexity of polar decoding in blind detection. Our scheme decreased the complexity of the process of blind detection.

Being different from the tail biting convolution code (TBCC) which is the coding scheme in LTE, all the existing decoders of polar codes are based on successive cancellation (SC) decoder [10], which allows the encoded bits to be decoded in given order. Taking advantage of successive property of polar decoders, decoding process can be paused after the first several bits being decoded and continued accordingly based on the value of first several bits. Based on the successive property of polar decoders, a novel design on DCI encoding and decoding is proposed in this paper. The proposed scheme could support dynamic configuration of transmission modes with decreasing the complexity of blind detection.

The rest of this paper is organized as follows. In Section 2 we introduce the foundation of proposed scheme. The scheme of DCI design is proposed in Section 3. In Section 4, we analyze the complexity, and simulation results are given. Finally, Section 5 concludes the paper.

#### 2. Preliminary

In this section, we introduce polar codes and DCI design of the LTE system briefly; these are the foundation of the proposed scheme.

##### 2.1. Polar Codes

Polar codes are based on channel polarization theory which is described as follows.

Theorem 1. *For any B-DMC , the channels polarize in the sense that, for any fixed , as goes to infinity through powers of two, the fraction of indices for which goes to and the fraction for which goes to , where is the length of code word which is equal to the length of polarized subchannels, denotes the th subchannel of subchannels, and denotes the channel capacity.*

According to Theorem 1, we set the information bits in the subchannel set in which and set the frozen bits in the other subchannels to construct the information block . Before setting the information bits and frozen bits, we should calculate the reliability of subchannels and decide which subchannels are good to be set as information bits. The common algorithms to calculate the reliability include algorithm based on Bhattacharyya parameters [11], density evolution (DE) [12], and Gaussian approximation (GA) [13]. And then send into polar encoder to be encoded. The polar encoding is denoted as , where is the code word, is the information block, and is the generator matrix of order . The recursive definition of is given bywhere is a permutation matrix.

##### 2.2. Successive Cancellation Decoder

All the existing decoders of polar codes are based on successive cancellation (SC) decoder. After receiving , the SC decoder generates its decision by computingwhereand denotes the received message, is the channel matrix, and denotes the noise of channel.

Through the above formula, we can see that the polar decoder decodes the information bit by bit from to . When we need to decode the th bit , it is decided as zero if is frozen bit; otherwise is decided by (2) with the prior information of and . This property of polar decoder is defined as successive property which makes it is possible to suspend the process of decoding when have been decoded.

##### 2.3. DCI Design

According to the latest MIMO-related progress in the 3rd-generation partnership project (3GPP), only one code word (CW) is transmitted for 1 to 4 layers and two CWs are transmitted for 5 to 8 layers. Thus, the actual number of transmission layers could implicitly indicate the number of CWs. Moreover, compared to 1-CW case, 2 CWs would add an additional block of bit fields to DCI, possibly containing MCS/RV/NDI and CBGTI/CBGFI if CBG-based transmission is configured, as shown in Figure 1. This is where the difference between DCI payload sizes mainly rises. Consequently, DCI formats with 1- to 4-layer transmission could strive to have the same DCI payload size and so are the DCI formats with 5- to 8-layer transmission. Different transport layers corresponded to different DCI formats. UE does not know which DCI format of information is selected by base station; therefore blind detection is needed. Constrained by UE processing capability on blind detection of DCI, two DCI formats are allowed to be configured to UE simultaneously. UE attempted to decode the information with one DCI format, if it can not perform decoding correctly, UE will attempt to decode the information with the other DCI format.