# Rateless Coded Uplink Transmission Design for Multi-User C-RAN
## 1 Introduction
### Motivation and Related Works  
There exist several issues for the implementation of C-RAN. Firstly, the limited fronthaul link capacity imposes restriction on the data rate between RRHs and the BBU pool. Therefore, signal compressors are necessary at both sides [9; 10; 11; 12]. Secondly, the additional latency introduced by fronthaul will cause stricter restriction on the decoding time at the BBU pool under the conventional hybrid automatic repeat-request (HARQ) framework [13]. Thirdly, for joint signal processing at the BBU pool, global information about the network states and channel states is needed. In consideration of the system overhead, it is nearly infeasible for the transmitter to obtain instantaneous global channel state information. This poses challenges for the application of traditional HARQ-based fixed-rate channel coding (such as low density parity check (LDPC) code and Turbo code) in C-RAN, with which the coding rate of the transmitter is chosen based on the current channel state feedback by the receiver, and the decoding time at the receiver is restricted in order to support HARQ [13]. On the other hand, rateless code (such as Luby transform (LT) code and Raptor code [14]) can encode the information into a codeword of infinite length. With rateless code, the transmitter continuously sends the codeword to the receiver until the latter feeds back an ACK. The performance of rateless code depends on the so-called output node degree profile [14]. The optimized rateless code can perform close to the channel capacity even without instant channel state information [15]. The above properties make rateless code a promising channel coding candidate for C-RAN.  
C-RAN has been widely investigated, and there has been a plenary of work on various aspects of C-RAN, e.g., [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31]. The network structure and the function split between RRHs and the BBU pool in C-RAN were discussed in [16; 17; 18]. Literature works [10; 20] investigated resource allocation and management in C-RAN. Non-orthogonal multiple access (NOMA) was considered in C-RAN [21; 22], where energy efficiency was analyzed and optimized. As for the physical layer, existing work mainly focused on the compressor and pre-coder design at the RRHs, wherein the signal dependence at the RRHs can be utilized to increase the compression rate [23; 24; 25], and the compressor and pre-coder can be jointly optimized for a higher system achievable rate [26; 27; 28]. Considering the channel coding in C-RAN, authors in [29; 30; 31] made efforts on the design and implementation of fixed-rate code, i.e., Turbo code and LDPC code, and the performances of decoding algorithms were evaluated on cloud computing platforms. On the other hand, rateless code, such as LT code and Raptor code, has been studied in various communications systems, including wireless broadcast systems [32], distributed antenna systems [33; 34] and relay systems [35; 36], but these cannot be directly applied to C-RAN. The application of rateless code as channel code in C-RAN has not been discussed until recently [37; 38; 39]. In [37], we considered Raptor code for the multi-user downlink of C-RAN, where both the precoder and the degree profile of Raptor code at the BBU pool are jointly optimized under an additive white Gaussian noise (AWGN) channel. In [38; 39], a rateless coded transmission scheme was designed for single-user uplink C-RAN, and the degree profile of the user was optimized under both AWGN and a block fading channel.