# CeRA-eSP: Code-Expanded Random Access to Enhance Success Probability of Massive MTC
## 1 Introduction  
With the emergence of 5G services, such as smart cities, smart grids, and smart homes/buildings, interest in Internet of Things (IoT) networks has increased significantly [1, 2, 3]. To ensure the connectivity of large-scale networks, the international telecommunication union radiocommunication sector (ITU-R) defined massive machine-type communications (mMTC) as one of the main use cases of 5G [4, 5]. The main features of mMTC are high connectivity of up to 1 million devices per square kilometer, small data volume, sporadicity, and delay tolerance [1, 6]. Because the characteristics of mMTC services are different to those of existing human-type communication (HTC)-based services, an mMTC-specialized study is required. A representative problem of mMTC is the preamble collision that occurs during the random access (RA) process. Preamble collisions occur when devices transmit the same preamble sequence during RA. In mMTC, massive devices transmit preamble sequences simultaneously, resulting in a higher collision rate in the preamble transmission phase. The access delay increases because most devices retransmit the preamble owing to the high collision rate. A high collision rate causes a decrease in the RA success rate. Therefore, the high collision rate problem is considered a representative problem of mMTC services [7, 8]. Extending the preamble resource is one approach to solving the high collision rate problem. Among the preamble extension schemes, a code-expanded random access (CeRA) scheme was proposed. The CeRA scheme operates based on a preamble codeword. Apreamble codeword refers to a longer preamble format composed of serial preamble sequences transmitted in consecutive RA slots. We assumed that the number of preamble sequences that can be transmitted in each random access occasion (RAO) is \(M\) and the number of preamble sequences constituting one codeword is \(L_{c}\). If the base station (BS) can recognize the preamble sequences for each RAO, the number of preamble codeword combinations is \(M^{L}\). The CeRA scheme is an innovative method that can exponentially increase preamble resources, considering that existing preamble extension methods linearly increase preamble resources [9; 10]. Because the preamble collision rate is significantly reduced as the number of selectable preamble codewords increases, the CeRA scheme can efficiently solve the high collision-rate problem. Furthermore, the CeRA scheme does not require significant changes in the preamble transmission and detection processes. To discuss the difference between the CeRA scheme and the existing RA scheme, it is necessary to know the preamble codeword transmission and identification processes. The preamble sequence transmitted in successive RA slots should be concatenated as long as the codeword length for the BS is able to successfully identify the preamble codeword. The CeRA scheme uses a virtual frame composed of consecutive RA slots for preamble codeword detection. The BS records the successfully transmitted preamble sequences in each RA slot that are included in the virtual frame. After the preamble transmission in the virtual frame is completed, the BS deduces the possible preamble codeword combinations based on the preamble sequence observations recorded for each RA slot. It can be observed that the preamble codeword reception process of the BS does not require any special change other than the transmission in units of virtual frames. With respect to the preamble transmitter, the devices transmit a random preamble sequence in the same manner as the conventional method. Because the receiver and transmitter do not require significant changes in the RA process, the CeRA scheme allows for the accommodation of the bursty RA load of the mMTC with minimal changes in the MAC layer [9]. Despite its various advantages, the CeRA scheme has unresolved problems. A representative problem is the codeword ambiguity problem that occurs during the deduction process of the preamble codeword.