# Power-Efficient Random Access Design for Machine Type Communication
## 2 LTE Random Access  
There are two forms of LTE random access procedure, contention-based and contention-free. For the initial access, contention-based random access procedure is the sole choice. For the handover or new downlink data transmission, the base station has the option to allocate a dedicated signature to a UE, resulting in contention-free access. In this paper, we focus on the contention-based random access procedure.  
As shown in Figure 1, the contention-based random access procedure in traditional LTE/LTE-eMTC system can be divided into four steps [20]. Firstly, a preamble is randomly selected by the device from a pool of Zadoff-Chu (ZC) sequences and sent to the base station (we define it as Tx 1). The preambles from different devices are code-division-multiplexed. The preambles are distinguished by the ZC sequence root or the cyclic shift of a ZC sequence. Moreover, the preambles are divided into two groups by the base station. The device's geometry and data packet size will influence the preamble selection. The device will monitor the random access response (RAR) dedicated to itself in a given time window after its preamble transmitted (Rx 1). The RAR contains a 16-bit temporary ID which is valid in further communication after the device wins the contention.  
If the corresponding RAR is not detected, the device will reselect a new preamble and transmit with a higher transmit power under the restriction of maximum transmit power. The minimum latency for the new preamble transmission after the end of the RAR window is 3 ms [20]. If the corresponding RAR is detected, the device will send a 40-bit contention resolution key (e.g., the device's unique ID in the network) to the base station to solve the potential collision in the preamble selection (Tx 2). Note that the transmit timing of contention resolution key has been adjusted according to the 11-bit timing advance command in RAR so that the data channel which is equipped with small cyclic prefix (CP) can be used. In addition, hybrid automatic repeat request (HARQ) is available in this step [20].  
The base station attempts to decode the contention resolution key from the access devices. Then the device will receive the contention resolution response from the base station which contains the successfully decoded contention resolution keys (Rx 2). A failure to receive the contention resolution response is caused by the preamble selection collision. Therefore, the device has to repeat the random access procedure from the very beginning. After the device wins the contention, it can send its uplink schedule request to the base station (TX 3).  
We can see that the random access contains a sequence of message exchanges both in the LTE and LTE-eMTC system even in the NB-IoT. The complex access process will present excessive overhead for MTC (considering the MTC application data per wakeup are typically small), which is not surprising since the LTE random access is not optimized for MTC but for HTC. It is now clear that there are two major issues in the random access of LTE-eMTC for applications in MTC: (1) excessive overhead that may incur unnecessary battery and resource consumption; and (2) non-scalable preamble bandwidth (fixed 1.08 MHz) that prevents its application in 200 kHz re-farmed GSM bands.