# Power-Efficient Random Access Design for Machine Type Communication
## 3 LTE-nMTC Random Access Design
### Simplified Random Access Signaling  
The preamble in LTE random access does not contain any kind of device identifier, thereby causing ambiguities among accessing devices. Then, the contention resolution message is used to solve the problem. As noted before, the objectives for the MTC are very different from these of the traditional mobile broadband LTE in that MTC is mainly for the delivery of relatively small datagrams with low cost devices, high energy efficiency (long battery life), and relaxed latency requirements. This is precisely in contrast to the mobile broadband LTE where the emphasis is on high data rates, low latency, and support of streaming data services. That is, only a small amount of application data needs to be transmitted per access. The overhead of the current LTE access method thus becomes significant, and may become a bottleneck in terms of battery life, considering that signaling takes much longer time to close the MTC link than in the legacy mobile broadband LTE due to the reduced capability of an MTC device and the need for coverage extension (we will see more detailed discussions in this regard in Section 4. Simplification of the random access procedure can potentially save device power as a result of the elimination of the preamble and the contention resolution from the random access procedure. There are two goals of the random access preamble, one is to indicate the presence of the device and the other is to provide a means of estimating the round-trip propagation delay of the device at the receiver. The information exchange between the base station and the device follows the preamble is to solve the potential collisions. We use a special orthogonal frequency division multiplexing (OFDM) signal to accomplish these goals. This idea is the extension of our previous work [21]. For ease of comprehension, we first briefly review some of our previous work in this subsection. Figure 2 shows the proposed random access signal and the corresponding arrive time at the base station. Guard Time (GT) is used to prevent the random access signal from distorting thefollowing data channel due to the round-trip propagation delay. Our proposed random access signal is a modulated signal that contains the random access message \(M\) just like the signal in the data channel. Apparently, random access signals from different devices arrive at the receiver at different time due to the round-trip propagation delay. So, we can use a long CP (e.g., \(T_{\rm CP}=35\,\upmu\)s for a cell size of 5 km) to absorb the round-trip delay. It makes the signal in the DFT window is a full ODFM symbol without the inter-symbol-interference (ISI) and the inter-carrier-interference (ICI) [20]. After the system acquisition in the downlink, the frequency offset is corrected. The residual frequency offset is small and has less of an effect on decoding an OFDM symbol. Besides this, our proposed random access signal can achieve a good PAPR by using single-carrier frequency division multiple access (SC-FDMA) just like the data channel. It is obvious that long CP will increase the overhead of an OFDM symbol, but it will be compensated by using a longer OFDM symbol (i.e., smaller subcarrier spacing) as will be seen later. We assume that the random access signal occupies \(s\) subcarriers in frequency domain and \(N_{\rm OFDM}\) OFDM symbols in time domain, the bandwidth for RACH channel is \(W\) which consists of \(S\) subcarriers. Then, the number of random access channels is \(N_{\rm RACH}=S/s\). Apparently, the subcarrier spacing is \(w_{0}=W/S\) and the transmission bandwidth of the access signal is \(w=w_{0}s=\frac{Ws}{S}\). The message \(M\) contains a 10-bit access ID, 4-bit resource request, and 10-bit CRC, resulting in a message that has \(|M|=24\) information bits (more details can be found in [21]). The device sends message \(M\) on one of \(N_{\rm RACH}\) random access channels which is randomly selected.