# Collision Detection Method Using Self Interference Cancelation for Random Access Multiuser MIMO
## 1 Introduction  
Access points (APs) are being increasingly deployed in wireless local area networks (WLANs) as a means of fulfilling cellular system offload requirements [1]. Since the service area covered by an AP in a WLAN system is considered to be a _small cell_, many APs must be deployed in areas with heavy traffic. However, since individual APs have limited frequency channels, collisions will occur between user terminals (UTs) when many UTs attempt to connect with an AP. To avoid such collisions, wired LAN systems can implement an access control scheme called carrier sense multiple access/collision detection (CSMA/CD) in which, prior to packet transmission, collisions are detected from voltage variations occurring within the Ethernet cable [2]. As this method for wired LAN transmission involves packet detection in advance and immediate retransmission, its efficiency is over 90% [3]. As a result, the issues surrounding this method have been intensively studied [4; 5]. On the other hand, another access control scheme called CSMA/collision avoidance (CA) is adopted in wireless LAN [6]. Unlike wired LAN, it is difficult to detect the packet collision with wirelesstransmission. Therefore, the reception characteristics are judged from the reply of acknowledgement (ACK) sent by a receiving station. Compared to wired LAN, the transmission efficiency of wireless LAN is very small, and its value is less than 65%, because retransmission cannot be started until ACK is judged [3]. Therefore, collision detection, which is used in wired communication systems, is essential for achieving high transmission efficiency. There are two types of collision detection schemes. The first collision detection method involves single-user MIMO transmission [7; 8]. For 2 \(\times\) 2 MIMO, this method utilizes the fact that the second antenna is idle when the first antenna transmit short preamble signals, which are used for timing synchronization with the UT. The _self-interference_ between these two antennas at the AP can be canceled by using dual polarized antennas, which are generally used in MIMO systems [9; 10; 11]. In this paper, this idea is applied for multi-user MIMO systems. In the second method, an approximate CSMA/CD for WLANs is realized using a scheme called CSMA/collision notification (CN) [12]. In CSMA/CN, a receiver detects the interfering signal while receiving a packet and immediately notifies the transmitter, which utilizes two antennas: one for normal transmission and the other dedicated to listening for notifications. Upon detection of a notification signal, the transmitter aborts its transmission, freeing up the channel for other transmitters in the vicinity [12]. Since the transmitter must use an additional antenna to discriminate between its own transmit signal and the notification signal from the receiver, the CSMA/CN procedure utilizes a correlation calculation based on a prior (training) signal received at the transmitter. When the notification signal arrives during the transmission of the own transmit signal, another antenna judges whether collision is detected, because the correlation value between the received signal (ideally only notification signal plus the noise) and the known training signal becomes high after canceling the own transmit signal [12]. However, there are two issues in the CSMA/CN procedure. Firstly, it is difficult to detect collision in a real propagation environment using only the correlation value, because the correlation value with limited number of samples is actually very small as the power level of the notification signal is much lower than that of the own transmit signal at another antenna on the transmitter. Secondly, since the power of the own transmit signal is much higher than that of the notification signal from the receiver, the received power becomes saturated at the amplifier and it causes the noise power to increase; [12] provided there is no concrete countermeasure for this effect.