## 2 Related Work  
Many studies have been conducted on the coexistence of heterogeneous wireless communication systems. The coexistence of WiFi and Bluetooth systems was dealt in [1-3], that of WiFi and ZigBee systems was discussed in [4,5]. [PERSON] [1] proposed two solutions for the coexistence of WiFi and Bluetooth systems. One involves WiFi STAs estimating the interference pattern, and transmitting a frame of the minimum payload size, 500 bytes, comparable to the duration of a single slot Bluetooth packet. This may reduce the probability of collisions between the two systems. However, it is hard to estimate the interference pattern. The second deals with frequency hopping, a suggested packet length and restricts the length of packets. Thus, the allowable data rate decreased. [PERSON] [2] introduced a backoff strategy and adaptive frequency hopping (AFH). This also reduced the collision probability of the two systems. The backoff strategy does not require changes to the Bluetooth specification. However, changing the frequency hopping pattern requires some changes to the Bluetooth specification. The benefits of AFH may not be as obvious for delay jitter and packet loss constrained applications, \begin{table}
\begin{tabular}{|c|c|c|c|} \hline \multirow{2}{*}{} & & \multicolumn{2}{c|}{**Collaboration**} \\ \cline{3-4}  & & **Not collaborative** & **Collaborative** \\ \hline \multirow{2}{*}{**Collocation**} & **Not collocated** & \multirow{2}{*}{Out of scope} & Figure 1(a) \\ \cline{2-2} \cline{4-4}  & **Collocated** & & Figure 1(b) \\ \hline \end{tabular}
\end{table} 
Table 1: Types of system for coexistence. Figure 1: Cont. such as voice or video services. [PERSON] [3] studied a dynamic WiFi system fragmentation technique. A WiFi station (STA) divides one frame into several fragments, and sends them. The WiFi STA should be acknowledged fragment by fragment. If the transmission of a specific fragment fails, the WiFi STA only retransmits the failed fragment. This decreases the allowable data rate, because it reduces the length of the collided fragment. [PERSON] [4] evaluated the effects of the mutual interference, and quantity coexistence issues. [PERSON] [5] introduced several coexistence models between ZigBee and WiFi systems, and evaluated their interference effects. References [6-11] deal with the coexistence of WiFi and WiMAX systems. [PERSON] [6] proposed a solution based on a circulator to reduce the mutual interference between them. It showed a width of the overlapped frequency band to tolerate the mutual interference. Even though both systems can endure a certain amount of mutual interference, it does not avoid but rather mitigates the mutual interference. [PERSON] [7,8] and [PERSON] [9] proposed solutions based on time division multiplexing (TDM) of WiMAX systems based on IEEE802.16 and WiFi systems based on IEEE802.11a/e. They improved spectral efficiency. However, no protocol controls the transmission of WiFi STAs. Even if a certain period is allocated to the WiFi system, a WiFi STA may send frames beyond the period. This can cause the mutual interference. [PERSON] [10] studied the coexistence between WiMAX systems based on IEEE802.16a and WiFi systems based on IEEE802.11b. A WiFi access pointer (AP) senses WiFi channels and selects a channel with the minimum received signal strength indicator (RSSI). The WiFi AP broadcasts the selected WiFi channel, and all of WiFi STAs move to it. However, if an interferer is not located within the range of a WiFi AP, the WiFi AP cannot recognize it. [PERSON] [11] also introduced a cognitive radio (CR) method. Each node announces information, such as frequency, power, modulation, duration, interference margin, and service type, through the common spectrum coordination channel. The method can reduce the influence of the mutual interference, but it needs much overhead. Two mechanisms termed Listen-Before-Talk (LBT) and Extended Quiet Period (EQP) are adopted to perform white space detection efficiently based on IEEE 802.16h [12].