## 3 Interference Mitigation Schemes
### MAC Approach
#### 3.2.6 Asynchronous Inter-Network Interference Avoidance  
In [30], a hybrid multiple-access algorithm of CSMA/CA and TDMA is used in a new MAC algorithm, asynchronous inter-network interference avoidance (AIIA). AIIA is time-independent synchronous algorithm to avoid interference by listening to the controlling message from other WBASNs then updating its own schedule.  
The superframe structure of AIIA is shown in Figure 4 and includes the contention-based regions (CR) and the scheduled regions (SR). The differences between AIIA superframe and IEEE 802.15.6 superframe [1] are: (1) The CR can correspond to the EAP, RAP, and CAP periods in IEEE 802.15.6,  
Figure 3: **(a)** An example of interference among WBASNs; and **(b)** channel assignment in DRA.  
where CSMA/CA is used for resource allocation; (2) the SR can be splitted into multiple periods in AIIA that are similar to the MAP periods in IEEE 802.15.6, and TDMA is used in SR.  
For each WBASN, the gateway device or coordinator runs AIIA to maintain two components: overlapping WBASN detection (OWD) and scheduled region relocation (SRR). The sequential operations of these components are described as follows.  
In the first operation, OWD is repeated periodically at the gateway to maintain the AIIA table. The scheduled region advertisement (SR_AD) message, which includes the AIIA table entries and the clock timer information, is created at every gateway to exchange information with the others. When many WBASNs are working at the same operating channel, the gateway of each WBASN broadcasts its SR_AD message before the start time of its SR period and listens to the SR_AD messages from the others. The gateway then constructs the AIIA table, which includes six fields. The first field is the gateway identifier (_GWID_) field to store the received device identifier. The second field is the _hop-count_ field to indicate the number of hops from the gateway to a neighboring gateway. The next field is the _superframe_offset_ to store the time difference between the GW's timer and a neighboring GW's timer. The _TdmaStarttime_ and the _TdmaDuration_ fields indicate the start time and the duration of SR period, respectively. The last field, _SeqNo_, indicates the latest version of the corresponding entries.  
In the second operation, SRR, the gateway reschedules its conflicting SR to another location within the CR period of the other WBASNs. SRR consists of three phases. Phase I is the \"searching for an idle period in CR\" phase in which the gateway searches for the CR periods by referring to its own AIIA table, remembers the start and end times of the CR, then relocates the conflicting SR period and transmits the SR_AD message. In phase II, the gateway finds the relocate-possible (RP) periods amongst the CRs of other WBASNs and assigns the indices to RP and remembers the location based on the index of the CR. In phase III, the gateway determines the new location of the SR period and the transmission time of the SR_AD message and, then the changed SR schedule is broadcasted to the other gateways of its neighbor WBASNs. An example of the SRR operation is shown in Figure 5.  
Figure 4: Superframe structure in AIIA.  
Figure 5: An example of the SRR operation.