Mechanism Design

AP Operation

AP can learn the type and the number of traffic flows every STA intends to transmit/receive during the contention-free periods (CFP) after the join/leaving handshaking. It manages the polling list, computes the appropriate WTS from the built-in database for each STA, and periodically announces EE-Multipoll frame after beacon or at scheduled time in every scheduled service interval (SI). The SI represents the interval between two successive multi-poll frames. To meet the QoS requirement of real-time traffic streams, the SI can be chosen such that every packet is served before its deadline. The guideline for SI selection can be found in [2] and [35].

The suggested frame format of EE-Multipoll is shown in Fig. 5.2. Each STA in the

Octets: 2 2 6 1 6 × RecordCount 4

Figure 5.2: Suggested frame format of EE-Multipoll.

polling list has a corresponding poll record for its possible uplink traffic and another record for its downlink traffic, if AP has data buffered for the STA. The number of poll records is indicated in the Record Count field. The Poll Record field contains the information of the association identifier (AID) in the BSS, the assigned backoff value, the wake-up time (in units of slot time) relative to the receiving time of this EE-Multipoll frame, and the maximal usable duration of an aggregate TXOP for a specified STA (in units of 32µs [2]). The backoff filed is filled with the all-1s value for downlink usage. As for the uplink phase, the backoff value assignment should make sure that no two STAs have the same backoff value at any time to avoid collisions. To reduce overhead, the backoff value (in units of slot time) bt_j of STA with access order j in the uplink phase follows the rule: bt₁ = 0 and bt_j = bt_j−1+ 1 for

Figure 5.3: The framework of the proposed EE-Multipoll mechanism.

Fig. 5.3 depicts the framework of the proposed mechanism. Multipoll-capable STAs should remain awake to check the multipoll frame and update their NAVs. The AP should first serve the downlink traffic which can be exactly scheduled. Then it replies the uplink traffic with ACKs during the uplink phase. Since a polled STA can perform physical carrier

sensing to determine the channel condition after it wakes up and collision can be avoided by the initial backoff assignment, the backoff value should be reduced more effectively for the busy case if the order of the ongoing transmitting STA can be learned. Hence, when AP replies ACKs to STAs, it should contain the order of the ongoing transmitting STA in the QoS Control field of the MAC header. After CFP, the remaining time of an SI will become contention period.

Scheduled STA Operation

Scheduled STAs should periodically awake to check the EE-Multipoll frame and achieve synchronization by listening to the beacon frames. They should keep awake to listen to the notification of EE-Multipoll frame and cannot fall back to sleep until any explicit information about their new wake-up time are successfully decoded. The announced information such as SI, backoff value, wake-up time and TXOP limit should be obeyed. To save energy, STAs not listed in the EE-Multipoll frame should enter the Doze state.

For an STA with downlink traffic, it should awake at the notified wake-up time to receive the buffered data and then back to sleep after its TXOP. As for the STA with uplink traffic, the assigned backoff value implies its access order during the uplink phase. When an STA wakes up at the assigned wake-up time, its backoff value count down process begins after the channel is sensed idle for an SIFS period. When the channel is busy, it sets the NAV and checks the access order of the ongoing STA from the overheard information. The obtained ongoing STA’s order can be utilized to adjust the possessed backoff value according to the difference between the STA’s access order and the obtained one. This adjustment keeps the possessed backoff values of awake STAs being unique and reduces the overhead. In case an STA cannot overhear a whole frame to identify the order of the ongoing STA or the medium is idle when the STA wakes up, it just keeps the original backoff value. Finally, when backoff value reaches zero, the STA transmits an uplink data frame to initiate its TXOP. When the frame exchanges are finished, the STAs can enter the Doze state till the transmission time of

the next multipoll frame. Note that it is possible that there is no uplink frame for a polled STA in some polling round and it will remain in the Doze state till the next multipoll frame.

Uplink Phase

Figure 5.4: Illustration of EEMP operations.

Error Recovery Issue

In a real environment, there could be transmission error and thus error recovery should be taken into account. For our scheme, the EE-Multipoll frame should be delivered at base rate to reduce error probability. Moreover, the first STA listed in the multipoll frame should take the responsibility of confirming the successful delivery of the multipoll frame. It should reply an ACK frame (or piggyback the confirmation on the uplink frame) when it receives the multipoll frame. In case the multipoll frame contains error and the first STA does not response as expected, the AP should retransmit the multipoll frame. To avoid failure of the first STA, the AP can change the order to let each STA take turn to be the first one. As for data frames, the immediate retransmission policy, i.e., retransmitting a data frame which is not correctly acknowledged after SIFS, can be adopted for error recovery.