Uplink Access Protocol In IEEE 802.11AC
IEEE 802.11ac amendment enhances WLANthroughput by exploiting spatial diversity of antennas in MUMIMO downlink transmission. Still, network resources remainunder-utilized in uplink transmission due to single user communication. This project proposes an Access Point-controlledMAC protocol (A-MAC) that enables simultaneous transmissions from multiple STAs in uplink. The protocol uses EDCAchannel access technique to initiate multi-user transmission and OFDMA method to transmit multiple RTSs simultaneously. Italso introduces explicit channel sounding technique by usingdedicated OFDM subcarrier blocks for each user. Performancemeasurement shows that network throughput of the A-MAC is150% higher than that of a single uplink transmission, thanksto the availability of concurrent multiple RTS transmissions inthe uplink.
IEEE 802.11 protocol is continuously evolving to keep pacewith the growing need for high speed broadband multimediacommunication. IEEE 802.11ac is a recent amendment thatsupports point-to-multipoint communication in the downlink(DL) using pioneering multi-input multi-output (MIMO) technique. DL-MIMOincreases the network throughput by allowing transmission to maximum four STAs simultaneouslyusing spatial multiplexing. The protocol still uses one-to-onecommunication in the uplink direction which keeps the abundant resources in AP under-utilized. In fact, the throughputof WLANs scales linearly with the multi packet reception(MPR) capability of the channel.
The technical challengesof multiple concurrent uplink transmissions require the APreceiver to perform per-user channel estimation, and carrierfrequency offset estimation due to RF mismatches betweenSTAs and AP. Without precise channel state information(CSI), space diversity through beamforming technique cannotbe utilized in the uplink due to overlapping of multipletransmitting signals at AP.
In IEEE802.11ac protocol, channel state information is obtained bythe training bits in the preamble. However, to determinechannel state effectively, preamble transmission must be ina clear channel state which means that, during multiple uplinktransmission, interference-free preamble detection cannot beguaranteed. Therefore, an explicit channel sounding techniqueis required for uplink MU-MIMO transmission.
Zhou and Z. Niu, “An Uplink Medium Access Protocol withSDMA Support for Multiple-Antenna WLANs,” in Proc. of IEEEWireless Communications and Networking Conference. pp. 1809–1814,Mar. 2008.
The MAC protocol proposed requires a majorchange in the IEEE 802.11 protocol. The inclusion of multipleRTS/CTS during random access period increases the overheadburden to the protocol and, consequently, reduces networkthroughput and efficiency.
Tandai, H. Mori, K. Toshimitsu, and T. Kobayashi, “An efficientuplink multiuser MIMO protocol in IEEE 802.11 WLANs,” in Proc.of IEEE 20th International Symposium on Personal, Indoor and MobileRadio Communications. pp. 1153–1157, Sept. 2009
An uplink MMSE detection based MU-MIMO protocol isproposed for the IEEE 802.11 WLAN. In this protocolSTAs use OFDMA technique to transmit access request to APand TDMA technique to transmit pilot signals.
An EDCAbased uplink transmission technique is proposed for non-HE devices thatallows multi-user transmission to improve spectral efficiencyin a coexistent WLAN network. An Access Pointcontrolled contentionbased MAC protocol (A-MAC) andexplicit channel sounding technique is proposed that allow multi-user concurrent uplink transmission within IEEE 802.11ac frameworkkeeping compatibility with the downlink MU-MIMO technique. The proposed protocol is a step towards implementingthe desired throughput enhancement using concurrent multipleuplink transmissions and MIMO-OFDMA techniques. Themajor contributions are:
- Introduce explicit channel sounding technique byusing dedicated subcarrier blocks.
Develop an analytical model using queuing model andMarkov chain model to evaluate the performance of theproposed A-MAC.
- Evaluate the performance metrics of proposed AMAC protocol for bothuniformly and non-uniformlyvarying packet arrival rates for different priority categories.
- Develop the stable operating criterion for the networkfor varying traffic conditions.
- The proposed protocol provides a green solution byreducing the backoff time and thereby increasing theefficiency of the network.
- Shortens the backoff timeby up to 50% for all traffic categories.
- Enhances battery life of thenodes.
- The smaller backoff windowof high priority traffic category enhances network throughput,higher intensity of high priority traffic drives the network fasterto saturation.
- Better network stability and fairnessamong different traffic categories can be achieved when thedominant traffic has low priority.
The frame exchange sequence of the proposed protocol medium becomes idle, all STAs start backoff process. When STA1 wins the contention,STA1 sends RTS. Since the network does not have anyhidden terminals and all STAs hear each other, the AP andSTAs have the knowledge about the collision of first roundRTS. If the collision happens, the primary STA initiates nextbackoff phase. On hearing RTS, secondary STAs suppress the backoff counting and update the NAV according to theduration information in RTS. Receiving STAs (including AP)decode RTS signal to find the intended destination address.
Ifthe RTS is intended for AP and AP decides to allow multiuser transmission, AP defers the transmission of G-CTS upto DIFS period. If AP cannot allow multiple transmission simultaneously, it sends CTS after SIFS period as shown in Fig3. If primary STA listens CTS after SIFS period, the primary STA starts data transmission after SIFS period and other STAskeep the backoff countdown suppressed until the medium isidle again. However, if no CTS is transmitted by AP to primary STA after SIFS period, other STAs are allowed to transmitconcurrent RTSs using previously allocated OFDM subcarrierblocks. It is fair to assume that AP is connected to the infrastructure and has no capability constrain in monitoring each subcarrier block to retrieve all secondary RTSs. Depending on the number of STAs, it is possible that AP receives larger number of RTSs than the number of antennas Mant in AP.
Tool : MATLAB 8.1
Platform : Windows/linux
Scripting Language : C / Matlab coding
Zulfiker Ali and Jelena,” Uplink Access Protocol in IEEE 802.11ac” IEEE Transactions on Wireless Communications, 2018