INTRODUCTION ac is the next evolution of the Wi-Fi standard which promises to deliver high data rates and sustain significantly higher throughput and lower latency than existing standards. It is also named as “Gigabit Wi-Fi” as it can deliver maximum data rate of 6.93 Gbps in 160MHz bandwidth mode. Wireless speed is generally the product of three factors: - channel bandwidth - Modulation techniques - number of spatial streams
PROBLEM The main concept of introducing ac is to improve the data rates and reduce the latency. HOW IS THIS ACHIEVED ??
SOLUTIONS Mandatory 5GHz operation Wider Bandwidth Higher Order Modulation Higher Order MIMO Multi User MIMO (MU-MIMO) RTS/CTS with Bandwidth Indication All A-MPDU’s Backwards Compatibility
Mandatory 5GHz operation: ac standard mandates operation only in 5Ghz band as it has relatively reduced interference and more number of non-overlapping channels available compared to 2.4ghz band. Wider bandwidth: Wider bandwidth allows higher data rates to be achieved. ac introduces 80Mhz and 160Mhz channel bandwidths in addition to 20Mhz and 40Mhz in n Higher Order Modulation: ac increased the constellation configuration to 256-QAM which increases data rate by 33% over 11n. Each symbol represents 8 coded bits. Higher Order MIMO: The speed is directly proportional to the number of spatial streams. STA can receive up to eight spatial streams to effectively double the total network throughput.
ENABLING MULTIPLE DATA STREAMS VIA DOWNLINK MIMO ac is the first Wi-Fi standard that introduced multi-user MIMO. In MU-MIMO, the AP can serve multiple STAs simultaneously. AP is able to use its antenna resources to transmit multiple frames to different clients, all at the same time and over the same frequency spectrum. AP STA
MU-MIMO In multi-user mode, the ac amendment supports up to four streams serving four different users simultaneously. Standard also specifies support for a different modulation and coding rate for each station being served in a downlink MU-MIMO transmission. The AP has to know Channel state information of all the users in order to decrease the amount of inter-user interference generated by the multiple simultaneous streams. Through Pre-processing of data streams at the transmitter, the interference from streams that are not intended for a particular station is eliminated at the receiver of each STA. So, every STA receives data free from interference. MU-MIMO Uses Combination of Beam forming and Null Steering to Multiple Clients in Parallel. ac specifies a single compressed beam forming method that relies on the use of explicit feedback to implement MU-MIMO
Transmit Beam forming beam forming allows a station to transmit multiple simultaneous data streams to a single, or multiple users. Beam forming is directly enabled by the support of “sounding” which is a process performed by the transmitter to acquire CSI from each of the different users by sending training symbols and waiting for the receivers to provide explicit feedback containing a measure of the channel. This feedback is then used to create a steering matrix that will be used to pre-code the data transmission by creating a set of steered beams to optimize reception at one or multiple receivers.
Protocol Description: The beamformer transmits a VHT Null Data Packet (NDP) Announcement frame that contains the addresses of the AP and the set of beamformees. After SIFS, the AP transmits a VHT NDP frame in order to sound the channel. Based on the NDP frame, the station will prepare the information that will be carried by the beamforming report. The targeted beamformees are required to reply with a VHT Compressed Beamforming Frame. The first intended station replies immediately whereas the others have to wait to be polled by the AP. In this way AP serves a set of users by forming various beams each transmitting a different data stream.
Primary and secondary sub-channels: Similar to n, channels consisting of 40 MHz or wider always require a primary 20 MHz wide sub-channel. Additionally, 80 MHz channels have a primary 40 MHz (which includes the primary 20 MHz) sub-channel and a secondary 40 MHz sub-channel. The primary sub-channel is used for carrier sensing in order to guarantee no other device is transmitting. The presence of the 20 MHz primary sub-channel is also necessary to guarantee coexistence and backward compatibility with legacy devices. Only the primary sub-channel performs full Clear Channel Assessment (CCA). Clear Channel Assessment (CCA) : The clear channel assessment tests the ability of the 11ac device to determine if a channel is free or occupied. If occupied, the PHY indicates this by setting a CCA indication signal field to “busy.”
Static and Dynamic Channel Access: An ac device in order to transmit an 80MHz PPDU - Primary channel must follow EDCA rules. - Secondary sub-channels must be IDLE for a duration of PIFS after back-off. In the case that any of the secondary sub-channels is busy, the station can follow either static or dynamic channel access rules. Static channel access: It is the legacy approach of accessing a medium. Here if any of secondary sub-channel is busy the STA will choose a random back-off period within the current CW size to restart the process and attempt only until no interferer is present in any of the sub-channels. Dynamic channel access: Here The ac station may attempt to transmit over a narrower channel using 20 or 40 MHz instead. - More efficient resource allocation because the station can still transmit over a fraction of the original bandwidth
RTS/CTS Mechanism Enhanced: To overcome the problem of collision between ac devices and Legacy AP’s, a Handshake protocol is introduced to handle both static and dynamic channel allocation. This handshake consists of a modified RTS/CTS mechanism that provides information about the current amount of available bandwidth.
Mechanism Consider the scenario where an initiating AP wants to transmit data to an associated client through an 80 MHz channel. The AP first checks if the channel is idle. If it is, then it transmits multiple RTS in the a PPDU format. Every nearby device receives an RTS on its primary channel. Each of these devices then sets its NAV. Client checks if any of the sub-channels in the 80 MHz band is busy before replying with a CTS The client only replies with a CTS on those sub-channels that are idle. On the other hand, if in a nearby AP is already transmitting before the initiating AP starts, i.e., channel is not idle Then, The client will inform the AP by replying with a CTS signals only on the idle sub channels which improves Overall Bandwidth Utilization
Frame Aggregation: All A-MPDU’s At the MAC, the standard specifies the use of different frame aggregation schemes, and capability negotiations to indicate channel width. In particular, it proposes mandatory use of frame aggregation via A-MPDU (Aggregate-MAC Protocol Data Unit), which was introduced in n. A-MPDUs are enhanced in ac by increasing their size thus packing several MPDUs within a single PPDU. This in turn increases channel utilization and MAC efficiency. Backward Compatibility : It is required to be fully compatible with n and a. ac only applies to 5 GHz band because there are no 80 MHz and 160 MHz channels available in the 2.4 GHz band. ac standard enables coexistence with n/a devices by requiring a backwards compatible preamble that has a section which is understandable by n/a devices. This would allow legacy devices to operate as intended.
CONCLUSION: ac is the future of wireless LANs, but Wi-Fi-certified ac APs are not yet available ac can provide full HD video at range to multiple users, higher client density, greater QoS, and higher power savings from getting on and off the network that much more quickly. IT administrators looking to invest in wireless LANs in the near term should strongly consider n APs that are field upgradable to ac.