Introduction

This post explains the basics of recent evolution of new WiFi standard 802.11ax, which is marketed by WiFi Alliance as WiFi 6.

WiFi 6 is named to notify that it is 6th generation WiFi technology, similar to the 4G and 5G in mobile GSM standard. The prior 802.11 standards, 802.11ac was named as WiFi 5 (5th generation), 802.11n was named as WiFi 4.

What is the need for new protocol - 802.11ax ?

In the past 20 years of continuous evolution of WiFi technology, many new standard protocols were developed, which were primarily targeted for higher peak speeds. But 802.11ax is the first protocol of its kind, that it is going to address most of the current challenges being faced by real world WiFi usage. One of the main challenge is to provide reliable & efficient WiFi connectivity for dense usage environments with ever increasing number of WiFi devices and diverse mix of traffic requirements.

802.11ax is mainly targeted to provide overall WiFi network performance improvements along with the speed, so it is named as "High Efficiency" (HE), as opposed to prior protocols' name 802.11n HT (High Throughput) or 802.11ac VHT (Very High Throughput)

What are the key features of 802.11ax ?

1. Support for new modulation scheme OFDMA (which is widely used in cellular standard) for both uplink and downlink traffic

2. Support for MU-MIMO (Multi User MIMO) for both uplink and downlink traffic

3. BSS Coloring

4. Target Wakeup Time (TWT)

5. Support for 1024-QAM (Quadrature Amplitude Modulation) and longer OFDM symbol (12.8 usec)

1. MU-OFDMA (Orthogonal frequency-division multiple access)

OFDMA is multi user version of OFDM (orthogonal frequency-division multiplexing) digital modulation scheme. This is the most important enhancement of 802.11ax for multi user traffic.

Think of data traffic between AP and Station as vehicles moving on a road between a source (STA) and destination (AP). In this example, road is a WiFi channel. Until 802.11ax, the road (channel) is one way, which means road can be used (accessed) only by one source at a time to send a vehicle (data). In 802.11ax's OFDMA, road (channel) is divided into multi lanes (RUs - Resource units) and it can be used to send multiple vehicles (data packets) at same time from different sources (stations) to single destination. Road lanes (RUs) are dynamic in allocation and can be of different sizes as well, based on the size of the vehicle.

OFDMA can be on both directions uplink (STA to AP) and downlink (AP to STA). For DL-OFDMA, AP allocates the sub channels (Resource units - RUs) to the stations based on the type of traffic scheduled and transmits the data simultaneously to multiple client devices. For UL-OFDMA also, AP allocates the RUs, based on the traffic statusat each client. All the clients notify the AP by sending BSR - Buffer status report frames, which assists the AP to perform the optimal RU allocation and stations will send the uplink data traffic in their allocated RUs.

In the earlier standards access to the channel was de-centralized and all the nodes (stations and AP) used to compete for the access of the channel, whereas in 802.11ax channel access is completely centralized and governed by AP. This systematic and optimized way of allocation of precious radio channel access, helps 802.11ax achieve the most important goals of greatly improved overall network throughput and deterministic quality of service assurance.

20MHz OFDMA channel consists of 256 subcarriers spaced at 78.2KHz and these subcarriers are grouped to form Resource Units (RUs). Combinations of grouping the subcarriers is defined by the 802.11ax standard. Table-1 showing allowed combinations for 20MHz, 40MHz, 80MHz and 160MHz channels. 20MHz channel can used to serve maximum of 9 clients at a time, whereas 40MHz channel can serve 18, 80MHz channel can serve 37 users and 160MHz channel can serve 74 users at a time.

Table-1 : RU - Resource Unit combinations and maximum number of stations supported with each channel width

2. MU-MIMO (Multi user MIMO)

In 802.11ax standard, MU-MIMO is supported in both up-link and down-link direction, whereas in 802.11ac standard it was supported only in down-link direction and not widely implemented. 802.11ax supports upto 8 clients for both UL-MU-MIMO and DL-MU-MIMO, whereas 802.11ac had support only for 4 clients in DL-MU-MIMO.

MU-MIMO allows transmission of multiple frames to be transmitted to different users at the same time on the same channel, using multiple antennas (spatial streams). MU-MIMO requires the clients to be spatially separated and AP uses sounding frames to identify the location of the client and form optimal groups for parallel transmission.

Main difference between MU-MIMO and MU-OFDMA is that MU-MIMO is suitable for low client density deployments with larger packets (larger bandwidth applications) and MU-OFDMA is suitable for high client deployments with smaller packets (low band width applications). MU-MIMO increases the efficiency and reduces latency, whereas MU-MIMO provides higher speed per user.

3. BSS Coloring

This is very interesting and useful feature for enterprise deployments, where multiple APs are deployed with channel reusing. Basic principle of WiFi channel access is CSMA/CA (Listen before Talk). If WiFi station listens energy above a threshold, due to overlapping neighbor network's transmission on same channel, current node will defer the access and wait for channel to become free. This overhead in medium contention is called co-channel interference (CCI).

To reduce CCI in enterprise AP deployments, BSS coloring allows each network to define color code - a number from 0 to 7, and include as part of PHY header. Whenever a station detects a frame on the medium as part of medium access, station will check for the color code and defer the access only if it is same color code. Otherwise station will proceed to use the medium. This way BSS coloring can reduce the CCI and improve the overall airtime utilization.

4. Target Wake Time (TWT)

Target Wake Time (TWT) is an enhancement to the existing power save mechanism. In the current standards, STA needs to wake up every DTIM interval and restrict the overall power savings. With TWT, AP allows the client to be in power save as long as possible and schedule the traffic as per the clients application requirement. This feature is most suitable for IoT devices, which generate very small data at larger time intervals.

5. 1024-QAM

1024-QAM (Quadrature Amplitude Modulation) is 802.11ax enhancement targeted for higher speed.

This new modulation scheme allows to modulate 10-bits per symbol, whereas 256-QAM in 802.11ac allows to modulate 8-bits per symbol. This increases 20% increase in data throughput. 1024-QAM can only be used with minimum of 242-subcarrier RUs or larger, which means at least a full 20 MHz channel will be needed for 1024-QAM. 802.11ax introduces two new MCSs: MCS-10 and MCS-11.

1024-QAM high speed transmissions are useful and effective only for low RF noise environments and shorter distances.

#11ax #WiFi6 #WiFi #HE #80211ax #OFDMA #MUMIMO #DLOFDMA #ULOFDMA #RU #ResourceUnit #TWT #TargetWakeupTime