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Introduction to 802.11ax High-Efﬁciency Wireless Updated Mar 19, 2019 Overview 802.11ax, also called High-Efficiency Wireless (HEW), has the challenging goal of improving the average throughput per user by a factor of at least 4X in dense user environments. This new standard focuses on implementing mechanisms to serve more users a consistent and reliable stream of data (average throughput) in the presence of many other users. This paper will explore the new mechanisms that will give the popular 802.11ax standard the title of High-Efficiency Wireless. View the WLAN Suite Learn more about 802.11ax Table of Contents 1. Introduction 2. Key Features and Applications 3. Current Challenges to Wi-Fi Throughput in Dense Environments 4. PHY Mechanisms for High Efficiency 5. MAC Mechanisms for High Efficiency 6. 802.11ax Test Challenges 7. Conclusion 8. About NI and the 802.11ax standard 9. References 10. Discover more Improving User Throughput in Dense User Environments Introduction In 2015 the iconic car manufacturer, Ferrari, released a new version of its entry-level model: the Ferrari California T. This sleek sports car has a 3.9 liter turbocharged V8 engine capable of generating more than 412 KW (553 Horsepower), good for smashing zero to 100 Km/h (0 to 62 mph) in 3.6 seconds. Keeping the foot on the accelerator will propel this engineering marvel to its top speed of 315 Km/h (196 mph). [1] Back to top The Ferrari designers considered many details of the engine, body and interior to make this vehicle a daily driver, while delivering the most precise handling, fluid motion, and performance at breakneck speeds. That kind of design would certainly make for an exhilarating – though significantly shorter – daily commute to the office. However, what good would that red Ferrari convertible be on the heavily congested streets of a large metropolitan area, with mostly stop-and-go traffic? Today many people find themselves in that kind of situation. Perhaps not as privileged to be driving an Italian sports car, but able to enjoy blazing fast wireless connectivity links. Consider that the first 802.11b Wi-Fi standard (1999), had a top link speed of 11 Mbps. A good first step, but significantly slower than a wired connection. Then a few years later the 802.11a/g revision (2003) increased the speed to 54 Mbps with the introduction of Orthogonal Frequency Division Multiplexing (OFDM) technology. The next link speed improvement came with 802.11n (2009) presenting users with single stream links up to 150 Mbps. The 802.11ac revision of the standard (2013) brought with it the possibility of link speeds around 866 Mbps on a single spatial stream with wider channels (160MHz) and higher modulation orders (256-QAM). Using the specified maximum number of 8 spatial streams, this engineering marvel would, in theory, reach its top speed of 6.97 Gbps. In theory, using 802.11ac is the equivalent of replacing your bicycle or even your family sedan with a souped-up Ferrari.

However, speeds approaching 7 Gbps might only be achievable in the controlled race-track environment of the RF lab. In reality, users commonly experience frustratingly slow data traffic when trying to check their email on a public Wi-Fi at a busy airport terminal. A new revision of the IEEE 802.11 wireless LAN standard – 802.11ax – seeks to remedy exactly this precise situation. 802.11ax, also called High-Efficiency Wireless (HEW), has the challenging goal of improving the average throughput per user by a factor of at least 4X in dense user environments. Looking beyond the raw link speeds of 802.11ac, this new standard implements several mechanisms to serve more users consistent and reliable data throughput in crowded wireless environments. Key Features and Applications High-Efficiency Wireless includes the following key features: Back to top Backwards compatible with 802.11a/b/g/n/ac Increase 4X the average throughput per user in high-density scenarios, such as train stations, airports and stadiums. Data rates and channel widths similar to 802.11ac, with the exception of new Modulation and Coding Sets (MCS 10 and 11) with 1024-QAM. Specified for downlink and uplink multi-user operation by means of MU-MIMO and Orthogonal Frequency Division Multiple Access (OFDMA) technology. Larger OFDM FFT sizes (4x larger), narrower subcarrier spacing (4X closer), and longer symbol time (4X) for improved robustness and performance in multipath fading environments and outdoors. Improved traffic flow and channel access Better power management for longer battery life High-Efficiency Wireless also serves the following target applications: Cellular data offloading: By 2020, 38.1 exabytes Wi-Fi offload traffic will be generated each month, continuing to exceed projected monthly mobile/cellular traffic (30.6 exabytes). [2] That’s equivalent to moving more than 6000 Blue-ray movies per minute on these networks. Environments with many access points and a high-concentration of users with heterogeneous devices (Airport Wi-Fi ≠ Home Wi- Fi) Outoors/outdoors mixed environments.

Figure 1. Example scenario of a stadium with high user density and mixed environments targeted for 802.11ax deployment Current Challenges to Wi-Fi Throughput in Dense Environments The 802.11 protocol uses a carrier sense multiple access (CSMA) method in which the wireless stations (STA) first sense the channel and attempt to avoid collisions by transmitting only when they sense the channel to be idle. That is, when they don’t detect any 802.11 signals. When an STA hears another one, it waits for a random amount of time for that STA to stop transmitting before listening again for the channel to be free. When they’re able to transmit, STAs transmit their whole packet data. Back to top Wi-Fi STAs may use Request to Send/Clear to Send (RTS/CTS) to mediate access to the shared medium. The Access Point (AP) only issues a CTS packet to one STA at a time, which in turn sends its entire frame to the AP. The STA then waits for an acknowledgement packet (ACK) from the AP indicating that it received the packet correctly. If the STA doesn’t get the ACK in time, it assumes the packet collided with some other transmission, moving the STA into a period of binary exponential backoff. It will try to access the medium and re-transmit its packet after the backoff counter expires.

Figure 2. Clear Channel Assessment Protocol Although this Clear Channel Assessment and Collision Avoidance protocol serves well to divide the channel somewhat equally among all participants within the collision domain, its efficiency decreases when the number of participants grows very large. Another factor that contributes to network inefficiency is having many APs with overlapping areas of service. Figure 3 depicts a user (User 1) that belongs to the Basic Service Set (BSS, a set of wireless clients associated to an AP) on the left. User 1 would contend for access to the medium with other users in its own BSS and then exchange data with its AP. However, this user would still be able to hear traffic from the overlapping BSS on the right. Figure 3. Medium access inefficiency from overlapping BSS In this case, traffic from the OBSS would trigger User 1’s backoff procedure. This kind of situation results in users having to wait longer for their turn to transmit, effectively lowering their average data throughput.

A third factor to consider is the shared use of wider channels. For example, for 802.11ac operation in North America there is only one 160 MHz channel available, and in Europe only two. Figure 4. Example 802.11ax channel allocation on the 5GHz band Planning dense coverage with a reduced number of channels becomes very difficult, forcing network managers to reuse channels in nearby cells. Without careful and deliberate power management, users will experience co-channel interference, which degrades performance and negates much of the expected gain from the wider channels. This is especially true for the top data rates of MCS 8, 9, 10, and 11, which are much more susceptible to low signal to noise ratio. Also, on the current implementation of 802.11 networks a 20 MHz channel overlapping an 80 MHz channel will basically render the 80 MHz channel useless, while a user transmits on the narrower channel. Implementing 802.11ac’s Channel Sharing in a high density network compromises the gains of the 80 MHz channel for transmissions on a 20 MHz channel. PHY Mechanisms for High Efﬁciency Back to top PHY Changes The 802.11ax specification introduces significant changes to the physical layer of the standard. However, it maintains backward compatibility with 802.11a/b/g/n and /ac devices, such that an 802.11ax STA can send and receive data to legacy STAs. These legacy clients will also be able to demodulate and decode 802.11ax packet headers – though not whole 802.11ax packets – and backoff when an 802.11ax STA is transmitting. The following table highlights the most important changes to this revision of the standard, in contrast to the current 802.11ac implementation: 802.11ac 802.11ax

BANDS CHANNEL BANDWIDTH FFT SIZES SUBCARRIER SPACING OFDM SYMBOL DURATION HIGHEST MODULATION DATA RATES 5 GHz 20 MHz, 40 MHz, 80 MHz, 80+80 MHz & 160 MHz 64, 128, 256, 512 2.4 GHz and 5 GHz 20 MHz, 40 MHz, 80 MHz, 80+80 MHz & 160 MHz 256, 512, 1024, 2048 312.5 kHz 78.125 kHz 3.2 us + 0.8/0.4 us CP 12.8 us + 0.8/1.6/3.2 us CP 256-QAM 1024-QAM 433 Mbps (80 MHz, 1 SS) 6933 Mbps (160 MHz, 8 SS) 600.4 Mbps (80 MHz, 1 SS) 9607.8 Mbps (160 MHz, 8 SS) Table 1. 802.11ac vs. 802.11ax Notice that the 802.11ax standard will operate in both the 2.4 GHz and 5 GHz bands. The specification defines a four times larger FFT, multiplying the number of subcarriers. However, one critical change with 802.11ax is that the subcarrier spacing has been reduced to one fourth the subcarriers spacing of previous 802.11 revisions, preserving the existing channel bandwidths. Figure 5. Narrower sub-carrier spacing

The OFDM symbol duration and cyclic prefix also increased 4X, keeping the raw link data rate the same as 802.11ac, but improving efficiency and robustness in indoor/outdoor and mixed environments. Nevertheless, the standard does specify 1024-QAM and smaller cyclic prefix ratios for indoor environment, which will increase the maximum data rate. Beamforming 802.11ax will employ an explicit beamforming procedure, similar to that of 802.11ac. Under this procedure, the beamformer initiates a channel sounding procedure with a Null Data Packet. The beamformee measures the channel and responds with a beamforming feedback frame, containing a compressed feedback matrix. The beamformer uses this information to compute the channel matrix, H. The beamformer can then use this channel matrix to focus the RF energy toward each user. Multi-User Operation: MU-MIMO and OFDMA The 802.11ax standard has two modes of operation: Single User: in this sequential mode the wireless STAs send and receive data one at a time once they secure access to the medium, as this paper has described above. Multi-User: this mode allows for simultaneous operation of multiple non-AP STAs. The standard divides this mode further into Downlink and Uplink Multi-user. Downlink multi-user refers to data that the AP serves to multiple associated wireless STAs at the same time. The existing 802.11ac standard already specifies this feature. Uplink multi-user involves simultaneous transmission of data from multiple STAs to the AP. This is new functionality of the 802.11ax standard, which did not exist in any of the previous versions of the Wi-Fi standard. Under the Multi-User mode of operation, the standard also specifies two different ways of multiplexing more users within a certain area: Multi-User MIMO and Orthogonal Frequency Division Multiple Access (OFDMA). For both of these methods, the AP acts as the central controller of all aspects of multi-user operation, similar to how an LTE cellular base station controls the multiplexing of many users. An 802.11ax AP can also combine MU-MIMO with OFDMA operation. Multi-User MIMO Borrowing from the 802.11ac implementation, 802.11ax devices will use beamforming techniques to direct packets simultaneously to spatially diverse users. That is, the AP will calculate a channel matrix for each user and steer simultaneous beams to different users, each beam containing specific packets for its target user. 802.11ax supports sending up to eight multi-user MIMO transmissions at a time, up from four for 802.11ac. Also, each MU-MIMO transmission may have its own Modulation and Coding Set (MCS) and a different number of spatial streams. By way of analogy, when using MU-MIMO spatial multiplexing, the AP could be compared to an Ethernet switch that reduces the collision domain from a large computer network to a single port. As a new feature in the MU-MIMO Uplink direction, the AP will initiate a simultaneous uplink transmission from each of the STAs by means of a trigger frame. When the multiple users respond in unison with their own packets, the AP applies the channel matrix to the received beams and separates the information that each uplink beam contains. The AP may also initiate Uplink multi-user transmissions to receive beamforming feedback information from all participating STAs as shown in Figure 7.

Figure 6. AP using MU-MIMO beamforming to serve multiple users located in spatially diverse positions Figure 7. A beamformer (AP) requesting channel information for MU-MIMO operation Multi-User OFDMA

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