Mobile users have continued to demand more data and greater availability of wireless connectivity networks since the early days of the first Wi-Fi devices. Subsequently, the IEEE 802.11 wireless networking standard has consistently evolved and adapted to meet this growing need.
Around 1999, the 802.11b implementation provided wireless links at about 11 Mbps using direct sequence spread spectrum (DSSS). In 2003 the 802.11a/g revision increased the link speed and wireless performance by adopting orthogonal frequency division multiplexing (OFDM). This implementation offered users data rates of up to 54 Mbps, a big improvement that spurred wider market adoption. The next performance jump came with 802.11n (2009), presenting users with single stream links up to 150 Mbps. The 802.11ac revision (2013), brought with it the possibility of link speeds around 866 Mbps on a single spatial stream (SS) with wider channels (80 and 160 MHz) and higher modulation orders (256-QAM). Using the specified maximum number of eight spatial streams, 802.11ac users would, in theory, benefit from link speeds of 6.97 Gbps, surpassing the data rates of wired Ethernet connections.
If that technology is already in place, then why do users commonly experience frustratingly slow data traffic when connected to a public Wi-Fi network at a busy train station or sports arena? Although there are several factors that affect signal quality and data rates, the way current access points (AP) and stations (STA) deal with overcrowded networks commonly cause the data flow to slow to a crawl.
A new revision of the IEEE 802.11 wireless LAN standard—802.11ax—seeks to remedy this situation. 802.11ax, also called high efficiency wireless (HEW), seeks to improve the average throughput per user by a factor of at least 4× in dense user environments. Looking beyond the raw link speeds of 802.11ac, this new standard implements several mechanisms to serve a consistent and reliable data throughput to more users in crowded places. This includes mixed-environment locations with many access points and a high concentration of users with different kinds of connectivity devices.
CHALLENGES IN DENSE ENVIRONMENTS
The 802.11 protocol uses a carrier sense multiple access (CSMA) method, where STAs first sense the channel and attempt to avoid collisions by transmitting only when they do not detect any 802.11 signals. When an STA hears another one, it waits for a random amount of time for the other STA to stop transmitting before listening again for the channel to be free. When able to transmit, STAs transmit their whole packet data.
Wi-Fi STAs may use “request to send” (RTS) or “clear to send” (CTS) packets to mediate access to the shared medium. The AP only issues a CTS packet to one STA at a time, which, in turn, sends its entire frame to the AP (see Figure 1). The STA then waits for an acknowledgment packet (ACK) from the AP, indicating that it received the packet correctly. If the STA does not get the ACK in time, it assumes the packet collided with some other transmission, moving the STA into a back off period. It will then try to access the medium and retransmit its packet after the backoff counter expires.
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 2 depicts a user (user 1) that belongs to the basic service set (BSS), a set of wireless clients associated with an AP, on the left. User 1 contends for access to the medium with other users in its own BSS (e.g., user 2) and then exchanges data with its AP. However, this user hears traffic from the overlapping BSS on the right. In this case, traffic from the overlapping BSS triggers user 1’s backoff procedure. This results in users waiting longer for their turns to transmit, effectively lowering their average data throughput.
PHY MECHANISMS FOR HIGH EFFICIENCY
The 802.11ax 0.1 draft specification1 introduces significant changes to the physical layer of the standard. However, it maintains backward compatibility with 802.11a/b/g/n/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, although not entire 802.11ax packets, and back off when an 802.11ax STA is transmitting. Table 1 highlights the most important parameters compared to the current 802.11ac implementation. Notice that the 802.11ax standard will operate in both the 2.4 and 5 GHz bands. The specification defines a 4× larger FFT, multiplying the number of subcarriers. However, one critical change with 802.11ax is reducing the subcarrier spacing to one fourth the spacing of previous 802.11 revisions, preserving the existing channel bandwidths (see Figure 3). The OFDM symbol duration and cyclic prefix (CP) also increased 4×, keeping the raw link data rate the same as 802.11ac while improving efficiency and robustness in indoor, outdoor and mixed environments. Nevertheless, the standard does specify 1024-QAM and smaller CP ratios for indoor environments, which will increase the maximum data rate.
To increase the ability to serve more users at the same time, the 802.11ax standard specifies two modes of operation: single user, a sequential mode where the wireless STAs send and receive data one at a time, and multi-user, a mode that allows for simultaneous communication with multiple STAs. The standard further divides this mode into downlink and uplink multi-user. Downlink multi-user refers to data that the AP serves to multiple associated STAs at the same time, which the existing 802.11ac standard already specifies. Uplink multi-user involves simultaneous transmission of data from multiple STAs to the AP, a new feature that the 802.11ax standard proposes, which did not exist in any of the previous 802.11 versions. 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. The 802.11ax designers have specified that 802.11ax support downlink and uplink MU-MIMO, MU-OFDMA or both to handle an even larger number of simultaneous users.
Borrowing from the 802.11ac implementation, 802.11ax devices will use beamforming techniques to direct packets simultaneously to spatially diverse users (see Figure 4). 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, and up to four of these streams to a single user. Also, each MU-MIMO transmission may have its own modulation and coding set (MCS) and a different number of spatial streams.
For MU-MIMO uplink, the AP will initiate a simultaneous transmission from each of the STAs to the AP 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 to separate the information from each uplink beam. The AP may also initiate uplink multi-user transmissions to receive beam forming feedback information from all participating STAs (see Figure 5).
The 802.11ax standard also uses OFDMA to multiplex more users in the same channel bandwidth. Building on the existing OFDM digital modulation scheme that 802.11ac already uses, the 802.11ax standard further assigns specific sets of subcarriers to individual users. That is, it divides the existing 802.11 channels (20, 40, 80 and 160 MHz wide) into smaller subchannels with a predefined number of subcarriers. The 802.11ax standard calls the smallest subchannel a resource unit (RU), with a minimum size of 26 subcarriers. Aware of multi-user traffic needs, the AP decides how to allocate the channel, always assigning all available RUs on the downlink. It may allocate the whole channel to only one user at a time, just as 802.11ac currently does, or it may partition the channel to serve multiple users simultaneously (see Figure 6). In dense user environments, where many users would normally contend inefficiently for a turn to use the channel, this OFDMA mechanism serves them simultaneously, with smaller, dedicated subchannels that improve the average throughput per user. Figure 7 illustrates how an 802.11ax system may multiplex the channel using different RU sizes. Note that the smallest division of the channel accommodates up to nine users for every 20 MHz of bandwidth.2 Table 2 shows the number of users able to get frequency-multiplexed access when the 802.11ax AP and STAs coordinate for MU-OFDMA operation.xz
To coordinate uplink MU-MIMO or uplink OFDMA transmissions, the AP sends a trigger frame to all users. This frame indicates the number of spatial streams and/or the OFDMA allocations (frequency and RU sizes) of each user. It also contains power control information, such that individual users can increase or reduce their transmitted power. This helps to equalize the power that the AP receives from all uplink users and improve reception of frames from users that are farther away. The AP also instructs all users when to start and stop transmitting. As Figure 8 depicts, the AP sends a multi-user uplink trigger frame that tells all users the exact moment at which they are to start transmitting and the duration of their frame, to ensure that they all finish transmitting simultaneously. Once the AP receives the frames from all users, it sends back a block ACK to finish the operation.
802.11ax TEST CHALLENGES
The 802.11ax standard specifies support for higher modulation orders (1024-QAM), in which more information is carried in ever-smaller differences in signal amplitude and phase. Considering that the subcarriers stand only 78.125 kHz apart, 802.11ax devices need cleaner oscillators with improved phase noise and RF front-ends with better linearity, to minimize spectral leakage, reduce bit error rates and achieve the high link speeds that the standard demands. The error vector magnitude (EVM) test provides designers with valuable information about the quality of the modulated signal. One of the biggest test challenges is to measure the EVM of new 802.11ax devices. The EVM noise floor of the test instruments must be significantly lower than the DUT’s, pushing current instrument designs to new levels of linearity and noise performance. Table 3 shows the EVM levels that 802.11ax-compliant devices will likely have to meet.
A second challenge is that OFDMA systems have very high susceptibility to frequency and clock offsets. Consequently, 802.11ax MU-OFDMA performance demands tight frequency synchronization and clock offset correction to ensure that all STAs operate exactly within their allocated subchannels. Additionally, the strict timing requirements guarantee that all STAs will transmit simultaneously in response to the AP’s multi-user trigger frames. 802.11ax APs will have to maintain system synchronization using their own built-in oscillators as the reference. Associated STAs will then adjust their internal clock and frequency references by extracting clock information from the trigger frames transmitted from the AP. Frequency and clock offset testing of 802.11ax devices will involve both absolute and relative frequency error tests. For the latter, the goal is to determine the ability of a non-AP STA participating in uplink multi-user transmission to correct its oscillator based on the frequency and clock information it derives from the AP’s trigger frame. The test methodology includes two steps: first, the test instrument acting as a reference (as the AP would be) sends trigger frames to the DUT. The DUT adjusts its clock to the reference and replies to the test instrument. As a second step, the test instrument measures the adjusted frequency of the DUT (see Figure 9).
Testing the receiver sensitivity of 802.11ax APs presents an additional challenge, considering that the AP acts as the clock and frequency reference. The AP (the DUT) initiates the test by sending a trigger frame. The test instrument adjusts its frequency and clock to match the DUT’s and then responds with a predetermined number of stimulus packets. The challenge here is with the strict relative frequency error limits of 802.11ax. The test instrument must derive very precise frequency and clock information from the trigger frames that the AP DUT sends, and it may be necessary to perform the calculation over multiple trigger frames to ensure proper frequency and clock synchronization. As a result, this method can add a significant delay to the test. One possible solution to speed the test is for the AP to export its clock reference, allowing the test equipment to lock its clock to it. This setup avoids the initial synchronization procedure based on trigger frames, leading to faster AP receiver sensitivity tests.
Another consideration arises when testing 802.11ax devices with up to eight antennas in MIMO operation. In this case, a DUT can produce very different results than when testing each signal chain individually and sequentially. For example, the signals from each antenna may interfere destructively with each other and affect power and EVM performance, with negative and potentially noticeable effects on the throughput. The test instrumentation must support sub-nanosecond synchronization of the local oscillators for each signal chain to ensure proper phase alignment and MIMO performance over many channels.
802.11ax aims to improve the average per-user data throughput by 4× in crowded environments. This new revision of the standard seeks to implement significant improvements to the physical and medium access layers to enable higher network efficiency. Thanks to multi-user technology, both in the form of MU-MIMO and MU-OFDMA, 802.11ax will have the ability to serve data simultaneously and consistently to multiple users in crowded environments. However, implementing this functionality will present a whole new set of challenges for the scientists, engineers and technologists developing and testing the 802.11ax multi-user ecosystem.
- IEEE 802.11-16/0024r1, Proposed TGax draft specification.
- IEEE 802.11-15/0132r16, Specification Framework Document.