In part one of this series, we explored the evolving 802.11 Wi-Fi standard and its support for various radio bands (2.4 GHz, 5 GHz, and 6 GHz), as well as peak PHY rates and spectrum utilization. In this blog post, we’ll be taking a closer look at MU-MIMO, OFDMA, and 1024-QAM.
One important metric for the 802.11 Wi-Fi standard remains the maximum number of single user streams. The original standard offered only a single transmit chain and a single receive chain that supported one stream of data. In sharp contrast, the upgraded Wi-Fi 4 (802.11n) standard offered four radio chains, effectively bolstering throughput and efficiency by supporting the transmission of four parallel streams to the same device. Subsequently, Wi-Fi 5 (802.11ac) provided support for up to 8 streams.
Wi-Fi 6 & MU-MIMO
Although both Wi-Fi 5 (802.11ac) and Wi-Fi 6 (802.11ax) support transmitting 8 streams, it should be noted that client devices will have to offer support for this mechanism. More specifically, wireless access points (APs) support 8 streams because they are relatively large and connect to a dedicated power source. However, consumer client devices are typically small and battery-operated. As such, Wi-Fi manufacturers don’t typically build 8 chain clients. In fact, clients usually have only one or two chains. So, even though an AP supports 8 streams, the average client device is likely only capable of transmitting and receiving two streams.
This is precisely why the IEEE introduced the Multi-User MIMO (MU-MIMO) mechanism, which divides 8 streams into four groups of two devices and transmits them to consumer devices such as smartphones. Indeed, Wi-Fi 5 (802.11ac) allows the AP to talk to four devices at the same time, while Wi-Fi 6 (802.11ax) extends this capability to 8 devices. Moreover, Wi-Fi 6 (802.11ax) supports MU-MIMO in both the upload and downlink directions, while the early Wi-Fi 5 (802.11ac) standard only supported MU-MIMO in the download direction.
OFDMA & 1024-QAM
Wi-Fi 6 (802.11ax) also introduces a new modulation scheme known as orthogonal frequency-division multiple access (OFDMA). Although this mechanism is new to Wi-Fi, it has been thoroughly vetted in LTE deployments, much like many other wireless technologies. In addition, Wi-Fi 6 (802.11ax) adds a modulation scheme of 1024-QAM which allows Wi-Fi to achieve higher data rates. More specifically, the maximum number of OFDM tones is increased from 64 in Wi-Fi 4 (802.11n) to 2,048 on the 160 MHz channel in Wi-Fi 6 (802.11ax). It should be noted that ‘tones’ and subcarriers are used interchangeably. For example, a 20 MHz OFDMA channel consists of a total of 256 subcarriers or ‘tones.’ Moreover, subcarrier spacing has been reduced by 4x from 312.5 kHz to 78.125 kHz. The narrower subcarrier spacing optimizes equalization and enhances channel robustness required for outdoor operation.