Validating 802.11ac Wave 2 MU-MIMO Performance Gain

The IEEE 802.11ac standard was introduced in two phases: Wave 1 and Wave 2.  The IEEE ratified 802.11ac Wave 2 as a wireless local area network (WLAN) standard in November 2013.  As the industry begins to adopt the Wave 2 products for either carrier and enterprise deployment or consumer home applications, testing 802.11ac Wave 2 becomes increasingly critical for equipment manufacturers and service providers who want to tap into the opportunities presented by today’s connected world. The relevant stakeholders who invested in IEEE 802.11ac Wave 2 products are now eager to know what exactly do these Wave 2 enhancements do, and how do we prove the Wave 2 performance gain.

The 802.11ac Wave 2 is a superset of 802.11ac Wave 1. As such, it supports all the data rates and features of 802.11ac Wave 1, and it comes with the addition of support for the features like downstream MU-MIMO, 160 MHz-wide channels, and up to 8 spatial streams. The Wave 2 gains a significant performance boost in the downlink from an access point (AP) to device clients than the Wave 1 with a feature called multiuser Multiple input, multiple output (MU-MIMO). This means that the spectrum can be more efficiently used with multiple connected clients being able to receive a transmission from an AP simultaneously.  Simply speaking, the Wave 1 supports a single user multiple input, multiple output (SU-MIMO) that sends a transmission only to a single client at any time specifically in the downstream direction. MU-MIMO can transmit to up to 4 Wave 2 clients simultaneously in downstream so that the performance can be improved by 4 times maximally, as shown in Figure 1.

Single and multi-user MIMO

Figure 1: IEEE 802.11ac Wave 1 SU-MIMO vs Wave 2 MU-MIMO

The performance advantage of MU-MIMO comes from its ability to share the entire available wireless frequency spectrum across multiple client devices. Although the Wave 2 MU-MIMO does not change the maximum possible PHY rate in each WLAN connection, it improves the aggregated throughput that each AP can support with an environment with multiple clients sharing the same AP. WLAN is a contention-based protocol and, by transmitting to multiple clients simultaneously, clients are on and off the network fast enough to allow the wireless spectrum to be used by other clients.  While most WLAN networks incorporate different client types from 802.11 a/n/ac on 5GHz band, the efficiency of MU-MIMO provides performance not only to the Wave 2 clients but also results in added performance to the legacy clients with 802.11 a/n/ac modes.

MU-MIMO works by taking advantage of beamforming to send frames to spatially diverse locations simultaneously. Therefore, beamforming in the downlink direction from the AP to the client was a challenge area for innovation in the 802.11ac standard. A beamforming process between an AP and a client can be simply described as that the AP is sending higher-level data such as IP packets to the client such as a laptop as the recipient in Figure 2. The channel calibration part of a beamforming process is also called sounding. Since 802.11ac beamforming is based on explicit channel measurements, both the transmitter and the receiver must support it. The sounding happens before any data traffic so it is also an overhead. The shorter the time a sounding takes, the better the MU-MIMO performance gets. Therefore, sounding timing can be an interesting parameter to pay attention to. Moreover, a MU-MIMO group refers to the clients that are receiving the different downstream transmissions at a time with the beamforming technology.

Wave2AP client

Figure 2: 802.11ac Beamforming Process

Spirent TestCenter WLAN offers multiple hardware platforms that combine both the Wave 2 WLAN interface cards or modules and new 2.5Gbps/5Gbps Base-T Ethernet solutions. These solutions support the highest performing and most realistic multi-client emulation for direct functionality and performance testing of Access Points (APs). More specifically, it can enable a tester to perform the following general AP test scenarios:

  • AP Personal and Enterprise security (802.1x) type
  • Medium capacity and maximum client loading
  • AP stability testing
  • AP interwork with various mixes of different 802.11 mode clients
  • Benchmark or baseline for UDP and TCP throughput, forwarding rate, and latency performance
  • RFC-style testing with clients across APs through the RF interface
  • Throughput vs packet size, throughput vs client numbers, IPv4, IPv6, DHCP scale and performance, etc.
  • Rate vs range testing

Wave 2 specific features supported are:

  • Validate 802.11ac 1.73Gbps PHY rate with 4x4/80MHz channels or 2x2/160MHz channels
  • Test above 1Gbps WiFi throughput with 4x4 MIMO
  • Validate AP’s Wave-2 MU-MIMO capability in downlink
  • Provide various flexible MU-MIMO client configurations for testing
  • Test the throughput (UDP/TCP) improvement with MU-MIMO clients
  • AP MU-MIMO stability and efficiency testing
  • AP MU-MIMO sounding timing testing
  • MU-MIMO rate vs range testing

For a more detailed discussion on how to test 802.11ac Wave2 features with a focus on MU-MIMO read the white paper: Understanding how to Test 80211ac Wave 2 MU-MIMO.

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