Beamforming

What Is Beamforming?

Beamforming is a technique that concentrates radio waves in precise directions toward wireless client devices, or STAs, resulting in a substantial enhancement of their received signal strength indicators (RSSIs). This improvement in RSSIs leads to an increase in the data transmission speeds for those stations. This technology was incorporated into the realm of Wi-Fi, known also as WiFi, starting with the Wi-Fi 4 (802.11n) standard.

Why Is Beamforming Necessary for Wi-Fi?

Beamforming has become a critical enhancement for Wi-Fi performance, geared at propelling data transmission speeds forward. Since the advent of Wi-Fi 4 (802.11n), which introduced both MIMO (multiple-input multiple-output) and beamforming technologies, transmission rates have skyrocketed to several hundred megabits per second and beyond.

MIMO achieves higher transmission rates through the use of multiple antennas. However, most STAs typically possess only one or two antennas, leading to discrepancies in the gains received and transmitted signals. Transmissions from an STA to an Access Point (AP) can benefit from the AP's multiple antennas, resulting in stronger received signals. However, the opposite isn’t true; without leveraging the full capacity of its antennas, the AP wouldn't make the most of potential gains when transmitting to an STA. Beamforming resolves this by allowing APs and STAs to negotiate better transmission rates via amplified RSSIs for STAs.

To maximize the use of AP multi-antenna capabilities, Wi-Fi 5 (802.11ac) introduced multi-user MIMO (MU-MIMO) technology, which enables an AP to communicate with several STAs simultaneously, thereby enhancing wireless transmission efficiency. MU-MIMO relies on beamforming to ensure that signals from the AP’s multiple antennas are constructively combined, so each STA receives only its intended signals, avoiding cross-STAs interference. Moreover, the Wi-Fi 6 (802.11ax) standard expanded the MU-MIMO capabilities, a feat made feasible with the implementation of beamforming.

How Does Beamforming Work?

Beamforming operates on a principle analogous to how beams of light intersect and interact. Just as layers of light from multiple flashlights can be combined to create a brighter and differently shaped composite beam, the same concept applies to radio signals in wireless communications.

In the context of multiple antennas, each acting like a flashlight emitting a beam, the configuration of their emissions can influence the form of the collective radio beam. When the multiple signal beams originating from these antennas converge at a given point, occasionally a 'spatial hole' might form if two signals with the same signal strength arrive with opposite phase shifts, effectively canceling each other out. Beamforming intelligently adjusts the phase of the signals from each transmitting antenna, enabling these beams to constructively overlap, enhancing their combined effect.

To fine-tune beamforming, parameters are derived by analyzing the channel state information (CSI), which offers insight into the transmission environment. Depending on how CSI is gathered, beamforming can be executed in one of two modes: explicit or implicit. Explicit beamforming involves direct communication of CSI from the receiver to the transmitter, while implicit beamforming relies on the transmitter deducing this information indirectly.

Explicit Beamforming

Explicit beamforming hinges on the reception of CSI feedback from a client device, commonly referred to as a station (STA). The process of explicit beamforming consists of CSI sounding and corresponding feedback, as outlined below:

1. An Access Point (AP) initiates the process by dispatching a sounding signal, also known as a training symbol, toward the STA. Under the 802.11n standard, this sounding can be carried out via null data packets (NDPs) or staggered preambles. However, the subsequent 802.11ac standard mandates the exclusive use of NDPs for this purpose.

• NDPs are essentially empty frames that contain no user data. The AP first transmits an NDP Announcement frame followed by the NDP itself. After the STA receives these two frames, it proceeds to provide its CSI feedback to the AP.

• Staggered preambles, utilized in 802.11n, embed a signaling channel within payload frames, which also transport a MAC frame, to facilitate the beamforming procedure.

2. The client device (STA) relays its CSI feedback back to the Access Point (AP). There are several feedback modes available in 802.11n, such as:

• CSI mode: The STA transmits the raw CSI data directly back to the AP, upon which the AP computes the optimal beamforming weights.

• Non-compressed beamforming weight: When the STA acquires the sounding signals, it calculates a beamforming weight and sends the resultant computation back to the AP. While effective, this mode does add to the overall system overhead, leading to the alternative of compressed feedback.

• Compressed beamforming weight: The STA still performs the calculation of the beamforming weight but applies certain measures to condense the feedback information, aiming to minimize system overhead.

In the evolution to the 802.11ac standard, the STA's role is to deliver CSI in the form of compressed beamforming weight feedback.

3. Armed with the weight data from the STA, or derived from its computations, the AP executes beamforming adjustments. This process aligns the multipath signals optimally towards the STA, creating signal gains and thereby enhancing the received signal quality.

Implicit Beamforming

Implicit beamforming is designed to simplify the process by not requiring client devices, known as stations (STAs), to send back channel state information (CSI) feedback. This approach takes advantage of the channel reciprocity in time-division duplex (TDD) systems, which assumes that the characteristics of the radio channel are symmetrical in both the uplink and downlink directions within the same frequency bandwidth. Consequently, CSI obtained from the uplink can be directly used to inform beamforming on the downlink path.

However, starting from the 802.11ac standard, implicit beamforming has not been supported due to its inherent complexity and limitations. The following illustrates the process that was utilized in the 802.11n standard:

In contrast to explicit beamforming, where STAs provide precise CSI feedback, implicit beamforming operates under the presumption that uplink CSI will mirror downlink CSI, even though the actual CSI for the downlink may differ. In reality, the use of uplink CSI for downlink beamforming can encounter issues as the uplink and downlink channels can exhibit dissimilar characteristics.

1. The AP begins the calibration by sending out a Calibration Start frame to the STA, which also serves as a Sounding Start frame.

2.Upon receipt of the Calibration Start frame, the STA responds with a Calibration Response frame, which includes a Sounding Response. This aids the AP in its estimation of the uplink channel.

3. Utilizing the STA's response, the AP performs its estimation of the uplink channel and then transmits a calibration frame back to the STA. This frame, which encompasses both Sounding information and a request for CSI Feedback, prompts the STA to analyze the downlink CSI and subsequently provide feedback to the AP regarding these channel conditions.