Understanding Spatial Multiplexing Power Save (SMPS) Method for Wi-Fi Power Management

Understanding Spatial Multiplexing Power Save (SMPS) Method for Wi-Fi Power Management

By Ankur Jain On 04/27/2023

Power Save Mode is an important feature for end users to save their battery power and use the device for extended hours.

Power Save Mode is an important feature for end users to save their battery power and use the device for extended hours.

In order to reduce battery drain, IEEE has come over different power save mechanisms. Many mechanisms that are described in the IEEE 802.11 standard allow a wireless device to reduce its power consumption, to turn off its radios, and to wake up at the correct time to retrieve its traffic. While the APs are generally connected to an external power source, the wireless clients are often running on batteries. The purpose of power-saving features is to increase the battery life and to allow longer performance. This battery life extension can be significant for low-powered devices such as smartphones, Voice over IP phones, or handheld barcode scanners.

Several techniques for power save have evolved over the years since Wi-Fi's inception:

1. Legacy Power Save
2. WMM-Power Save
3. Spatial Multiplexing Power Save (SMPS) Method
4. TXOP Power Save
5. TWT (Target wake time)

Spatial Multiplexing Power Save (SMPS) Method

• This is also known as MIMO PS Method, which is related to MIMO transmission.
• As we know, from 11n onwards, devices can support multiple radio chains on transmit and receive sides.
• Due to multiple chains, multiple streams can be transmitted and received, and hence overall performance is increased however, it comes at the cost of battery power since it will consume higher current.
• So there is a trade-off between Battery Power and Performance, and if someone gives priority to power, then the need for power save arises.

SMPS – How it Works

SMPS is a process that allows an HT client which is capable of receiving multiple streams to operate with only one active receive chain. Other receive chains are disabled, and so Power Save is achieved.

SMPS Methods

1. Static SM Power Save
2. Dynamic SM Power Save

The client may invoke any method depending on the support and need. And the client may intimate to AP during the initial Association or anytime after the association.

1. Static SM Power Save

• This method allows the client to disable the receive of MIMO rates completely, and only one receiver chain will be active on the client.
• When the client informs the AP (either at association time or anytime after using the MIMO power save HT Action management frame) that it is in static MIMO power save mode, the AP must not transmit any frames at MIMO rates to that particular STA. Because only one receiver chain is active, the client will not understand MIMO data (transmissions higher than MCS7 will be MIMO transmissions).
• Once the STA informs the AP that it has come out of static MIMO power save, the AP may resume sending frames to that STA at MIMO rates.

2. Dynamic SM Power Save

• In this method, the client is allowed to use one receiver chain or multiple receive chains dynamically based on the AP's RTS packet.
• In other words, it allows the STA to disable the receive of MIMO rates without protection and enable receiving of MIMO rates with protection.
• When the STA informs the AP (either at association time or anytime after that using the MIMO power save HT Action management frame) that it is in dynamic MIMO power save mode, then the AP must not transmit any frames at MIMO rates without a preceding RTS/CTS to that particular STA.
• The AP may send frames at MIMO rates after it sends an RTS frame at one of the non-MIMO rates. The STA, which implements dynamic MIMO power save, is required to turn on any inactive receive chain when it receives an RTS frame in order to be able to receive a MIMO transmission.

Client Requirements to Support SMPS

1. Trigger to enter MIMO power save:

The trigger to enter MIMO power save can be based on a user configuration through the GUI or an automatic algorithm which decides when it makes sense to enter MIMO power save. For the user configuration, a knob needs to be exposed which allows the user to choose between the following

• Disable MIMO power save
• Enable dynamic MIMO power save
• Enable static MIMO power save

2. Inform the AP:

When the user (or the algorithm) triggers any change in dynamic/static MIMO power save, the client needs to do the following

• If it is already associated, generate a MIMO power save HT Action management frame to communicate the new state to the AP
• If not currently associated, save the changed state and communicate it during the association in association requests

2.1. Inform AP during Association:

• A client may use the SM Power Save subfield of the HT Capability Information field in the HT Capabilities element of the (Re)Association Request frame to change the SM Power Save mode.
• This method enables only one single receive chain after (Re)Association.
• The 2 bits of the SM Power Save subfield indicate the SM Power Save mode that is in operation immediately after (Re)Association and a capability.

Set to 0: for Static SM Power Save mode
set to 1: for Dynamic SM Power Save mode
set to 3: when SM Power Save is disabled or not supported.

2.2. Inform AP After Association:

• A client may also use an SM Power Save Action frame (HT Action frame) to manage the SM Power Save transition.
• The SM Power Save Enabled subfield is set to 1 when SM power saving is enabled at the client.
• The SM Mode subfield is set to 1 for Dynamic SM Power Save and set to 0 for Static SM Power Save mode.
• The change is validated after the frame has been successfully acknowledged.

Sniffer Capture (Association Request):

client using the association request to update the SM power save state during association to AP.

Sniffer Capture (Action Frame)

client using the action frame to update the SM power save state after association.

3. Local Actions:

• A client entering MIMO PS may change its PHY configuration to keep only one receive chain active.
• It should also disable the use of MIMO rates for its own transmissions.
• When the STA comes out of MIMO PS, it should turn on all the receive chains and can resume the use of MIMO rates for transmissions

AP Requirements to Support SMPS

1. Maintain per-STA state

The AP needs to track whether each associated client is in dynamic/static MIMO PS or not. It needs to update the state every time it receives an association frame or a MIMO PS HT Action Management frame from any STA. It needs to process the MIMO PS HT Action management frames for this.

2. Honoring Static MIMO PS

If a client is in static MIMO PS, the AP must not send any frame at MIMO rates to that STA. This implies disabling all MIMO rates from the rate adaptation table for that STA.

3. Honoring dynamic MIMO PS

If a client is in dynamic MIMO PS, the AP must precede all transmissions at MIMO rates to that STA by an RTS/CTS exchange at a legacy rate.

References:

Understanding WLAN Power Save Mode by Prasanna Chamala https://alethea.in/wlan-power-save-mode/#:~:text=In%20order%20to%20reduce%20the,sleep%20states%20to%20conserve%20energy.

802.11 Power Management with packet capture examples by dot11zen blogs On February 21, 2018 https://dot11zen.blogspot.com/2018/02/80211-power-management-with-packet.html

IEEE Standard 802.11-2016: https://standards.ieee.org/findstds/standard/802.11-2016.html

Wi-Fi Alliance WMM Specification v1.2: https://www.wi-fi.org/file/wmm-specification-v12

Tagged with: cwnp, smps, mimo, wi-fi power management, battery life

Blog Disclaimer: The opinions expressed within these blog posts are solely the author’s and do not reflect the opinions and beliefs of the Certitrek, CWNP or its affiliates.