RFWi-Fi 8 Frame Formats: UHR MU, TB and ELR PPDU Guide
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Introduction : The transition to Wi-Fi 8 (IEEE 802.11bn) introduces a new Physical Layer Protocol Data Unit (PPDU) architecture designed for Ultra High Reliability (UHR). While it maintains backward compatibility with legacy devices, the framing structure is optimized for multi user coordination and long range stability. Following are the three primary wi-fi 8 frame formats based on IEEE 802.11bn specifications.
UHR MU PPDU (Multi-User)
The UHR MU PPDU is primarily used for transmissions from an Access Point (AP) to one or multiple Stations (STAs). It contains the “Resource Map” (UHR-SIG) required for users to identify their data within the transmission.
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| Field Name | Description/Purpose |
|---|---|
| Legacy Fields (L-STF, L-LTF, L-SIG) | Ensures backward compatibility; allows legacy devices to detect the frame and avoid collisions. |
| RL-SIG | Repeated non-HT Signal field; used for auto detection of the high efficiency format. |
| U-SIG (Universal SIG) | Introduced in Wi-Fi 7; contains version identifiers (set to 1 for Wi-Fi 8) and general PHY info. |
| UHR-SIG | Specific to Wi-Fi 8 MU: Contains the resource allocation map and user-specific decoding info. |
| UHR-STF | Short Training Field used to set the receiver’s gain (AGC) for the UHR modulated portion. |
| UHR-LTF | Long Training Field used for channel estimation and MIMO synchronization. |
| Data | The actual payload (PSDU) containing the user information. |
| PE (Packet Extension) | Provides extra processing time for the receiver to decode the final data symbols. |
UHR TB PPDU (Trigger-Based)
The UHR TB PPDU is used for coordinated uplink transmissions. It is sent by one or more stations only after receiving a Trigger Frame from the AP. Because the AP has already provided instructions in the Trigger Frame, this PPDU is leaner.
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| Field Name | Description/Purpose |
|---|---|
| Legacy & U-SIG Fields | Standard preamble components for compatibility and PHY version identification. |
| UHR-STF (Uplink) | Synchronizes the power levels of multiple stations talking at once. |
| UHR-LTF (Uplink) | Allows the AP to distinguish and decode signals from multiple users simultaneously. |
| Data | The uplink information being sent back to the Access Point. |
| PE | Final padding to ensure the AP has sufficient time to finish signal processing. |
Note: UHR TB frame does not require UHR-SIG as scheduling info was already sent in the Trigger Frame.
ELR PPDU (Enhanced Long Range)
The ELR PPDU is a specialized format used to maintain a connection at extreme ranges. It uses a restricted 20 MHz bandwidth and power boosted fields to ensure the receiver can “hear” the signal even in very weak conditions.
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| Field Name | Description/Purpose |
|---|---|
| L-STF & L-LTF | Power Boosted (+3 dB): Strengthened to ensure detection at long distances. |
| U-SIG | Identifies the PPDU as ELR (using a specific PHY version and compression mode). |
| ELR Mark | New for Wi-Fi 8: Predefined tone patterns used for cross correlation and coherent combining at the receiver. |
| UHR STF & UHR LTF | Power Boosted (+3 dB): Critical for establishing a stable link in low SNR environments. |
| ELR SIG | Carries the specific MCS and coding parameters for the long range transmission. |
| Data | Repeated/duplicated data symbols to ensure reliable delivery at the cell edge. |
| PE | Standard packet extension field |
Summary Comparison between Wi-Fi 8 Frame Formats
| Feature | UHR MU PPDU | UHR TB PPDU | ELR PPDU |
|---|---|---|---|
| Primary Use | Downlink (AP to STAs) | Uplink (STA to AP) | Extreme Range / Reliability |
| Trigger Requirement | Independent | Mandatory (Follows Trigger) | Independent |
| Signaling | UHR-SIG (Full Map) | No SIG field in frame | ELR-SIG (Robust Signaling) |
| Power Boost | Standard | Standard | + 3 dB on Training Fields |
| Bandwidth | Up to 320 MHz | Up to 320 MHz | Fixed 20 MHz |
| Special Field | UHR-SIG | None | ELR Mark |
| Data Strategy | Standard Efficiency | Multi-User Coordination | 4x Data Duplication |
Summary
The framing strategy in Wi-Fi 8 highlights the “Reliability First” philosophy. The MU PPDU handles the distribution of complex data, the TB PPDU ensures perfectly timed multi user responses, and the ELR PPDU acts as a robust “safety net” that trades high speed throughput for an unbreakable connection at a distance.
References
- IEEE P802.11bn /D1.0: Amendment 6: Enhancements for ultra high reliability (UHR).
- Amendment to IEEE Standard 802.11-2020 – Ultra High Reliability.
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