Changes between Version 13 and Version 14 of 802.11/PHY


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Timestamp:
Dec 2, 2013, 5:16:51 AM (10 years ago)
Author:
murphpo
Comment:

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  • 802.11/PHY

    v13 v14  
    4949[[Image(wiki:802.11/files:wlan_phy_rx_blk_diag.png)]]
    5050
    51  * '''Packet Detection''': implements two packet detection schemes: simple energy detection based on RSSI and auto-correlation of the I/Q samples searching for the preamble STS, based on the well-known [ieeexplore.ieee.org/iel3/26/14182/00650240.pdf‎ Schmidl-Cox algorithm]. When selection diversity is enabled parallel packet detectors are enabled so that either antenna can trigger a detection.
     51 * '''Packet Detection''': implements two packet detection schemes: simple energy detection based on RSSI and auto-correlation of the I/Q samples searching for the preamble STS, based on the well-known [http://ieeexplore.ieee.org/iel3/26/14182/00650240.pdf‎ Schmidl-Cox algorithm]. When selection diversity is enabled parallel packet detectors are enabled so that either antenna can trigger a detection.
    5252
    5353 * '''Antenna Selection''': automatic selection of which I/Q stream feeds the rest of the PHY pipeline, using AGC gain selections as an indicator of received SNR.
    … …  
    6161 * '''FFT''': translates the time domain received samples into the frequency domain. Each FFT consumes 64 time domain samples and produces 64 frequency domain samples. The boundary of each FFT is established by the synchronization blocks above. The cyclic prefix of each OFDM symbol is removed by advancing the boundary of each FFT 16 samples per transform.
    6262
    63  * '''Channel Estimation''':
     63 * '''Channel Estimation''': a complex channel coefficient is calculated for each non-zero subcarrier by averaging the estimates from the two LTS.
    6464
    65  * '''Phase Error Estimation''':
    66  * Equalization:
    67  * Soft Demod:
    68  * De-Interleaving:
    69  * Decoding:
    70  * Descrambling:
     65 * '''Phase Error Estimation''': the pilot tones embedded in each OFDM symbol are used to calculate a phase error estimate per OFDM symbol. Every subcarrier in the OFDM symbol is then de-rotated by the estimated phase error.
    7166
    72 '''DSSS:''' The PHY receiver also implements the 1Mbps DSSS rate specified in the original 802.11 standard (section 16.2 of the 802.11-2012 standard). This receiver allows reception of management frames transmitted by 802.11 devices at 1Mbps. These transmissions are common in deployments of 802.11 hardware at 2.4GHz. For example, Beacon and Probe Request frames are frequently transmitted at 1Mbps by commercial devices. The basic STA/AP association handshake requires reception of these frames. The 802.11 Reference Design does not implement a DSSS transmitter, as modern 802.11 devices are able to receive management frames at higher rates (including 6Mbps, the lowest OFDM rate, which is commonly used for management frames at 5GHz).
     67 * '''Equalization''': the channel estimates and phase-corrected data symbols are fed into the equalizer to remove amplitude and phase errors incurred by propagation through the wireless channel. The current implementation uses a simple zero-forcing equalizer, dividing each subcarrier by the corresponding channel coefficient and using the same channel coefficients for the full packet.
    7368
    74 '''Synchronization:''' The PHY implementation requires no "cheating"- all synchronization is implemented in the FPGA and operates per-packet in real time.
    75  * The AGC block selects Rx gains per-packet and makes no assumptions about inter-packet receive powers.
    76  * The CFO (carrier frequency offset) block estimates and corrects CFO per-packet. CFO estimates are extracted from the preamble long training symbols and correction is applied pre-FFT.
    77  * The symbol sync block establishes sample-level synchronization using a complex cross correlator tuned to the preamble long training symbols. All Rx timing is established per-packet based on the correlator output.
    78  * The channel estimation block computes a complex channel estimate (magnitude and phase) for each subcarrier per-packet. The equalizer applies the channel estimate per-subcarrier. The current Rx PHY uses the same channel estimates for the full packet. Extending this to a decision feedback scheme (where channel estimates are updated intra-packet) would be a straightforward extension.
     69 * '''Soft Demod''': each data symbol is then demodulated to a soft value per coded bit
    7970
    80 '''Multi-antenna support:''' The current PHY Tx/Rx pipelines are SISO, supporting the modulation/coding rates specified in section 18 of the standard. The PHY antenna interfaces implement selection diversity across the two RF interfaces on WARP v3 hardware. The antenna selection is made per packet. For transmissions the antenna selection is always controlled by C code in CPU Low. For receptions the PHY can automatically select the higher-SNR antenna based on the AGC gain selections. Alternatively the C code in CPU Low can force the receive antenna selection.
     71 * '''De-Interleaving''': the coded bits, represented as soft 4-bit confidence values, are de-interleaved along OFDM symbol boundaries using the interleaving pattern specified in the standard
     72
     73 * '''Decoding''': the de-interleaved soft values are decoded using a standard Viterbi decoder
     74
     75 * '''Descrambling''': the de-coded bits are finally descrambled using the LFSR specified in the standard
     76
     77All logic in the WLAN receiver core is clocked at 160MHz and supports a maximum bandwidth of 20MHz (clock rate = 8x max sample rate). Smaller bandwidths are supported by reducing the duty cycle of the sample_valid signal at the input to the Rx pipeline.
     78
     79=== DSSS ===
     80The PHY receiver also implements the 1Mbps DSSS rate specified in the original 802.11 standard (section 16.2 of the 802.11-2012 standard). This receiver allows reception of management frames transmitted by 802.11 devices at 1Mbps. These transmissions are common in deployments of 802.11 hardware at 2.4GHz. For example, Beacon and Probe Request frames are frequently transmitted at 1Mbps by commercial devices. The basic STA/AP association handshake requires reception of these frames. The 802.11 Reference Design does not implement a DSSS transmitter, as modern 802.11 devices are able to receive management frames at higher rates (including 6Mbps, the lowest OFDM rate, which is commonly used for management frames at 5GHz).
     81
     82=== Multi-antenna Support ===
     83The current PHY Tx/Rx pipelines are SISO, supporting the modulation/coding rates specified in section 18 of the standard. The PHY antenna interfaces implement selection diversity across the two RF interfaces on WARP v3 hardware. The antenna selection is made per packet. For transmissions the antenna selection is always controlled by C code in CPU Low. For receptions the PHY can automatically select the higher-SNR antenna based on the AGC gain selections. Alternatively the C code in CPU Low can force the receive antenna selection.
    8184
    8285We are also considering implementing some of the MIMO modes from 802.11n/11ac. Let us know if these would be especially useful for your research.