| | 1 | [[TracNav(802.11/TOC)]] |
| | 2 | |
| | 3 | = 802.11 Reference Design: PHY = |
| | 4 | |
| | 5 | Our physical layer implementation is based on the OFDM PHY specified in section 18 of of the 802.11-2012 standard. This PHY is commonly referred to as "802.11a" (at 5GHz) and "802.11g" (at 2.4GHz). Our implementation is OFDM-only (with one exception; see below). |
| | 6 | |
| | 7 | '''Bandwidth:''' 20MHz |
| | 8 | |
| | 9 | '''OFDM Specs:''' 64 subcarriers (48 data, 4 pilots), 16-sample cyclic prefix |
| | 10 | |
| | 11 | '''Frame Format:''' As specified in section 18.3.2 of 802.11-2012: |
| | 12 | * Preamble (10 repetitions of 16-sample short training symbol, 2.5 repetitions of 64-sample long training symbol) |
| | 13 | * SIGNAL field as first OFDM symbol (3 bytes as BSPK, rate 1/2 code) |
| | 14 | * Remaining OFDM symbols filled with SERVICE field (2 bytes) and payload (up to 1500 bytes) at one of the rates listed below |
| | 15 | |
| | 16 | '''Rates:''' The following OFDM data rates are implemented. Each data rate is realized by a combination of modulation and coding rates. |
| | 17 | ||= Modulation[[BR]]Rate =||= Code[[BR]]Rate =||= Data Rate[[BR]](Mbps) =|| |
| | 18 | || BPSK || 1/2 || 6 || |
| | 19 | || BPSK || 3/4 || 9 || |
| | 20 | || QPSK || 1/2 || 12 || |
| | 21 | || QPSK || 3/4 || 18 || |
| | 22 | |
| | 23 | The following rates will be implemented in a future release: |
| | 24 | |
| | 25 | ||= Modulation[[BR]]Rate =||= Code[[BR]]Rate =||= Data Rate[[BR]](Mbps) =|| |
| | 26 | || 16-QAM || 1/2 || 24 || |
| | 27 | || 16-QAM || 3/4 || 36 || |
| | 28 | || 64-QAM || 2/3 || 48 || |
| | 29 | || 64-QAM || 3/4 || 54 || |
| | 30 | |
| | 31 | 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. |
| | 32 | |
| | 33 | The current WARP 802.11 ref design does not implement a DSSS transmitter, as 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). |
| | 34 | |
| | 35 | '''Synchronization:''' Our PHY implementation requires no "cheating"- all synchronization is implemented in the FPGA and operates per-packet in real-time. |
| | 36 | * The AGC block selects Rx gains per-packet and makes no assumptions about inter-packet receive powers. |
| | 37 | * 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. |
| | 38 | * 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. |
| | 39 | * 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. |
| | 40 | |
| | 41 | '''Multi-antenna support:''' The current PHY Tx/Rx pipelines are SISO, supporting only the modulation/coding rates specified in section 18 of the standard. |
