OFDM Modem (802.11a-Compliant)

Features

• Fully compliant with IEEE 802.11a PHY standard
• 40 MHz system clock
• Power-save and power-down modes
• 2kB memory
• <400k system gates
• Synthesizable Verilog source code
• MATLAB simulation model
• Zero IF or heterodyne (IF up to 140 MHz) interface to RF section
• Configurable Tx & Rx sample rates, with support of subsampling
• Simple interface to 3rd-party MAC layer implementation
• Support of IEEE 802.11a Physical Medium Dependent primitives

Block Diagram

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Description

The CommStack OFDM Modem is in full compliance with the IEEE 802.11a PHY standard, supporting the following combination of data rates, modulation types, and code rates:

With a simple and flexible Tx and Rx Media Access Control (MAC) interface toward the data source and sink, the OFDM Modem allows the MAC function to be either compliant with the 802.11a standard or non-standard to support custom OFDM applications.  Similarly, the RF interface supports I and Q channels at zero IF or at an IF up to 240 MHz, thereby allowing considerable flexibility in the choice of RF section design.

The 802.11a PHY standard specifies performance for the combination of both the baseband OFDM modem and the RF section. CommStack’s OFDM Modem’s performance has minimal implementation loss, allowing more flexibility for designers in the implementation of the RF section: 

• Spectral purity - transmit spectral purity as measured at the RF interface is at least 8 dB below the specified spectral mask
• Responsiveness - transmit bursts appears at the outputs within 100 nsec after being strobed at the MAC interface, while Clear Channel Assessment indicates busy with greater than 98% probability within 1 μsec
• Robustness - receive adjacent channel rejection performance exceeds the specification by more than 6 dB, while the acquisition circuitry can pull in offsets greater than 500 kHz even in the presence of large I/Q imbalances at low SNR
• Accuracy - transmit constellation error (Error Vector Magnitude) as measured at the RF interface exceeds the specification by at least 12 dB, where accuracy is limited by the measurement device itself

The CommStack OFDM Modem requires less than 20,000 Altera logic elements and is thus estimated to be less than 400k gates when implemented as an ASIC, making it one of the smallest designs of its type available today. Such a small footprint implies reduced per-unit costs, even if the design is customized with expanded features and extensions. At the same time, power consumption is kept low by a synchronous pipelined design that disables those modules not currently in use on a symbol-by-symbol basis.