MIMO OFDM | Documentation? | OFDM Frame Format

Overview

The frame format for the WARP MIMO OFDM core is based loosely on the frame format specified in the ​IEEE 802.11a wireless networking standard.

Each frame is divided into four sections:

The actual format depends on the antenna mode:

SISO Frame Format

Alamouti Frame Format

Multiplexing Frame Format

Preamble

The preamble is a hard-coded 320-sample sequence used by the receiver for AGC, carrier frequency offset estimation and symbol timing estimation. The preamble consists of two 160-sample sections. The first is a repitition of 10 16-sample sequences, called short training symbols (STS). The second half is 2.5 repititions of a 64-sample sequence, called the long training symbol (LTS). The preamble is defined in the OFDM models's initialization script.

In SISO mode, the preamble is transmitted only from the active antenna. In Alamouti and multiplexing modes, the preamble is always transmitted from antenna A and optionally from antenna B. This option is controlled by user software.

In both modes, the preamble is scaled by a programmed constant. This scaling allows the full-scale swing of the preamble (+/-1 default) to match the swing of the IFFT output. The scaling constant is programmed by user software.

Training

The OFDM core uses hard-coded sequence of BPSK symbols for channel training. Each repetition of the sequence is treated as a full OFDM symbol at the receiver. The number of repetitions is programmable but must be known to both the transmitter and receiver ahead of time. The training sequence is defined in the OFDM model's initialization script.

For SISO mode, the programmed number of training symbols are inserted between the preamble and base-rate symbols. For Alamouti and multiplexing modes, the programmed number of training symbol periods are dedicated to channel estimation and are split evenly between the transmit antennas. See the frame format section below for more details.

Base Rate

In order to maintain compatibility with nodes communicating over various distances (i.e. capable of communicating at various rates), the OFDM core allows a number of OFDM symbols to be modulated using a base-rate. The number of base-rate symbols and the assignment of base-rate modulation schemes to subcarriers must be known to both the transmitter and receiver ahead of time. In multiplexing mode, the base-rate symbols are transmitted only from antenna A.

Generally, these base-rate symbols contain the packet's header, which enables every receiving node to know the packet's source, destination, full-rate modulation and length.

The number of base-rate symbols and base-rate modulation scheme is programmed by user code and must correspond to the length of the header defined by the user's packet format.

Full Rate

The remaining OFDM symbols in each packet contain data modulated at the full-rate. The full-rate modulation schemes are programmable across subcarriers and antennas. The assignment of full-rate modulation schemes to subcarriers and antennas must be known to both the transmitter and receiver ahead of time.

The number of full-rate symbols is calculated automatically by the transmitter based on the modulation rate and packet length specified by user code. The maximum number of full-rate symbols is 2047 (way more than enough for very long packets).

The number of full-rate symbols can be zero. This enables header-only packets. User code selects this mode by transmitting a packet whose length equals the number header bytes specified during the OFDM core initialization.

Checksums

The OFDM transmitter subsystem includes a checksum calculation and insertion block. This block will overwrite the last two bytes of the header with a 16-bit CRC calculated over the full contents of the base-rate symbols. The checksum block will overwrite the last four bytes of the packet with a 32-bit CRC calculated over the full packet, including the header. The 32-bit full-packet checksum is not used for header-only packets.

User code must accommodate these checksum insertions by leaving the corresponding bytes empty in packets loaded for transmission, and by ignoring these bytes when processing received packets.