Legacy PHY - FHSS, DSSS, HR-DSSS, ERP

The transmitter hops between 79 channels (1 MHz each) in the 2.4 GHz band according to a pseudo-random hopping sequence. Both transmitter and receiver must be synchronised to the same sequence - the Hop Index is carried in Beacon frames. Data is transmitted on each frequency for a short dwell time (20–400 ms) before hopping to the next.

Because any narrowband interferer only affects the signal during the fraction of time spent on that frequency, FHSS is inherently resistant to narrowband interference. It was the original 802.11 PHY alongside DSSS.

Each data bit is multiplied by an 11-chip Barker sequence before transmission. Each chip is transmitted at 11 Mchips/sec, spreading the signal across a 22 MHz bandwidth - 11× wider than the data rate alone would require. The receiver correlates the received signal with the same Barker sequence to recover the original bits, achieving a processing gain of approximately 10.4 dB.

The Barker code {+1,-1,+1,+1,-1,+1,+1,+1,-1,-1,-1} has excellent autocorrelation properties - the receiver can reliably distinguish signal from noise even at very low SNR. This gave DSSS superior range compared to FHSS.

CCK (Complementary Code Keying) replaces the 11-chip Barker sequence with 8-chip complex codewords selected from a 64-codeword set. At 5.5 Mbps, 4 bits per symbol chip are encoded using DQPSK + CCK-16. At 11 Mbps, 8 bits per symbol chip use DQPSK + CCK-256. The same 22 MHz bandwidth and 11 Mchips/sec chipping rate is maintained - the increased throughput comes from the higher information density in each chip rather than wider bandwidth.

802.11b remains backward compatible with DSSS (1/2 Mbps) via optional PBCC (Packet Binary Convolutional Coding). The long preamble (192 µs) ensures legacy DSSS STAs can detect the channel busy. Short preamble (96 µs) provides efficiency gains in pure-b networks.

Instead of one wide carrier (DSSS/FHSS), OFDM divides the channel into 64 orthogonal subcarriers, each only 312.5 kHz wide. 52 subcarriers carry data (48 data + 4 pilot). Because each subcarrier has a very long symbol period (3.2 µs) relative to its narrow bandwidth, it is highly resistant to multipath - the key advantage over spread spectrum in reflective indoor environments.

802.11a-1999 defined the same OFDM structure (64-point FFT, 312.5 kHz subcarrier spacing) still used in every subsequent generation. 802.11n doubled it to 40 MHz (128-point FFT), 802.11ac to 80 MHz, and 802.11ax narrowed each subcarrier to 78.125 kHz for 4× more frequency resolution.

802.11g brought OFDM to the 2.4 GHz band. The ERP (Extended Rate PHY) defines three mandatory sub-PHYs: ERP-OFDM (54 Mbps, same OFDM parameters as 802.11a), ERP-DSSS/CCK (11 Mbps, mandatory backward compat with 802.11b), and optional ERP-PBCC.

The critical operational issue: when 802.11b clients are present in the BSS, the AP must enable Protection Mode - adding CTS-to-self (or RTS/CTS) before every HT/ERP-OFDM frame. This overhead reduces effective 802.11g throughput from ~22 Mbps to ~11 Mbps in mixed b/g networks.