Direct-Sequence Spread Spectrum - Motorola AP-51 Series Product Reference Manual

1-24 AP-51xx Access Point Product Reference Guide
supports multiple-cell operations with fast roaming between cells. Within a direct-sequence system,
each cell can operate independently. Adding cells to the network provides an increased coverage area
and total system capacity.
The RS-232 serial port provides a Command Line Interface (CLI) connection. The serial link supports
a direct serial connection (assuming a DB9 connector is used). The access point is a Data Terminal
Equipment (DTE) device with male pin connectors for the RS-232 port. Connecting the access point to
a PC requires a null modem serial cable.

1.3.4 Direct-Sequence Spread Spectrum

Spread spectrum (broadband) uses a narrowband signal to spread the transmission over a segment
of the radio frequency band or spectrum. Direct-sequence is a spread spectrum technique where the
transmitted signal is spread over a particular frequency range. The access point uses Direct-
Sequence Spread Spectrum (DSSS) for radio communication.
Direct-sequence systems communicate by continuously transmitting a redundant pattern of bits
called a chipping sequence. Each bit of transmitted data is mapped into chips by the access point and
rearranged into a pseudorandom spreading code to form the chipping sequence. The chipping
sequence is combined with a transmitted data stream to produce the output signal.
MUs receiving a direct-sequence transmission use the spreading code to map the chips within the
chipping sequence back into bits to recreate the original data transmitted by the access point.
Intercepting and decoding a direct-sequence transmission requires a predefined algorithm to
associate the spreading code used by the transmitting access point to the receiving MU. This
algorithm is established by IEEE 802.11 specifications. The bit redundancy within the chipping
sequence enables the receiving MU to recreate the original data pattern, even if bits in the chipping
sequence are corrupted by interference.
The ratio of chips per bit is called the spreading ratio. A high spreading ratio increases the resistance
of the signal to interference. A low spreading ratio increases the bandwidth available to the user. The
access point uses different modulation schemes to encode more bits per chip at higher data rates.
The access point is capable of a maximum 54Mbps data transmission rate (802.11a radio), but the
coverage area is less than that of an access point operating at lower data rates since coverage area
decreases as bandwidth increases.