1. Introduction
Chapter 1 - Introduction
12
SpaceLink Home Gateway
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 Space Link Home Gateway
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 Home Gateway 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 Home
Gateway output signal.
Wireless mobile clients 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 Home Gateway.
Intercepting and decoding a direct-sequence transmission requires a
predefined algorithm to associate the spreading code used by the transmitting
Home Gateway to the receiving wireless mobile client.
This algorithm is established by IEEE 802.11b specifications. The bit
redundancy within the chipping sequence enables the receiving wireless
mobile client 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 Home Gateway uses a constant chip rate of
11Mchips/s for all data rates, but uses different modulation schemes to
encode more bits per chip at the higher data rates. The Home Gateway is
capable of an 11 Mbps data transmission rate, but the coverage area is less
than a 1 or 2 Mbps Home Gateway since coverage area decreases as
bandwidth increases.
