Asus WL-500b Benutzerhandbuch - Seite 12

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Chapter 1 - Introduction

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.

SpaceLink Home Gateway