Direct sequence | Spread-Spectrum LANs

In direct sequence spreading, the radio energy is spread across a larger portion of the band than is actually necessary for the data. This is done by breaking each data bit into multiple sub-bits called "chips" to create a higher modulation rate. The higher modulation rate is achieved by multiplying the digital signal with a chip sequence. If the chip sequence is ten, for example, and it is applied to a signal carrying data at 300 Kbps, then the resulting bandwidth will be ten times wider. The amount of spreading is dependent upon the ratio of chips to each bit of information. Because data modulation widens the radio carrier to increasingly larger bandwidths as the data rate increases, this chip rate of 10 times the data rate spreads the radio carrier to 10 times wider than it would otherwise be for data alone. Figure 8.2 compares the bandwidth required for 300 Kbps of data and the 10:1 increase in bandwidth due to spreading.

Figure 1: Spreading 300 Kbps of data across a wider band causes it to resemble random noise during transmission, making it difficult to intercept. (a) Original signal. (b) Spread signal.

The rationale behind this technique is that a spread-spectrum signal with a unique spread code cannot create the exact spectral characteristics as another spread-coded signal. Using the same code as the transmitter, the receiver can correlate and collapse the spread signal back down to its original form, while other receivers using different codes cannot.

This feature of spread spectrum makes it possible to build and operate multiple networks in the same location. By assigning each one its own unique spreading code, all transmissions can use the same frequency band, yet remain independent of each other. The transmissions of one network appear to the other as random noise and are filtered out because the spreading codes do not match.

This spreading technique would appear to result in a weaker signal-to-noise ratio, since the spreading process lowers the signal power at any one frequency. Normally, a low signal-to-noise ratio would result in damaged data packets that would require retransmission. However, the processing gain of the despreading correlator recovers the loss in power when the signal is collapsed back down to the original data bandwidth. This process is not the same as the signal enhancement techniques used by certain devices on wireline networks, since the signal is not strengthened beyond what would have been received had the signal not been spread. (In wireline data transmission, DSUs, for example, perform data regeneration by reshaping the transmit signal before it is sent out over the digital facility to ensure optimal network performance.)

The FCC has set rules for direct sequence transmitters. Each signal must have ten or more chips. This rule limits the practical raw data throughput of transmitters to 2 Mbps in the 902-MHz band and 8 Mbps in the 2.4-GHz band. Unfortunately, the number of chips is directly related to a signal's immunity to interference. In an area with a lot of radio interference, users will have to give up throughput to successfully limit interference.