SPREAD SPECTRUM COMMUNICATION - FARAZDUK HAFIZEE ,B.E. TELECOM
Spread
spectrum is a type of modulation that scatters data transmissions across the available frequency band in a 'pseudo random' pattern. This type of communication makes the signal resistant to noise, interference, and snooping. Today, spread spectrum holds the potential to revolutionise wireless communications, because it renders radio spectrum - a resource currently deemed so precious that only the largest of corporations can afford to buy it, plentiful enough for all of us. Spread spectrum is a highly efficient way of using radio waves to communicate, because it enables multiple users to share radio frequencies at the same time, without interfering with each other.In this system, the transmitted signal is spread over a frequency much wider than the minimum bandwidth required to send the signal. The fundamental premise is that, in channels with narrow band noise, increasing the transmitted signal bandwidth results in an increased probability that the received information will be correct. If total signal power is interpreted as the area under the spectral density curve then signals with equivalent total power may have either a large signal power concentrated in a small bandwidth or a small signal power spread over a large bandwidth. From a system viewpoint, the performance increase for very wide band systems is referred to as "process gain". This term is used to describe the received signal fidelity gained at the cost of bandwidth. The numerical advantage is obtained from Claude Shannon's equation describing channel capacity:C=W log2 (1+ S/N)
Where, C = Channel capacity in bits, W = Bandwidth in Hertz, S = Signal Power, and N = Noise Power
From this equation the result of increasing the bandwidth becomes apparent. By increasing W in the equation, the S/N may be decreased without decreased system performance. The process gain (GP) is what actually provides increased system performance without requiring a high S/N. This is described mathematically as:
GP = BWRF/RINFO
Where, BWRF = RF Bandwidth in Hertz and RINFO = Information rate in bits/second.
The base band signal is spread out to BWRF over the channel. Then at the receiving end, the signal is de-spread by the same amount by a correlation with a desired signal generated by the spreading technique (more on the different spreading techniques later). When the received signal is matched to the desired signal the base band/information signal is retrieved.
 | Bandwidth Spreading : Signal Spreading works quite well in situations with strong narrow band interference signals since the SS signal has a unique form of frequency diversity. The actual signal spreading may be achieved with one of three basic techniques. These include direct sequence, frequency hopped and pulsed FM or hybrid forms. |
| Direct Sequence Spread Spectrum (DSSS) : This is probably the most widely recognised form of spread spectrum. The DSSS process is performed by effectively multiplying an RF carrier and a pseudo-noise (PN) digital signal. First the PN code is modulated onto the information signal using one of several modulation techniques (e.g. BPSK, QPSK, etc.). Then, a doubly balanced mixer is used to multiply the RF carrier and PN modulated information signal. This process causes the RF signal to be replaced with a very wide bandwidth signal with the spectral equivalent of a noise signal. The demodulation process (for the BPSK case) is then simply the mixing/multiplying of the same PN modulated carrier with the incoming RF signal. The output is a signal that is a maximum when the two signals exactly equal one another or are "correlated." The correlated signal is then filtered and sent to a BPSK demodulator. |
| Frequency Hopped Spread Spectrum (FHSS) : Frequency hopping relies on frequency
diversity to combat interference. This is accomplished by multiple frequency, code
selected, FSK. Basically, the incoming digital stream is shifted in frequency by an amount
determined by a code that spreads the signal power over a wide bandwidth. In comparison to
binary FSK, which has only two possible frequencies, FHSS may have 2x1020 or more. The
FHSS transmitter is a pseudo-noise PN code controlled frequency synthesiser. The
instantaneous frequency output of the transmitter jumps from one value to another based on
the pseudo-random input from the code generator. Varying the instantaneous frequency
results in an output spectrum that is effectively spread over the range of frequencies
generated. |
| Spread Spectrum and The Internet : Spread spectrum's efficient use of available bandwidth helps to connect small rural communities and other outlying areas to the Internet. Today, one can access the Internet using a spread-spectrum radio at a T-1 level connection without incurring any telephone charges or monthly service fees, at just the cost of the radio, making spread spectrum highly affordable. |
