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spread spectrum

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• The signals gain immunity from various kinds of noise and multipath distortion. The earliest applications of spread spectrum were military, where it was used for its immunity to jamming.

• It can also be used for hiding and encrypting signals. Only a recipient who knows the spreading code can recover the encoded information.

• Several users can independently use the same higher bandwidth with very little interference. This property is used in cellular telephony applications, with a technique known as code division multiplexing (CDM) or code division multiple access (CDMA).

Typically, a large number of frequencies is used in FHSS so that bandwidth of the FHSS signal is much larger than that of the original MFSK signal. One benefit of this is that a large value of k results in a system that is quite resistant to jamming. If frequency hopping is used, the jammer must jam all 2k frequencies. With a fixed power, this reduces the jamming power in any one frequency band to Sj/2k. In general, fast FHSS provides improved performance compared to slow FHSS in the face of noise or jamming, as we will discuss shortly.

s(t) = A d(t)c(t) cos(2πfct): Equation (9.5)

At the receiver, the incoming signal is multiplied again by c(t). But c(t) c(t) = 1 and therefore the original signal is recovered. Equation (9.5) can be interpreted in two ways, leading to two different implementations. The first interpretation is to first multiply d(t) and c(t) together and then perform the BPSK modulation. That is the interpretation we have been discussing. Alternatively, we can first perform the BPSK modulation on the data stream d(t) to generate the data signal sd(t). This signal can then be multiplied by c(t). An implementation using the second interpretation is shown in Stallings DCC8e Figure 9.7 above.

As with FHSS, we can get some insight into the performance of DSSS by looking at its effectiveness against jamming. Let us assume a simple jamming signal at the center frequency of the DSSS system. Can show the carrier power Sj is spread over a bandwidth of approximately 2/Tc. However, the BPSK demodulator (Figure 9.7) following the DSSS despreader includes a bandpass filter matched to the BPSK data, with bandwidth of 2/T. Thus, most of the jamming power is filtered. The jamming power has been reduced by a factor of (Tc /T) through the use of spread spectrum. The result is similar to the result for FHSS.

At the base station the receiver decodes the chip patterns. If the decoder is linear and if A and B transmit signals sA and sB, respectively, at the same time, then SA (sA + sB) = SA (sA) + SA (sB) = SA (sA) since the decoder ignores B when it is using A’s code. The codes of A and B that have the property that SA (cB) = SB (cA) = 0 are called orthogonal. Using the decoder, Su, the receiver can sort out transmission from u even when there may be other users broadcasting in the same cell. In practice, the CDMA receiver can filter out the contribution from unwanted users or they appear as low-level noise. However, if there are many users competing for the channel with the user the receiver is trying to listen to, or if the signal power of one or more competing signals is too high, perhaps because it is very near the receiver (the “near/far” problem), the system breaks down.

- 1. Data and Computer Communications Chapter 9 – Spread Spectrum
- 2. Spread Spectrum important encoding method for wireless communications analog & digital data with analog signal spreads data over wide bandwidth makes jamming and interception harder two approaches, both in use: Frequency Hopping Direct Sequence
- 3. General Model of Spread Spectrum System
- 4. Spread Spectrum Advantages immunity from noise and multipath distortion can hide / encrypt signals several users can share same higher bandwidth with little interference CDM/CDMA Mobile telephones
- 5. Pseudorandom Numbers generated by a deterministic algorithm not actually random but if algorithm good, results pass reasonable tests of randomness starting from an initial seed need to know algorithm and seed to predict sequence hence only receiver can decode signal
- 6. Frequency Hopping Spread Spectrum (FHSS) signal is broadcast over seemingly random series of frequencies receiver hops between frequencies in sync with transmitter eavesdroppers hear unintelligible blips jamming on one frequency affects only a few bits
- 7. Frequency Hopping Example
- 8. FHSS (Transmitter)
- 9. Frequency Hopping Spread Spectrum System (Receiver)
- 10. Slow and Fast FHSS commonly use multiple FSK (MFSK) have frequency shifted every T c duration of signal element is T s Slow FHSS has Tc seconds ≥ Ts Fast FHSS has Tc seconds < Ts FHSS quite resistant to noise or jamming with fast FHSS giving better performance
- 11. Slow MFSK FHSS
- 12. Fast MFSK FHSS
- 13. Direct Sequence Spread Spectrum (DSSS) each bit is represented by multiple bits using a spreading code this spreads signal across a wider frequency band has performance similar to FHSS
- 14. Direct Sequence Spread Spectrum Example
- 15. Direct Sequence Spread Spectrum System
- 16. DSSS Example Using BPSK
- 17. Approximate Spectrum of DSSS Signal
- 18. Code Division Multiple Access (CDMA) a multiplexing technique used with spread spectrum given a data signal rate D break each bit into k chips according to a fixed chipping code specific to each user resulting new channel has chip data rate kD chips per second can have multiple channels superimposed
- 19. CDMA Example
- 20. CDMA for DSSS
- 21. Problem # Consider the seven-channel CDMA shown in the following figure. Enlist the user code for each channel. A positive sum is decoded (at the receiver) as ‘1’ and a negative sum is decoded as ‘0’. If all the channels are transmitting as shown, determine whether the receiver detects the correct bit of channel 1.
- 22. Problem
- 23. Orthogonal Sequence Sequence is generated by Walsh Table: W1 = [+1] W2 = W1 W1 W1 !W1 W4 = W2 W2 W2 !W2
- 24. Summary looked at use of spread spectrum techniques: FHSS DSSS CDMA

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