Mobile comm. 1
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Mobile communication faces challenges due to changing propagation characteristics as terminals move and encounter obstacles. Spread spectrum techniques are used to improve wireless communication. There are two main types: frequency hopping spread spectrum rapidly switches a signal between frequencies, while direct sequence spread spectrum represents each bit with a chipping code to spread the signal across a wider bandwidth. Both require synchronization between transmitters and receivers.
Mobile comm. 1
- 1. Mobile Communications Course: Communication Systems Dr. Muhammad Saleem Awan
- 42. Mobile communication is burdened with particular propagation complications, making reliable wireless communication more difficult than fixed communication between and carefully positioned antennas. • The antenna height at a mobile terminal is usually very small, typically less than a few meters. Hence, the antenna is expected to have very little 'clearance', so obstacles and reflecting surfaces in the vicinity of the antenna have a substantial influence on the characteristics of the propagation path. • The propagation characteristics change from place to place and, if the terminal moves, from time to time.
- 49. Spread Spectrum An increasingly popular form of encoding for wireless communications is known as spread spectrum. • The use of spread spectrum makes jamming & interception more difficult and improves reception. • The basic idea of spread spectrum is to modulate the signal so as to increase significantly the bandwidth (spread the spectrum) of the signal to be transmitted.
- 50. Spread Spectrum A comment about pseudorandom numberspseudorandom numbers: These numbers are generated by an algorithm using some initial value called the seed. The algorithm is deterministic and therefore produces sequences of numbers that are not statistically random. • The important point is that unless you know the algorithm and the seed, it is impractical to predict the sequence. Hence, only a receiver that shares this information with a transmitter will be able to successfully decode the signal.
- 52. Frequency Hopping Spread Spectrum The signal is broadcast over a seemingly random series of radio frequencies, hopping from frequency to frequency at split-second intervals. A receiver, hopping between frequencies in synchronization with the transmitter, picks up the message.
- 53. Frequency Hopping Spread Spectrum
- 54. Frequency Hopping Spread Spectrum
- 55. Direct Sequence Spread Spectrum Each bit in the original signal is represented by multiple bits in the transmitted signal using a spreading code known as chipping code. The chipping code spreads the signal across a wider frequency band in direct proportion to the number of bits used. Therefore, a 10-bit chipping code spreads the signal across a frequency band that is 10 times greater than a 1-bit chipping code.
- 56. Direct Sequence Spread Spectrum Note that an information bit of 1 inverts the pseudorandom bits in the combination, while an information bit of 0 causes the pseudorandom bits to be transmitted without inversion.
- 57. Direct Sequence Spread Spectrum The amount of spreading that is achieved is a direct result of the data rate of the pseudorandom stream; the greater the data rate of the pseudorandom input, the greater the amount of spreading.
- 58. Direct Sequence Spread Spectrum In this case, the information stream and the pseudorandom stream are both converted to analog signals and then combined, rather than performing the exclusive-or of the two streams and then modulating.