UNIT 4 Spread Spectrum and Multiple. Access Technique

Size: px

Start display at page:

Download "UNIT 4 Spread Spectrum and Multiple. Access Technique"

Lucinda Lambert
5 years ago
Views:

1 UNIT 4 Spread Spectrum and Multiple Access Technique

2 Spread Spectrum lspread spectrumis a communication technique that spreads a narrowband communication signal over a wide range of frequencies for transmission then de-spreads it into the original data bandwidth at the receive. l Spread spectrum is characterized by: 4wide bandwidth and 4low power ljamming and interferencehave less effect on Spread spectrum because it is: 4 Resembles noise 4Hard to detect 4Hard to intercept February 2005 Copyright 2005 All Rights Reserved 2

3 These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference, noise and jamming

4 Definition of Spread Spectrum Modulation in two parts means of transmission in which the data sequence occupies a bandwidth in excess of minimum bandwidth necessary to send it. Spread spectrum is accomplished before transmission through the use of a code that is independent of the data sequence. The same code is used in the receiver that is synchronized with the transmitter to de-spread the received signal so that the original data sequence may be recovered.

5 General Model of Spread Spectrum System

6 Pseudorandom Numbers A pseudo-noise (PN) sequence is a periodic binary sequence with a noiselike waveform that is usually generated by means of a feedback shift register

7 Classification Direct Sequence Frequency Hopping

8 Direct Sequence (DS) Spread Spectrum Technique two stages of modulation The incoming data sequence is used to modulate a wideband code. This code transforms the narrow band data sequenceinto a noise like wide band signal. The resulting wide band signal undergoes a second stage of modulation using a phase shift keying (PSK) technique.

9 Direct Sequence Spread Spectrum Transmitter

10 Direct Sequence Spread Spectrum Receiver

11 Direct Sequence Spread Spectrum Example

12 Frequency Hopping Spread Spectrum (FHSS) Signal 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

13 Frequency Hopping Spread Spectrum System (Transmitter)

14 Frequency Hopping Spread Spectrum System (Receiver)

15 Multiple Access Techniques multiple-access method,inwhich the available bandwidth is shared in time, frequency, or through code, between different stations. Frequency division multiple access (FDMA) Time division multiple access (TDMA) Spread spectrum multiple access (SSMA) : an example is Code division multiple access (CDMA) Space division multiple access (SDMA)

16 FDMA In FDMA, the available bandwidth of the common channel is divided into bands that are separated by guard bands.

18 TDMA In TDMA, the bandwidth is just one channel that is timeshared between different stations.

20 Repeating Frame Structure One TDMA Frame Preamble Information Message Trail Bits Slot 1 Slot 2 Slot 3 Slot N Trail Bits Sync. Bits Information Data Guard Bits The frame is cyclically repeated over time.

21 CDMA In CDMA, one channel carries all transmissions simultaneously. All the users in a CDMA system, use the same frequency and may transmit simultaneously. Unique orthogonal code is assigned to each channel For recover the signal,the receiver needs to know the code used by the transmitter.

24 Use spot beam antennas SDMA The different beam area can use TDMA, FDMA, CDMA Sectorized antenna can be thought of as a SDMA Adaptive antennas can be used in the future (simultaneously steer energy in the direction of many users) spot beam antenna

25 Features: A large number of independently steered high-gain beams can be formed without any resulting degradation in SNR ratio. Beams can be assigned to individual users, thereby assuring that all links operate with maximum gain. Adaptive beam forming can be easily implemented to improve the system capacity by suppressing co channel interference.

26 Comparison SDMA/TDMA/FDMA/CDMA Approach SDMA TDMA FDMA CDMA Idea Terminals Signal separation segment space into cells/sectors only one terminal can be active in one cell/one sector cell structure, directed antennas segment sending time into disjoint time-slots, demand driven or fixed patterns all terminals are active for short periods of time on the same frequency synchronization in the time domain segment the frequency band into disjoint sub-bands every terminal has its own frequency, uninterrupted filtering in the frequency domain spread the spectrum using orthogonal codes all terminals can be active at the same place at the same moment, uninterrupted code plus special receivers Advantages very simple, increases capacity per km² Disadvantages Comment inflexible, antennas typically fixed only in combination with TDMA, FDMA or CDMA useful established, fully digital, flexible guard space needed (multipath propagation), synchronization difficult standard in fixed networks, together with FDMA/SDMA used in many mobile networks simple, established, robust inflexible, frequencies are a scarce resource typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse) flexible, less frequency planning needed, soft handover complex receivers, needs more complicated power control for senders still faces some problems, higher complexity, lowered expectations; will be integrated with TDMA/FDMA 26

Wireless Transmission & Media Access

Wireless Transmission & Media Access Signals and Signal Propagation Multiplexing Modulation Media Access 1 Significant parts of slides are based on original material by Prof. Dr.-Ing. Jochen Schiller,