Part A: Spread Spectrum Systems

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1 1 Telecommunication Systems and Applications (TL - 424) Part A: Spread Spectrum Systems Dr. ir. Muhammad Nasir KHAN Department of Electrical Engineering Swedish College of Engineering and Technology March 11, 2014

2 Frequency Hopping Spread Spectrum Also FH-SS modulation was originally developed for military communication systems between for covered communications. In FH-SS modulation, the information signal is spread over a much wider bandwidth than required based on the symbol rate. Again, a narrowband signal is made wide-band! Spreading of the signal is now achieved by randomly changing the carrier frequency of the narrowband signal according to a hopping code. Why Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 11, / 1 2

3 Frequency Hopping Spread Spectrum Also FH-SS modulation was originally developed for military communication systems between for covered communications. In FH-SS modulation, the information signal is spread over a much wider bandwidth than required based on the symbol rate. Again, a narrowband signal is made wide-band! Spreading of the signal is now achieved by randomly changing the carrier frequency of the narrowband signal according to a hopping code. Why To create robustness against: frequency selective fading, interference, detection and eavesdropping. Because of the randomly changing carrier frequency, the presence and contents of the signals are difficult to detect. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 11, / 1 3

FH-SS Transmitter In FH-SS, spreading of the signal over a much wider bandwidth is achieved by regular changing the transmit frequency according to a hopping-code, within a set of available

4 FH-SS Transmitter In FH-SS, spreading of the signal over a much wider bandwidth is achieved by regular changing the transmit frequency according to a hopping-code, within a set of available frequencies. The set of available frequencies c(t) {c 1, c 2,, c N } is called the hop-set. For a BPSK modulated signal: s ck (t) = A c d(t) cos ω ck t Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 11, / 1 4

5 FH-SS Transmitter (Cont d) We distinguish fast hopping and slow hopping. Fast hopping: the transmission of a single symbol lasts several hops. Slow hopping: multiple symbols are transmitted per hop. The dwell-time is the duration the signal stays at a certain frequency. The ratio G p = BWFH SS BW data = 2NRs 2R s = N where N is the hop-set size. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 11, / 1 5

6 FH-SS Transmitter (Cont d) The of the FH-SS in a frequency selective channel. The channel quality changes per hopping period. By applying special coding techniques (interleaving, the effect of the bad channel can be compensated by the good channel. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 11, / 1 6

7 FH-SS Reception Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 11, / 1 7

8 FH-SS Reception (Cont d) Robustness of FH-SS against interference is due to the fact that an occupied channel will be used during a small fraction of time. For a hopping set of N channels, which includes the interfering channel, the probability that the FH-frequency coincides with the occupied channel (a hit or collision) is: 1 G p = 1 N. Effective interference power: I eff = PI G p = PI N. Frequency hopping with narrowband interference de- After freq. hoppinhg frequency de-hopping Time Time A hit t2 t2 t1 t1 f1 f2 fn fif Frequency hop-set 8 Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 11, / 1

9 Spectral Efficiency of FH-SS For FH-SS, with M-level modulation, the spectral efficiency is given by: η R b B T = lrs B T = lrs 2NR s = l 2G p [bit/s/hz] The price, we pay for the increased robustness (large N) is a very low spectral efficiency. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 11, / 1 9

10 Spectral Efficiency of FH-SS For FH-SS, with M-level modulation, the spectral efficiency is given by: η R b B T = lrs B T = lrs 2NR s = l 2G p [bit/s/hz] The price, we pay for the increased robustness (large N) is a very low spectral efficiency. This can be improved by letting multiple users share the same bandwidth (hop-set)! Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 11, / 1 10

11 Hopping Codes The FH-SS hopping code determines the order in which the channels of the hop-set are used. Different hopping-codes allow multiple users to share the same hop-set simultaneously. This is called frequency hopping multiple access (FHMA). FHMA In FHMA, it is important to minimise the probability that two or more users occupy the same channel at the same time. With a high probability, collision will result in loss of the transmitted information. Two types of FH-SS codes are distinguished, orthogonal codes. random codes. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 11, / 1 11

Part A: Spread Spectrum Systems

1 Telecommunication Systems and Applications (TL - 424) Part A: Spread Spectrum Systems Dr. ir. Muhammad Nasir KHAN Department of Electrical Engineering Swedish College of Engineering and Technology March