Direct sequence Spread Spectrum• A direct sequence spread spectrum signal is one in

which the amplitude of an already modulated signal is amplitude modulated by very high rate NRZ binary stream of digits.

• Thus if the original signal is s(t) then

• The DS-SS Signal is

Where g(t) is a pseudo-random noise (PN) binary sequence having the values ±1

0( ) 2 ( ) cosss t P d t tω=

0( ) ( ) ( ) 2 ( ) ( ) cossv t g t s t P g t d t tω= =

• Here we assume that g(t) is binary sequence as is the data d(t). The sequence g(t) is generated in deterministic manner and is repetitive. However the sequence length before repetition is usually extremely long. Sequence is completely random in nature means no correlation between the bits of the sequence.

• Furthermore the bit rate fc of g(t) is usually much greater than the bit rate fb of d(t), we can say that g(t) “chops the bits of data into chips”

• To see that multiplying the BPSK sequences s(t) by g(t) spreads the spectrum.

• Figure shows a data sequence d(t) , a pseudo-random sequence g(t) and the product sequence g(t). d(t).

• Note that the edges of g(t) and d(t) are aligned , that is each transition in d(t) coincides with transition on g(t).

• Since the bandwidth of the BPSK signal s(t) is nominally 2fb the bandwidth of the BPSK spread spectrum signal v(t) is 2fc and the spectrum has been spread by the ratio fcfb.

• Since the power transmitted by s(t) and v(t) is the same, the power spectral density Gs(f) is reduced by the factor fb/fc.

• To recover the DS spread spectrum signal, the receiver shown in fig, first multiplies the incoming signal with the waveform g(t) and then by the carrier . The resulting waveform is then integrated for the bit duration and the output of the integrator is sampled, yielding the data d(kTb).

02 cos tω

Effect of Thermal Noise

FHSS• FHSS, or code-controlled multifrequency-FSK

modulation, is another type of spectrum spreading technique. Unlike DS-CDMA which occupies the whole frequency band, FHSS uses only one among a set of narrow channels and hops through all of them in a predetermined spreading sequence at each predefined time interval.

• In the FHSS system, as in the DSSS system, the processing gain Gp is also defined as the ratio of the spread bandwidth of the transmitted signal to the data rate. The principle of FHSS is shown in Fig. 8.15, where B is the spread bandwidth, and N = Gp is the period of the code.

• At the transmitter, the modulated signal is mixed with the synthesizer output pattern to produce the FH signal. For an angle-modulation scheme, the transmitted signal for the kth hop is given by

• where Pavg is the average power, fi is the carrier frequency for the kth hop, φ(t) is a continuous phase term, φk is a random phase angle for the kthhop, and Th is the interval of each hop.

Principles of FHSS

B

ct

f

NT

Block diagram of FHSS

FH signal

Channel

Synchr.

Dehoppedsignal

DemodulatorSymbols

Code

Modulator

synthesizergeneratorFrequency Code

filter

synthesizerFrequency

Bandpass

generator

Transmitter Receiver

streamData

• At the receiver, the mixing operation removes the frequency-hopping pattern from the received signal, and the dehopped signal is obtained.

• FH can be either slow FH or fast FH. The time spent in one frequency is called dwell time or hop interval, Th. Fast FH changes the carrier frequency several times during one symbol period, the chip period Tc = Ts/k for some integer k. Thus, each symbol is spread over a large bandwidth, and there is frequency diversity on each symbol that combats fading or interference.

• Slow FH transmits one or multiple symbols over each frequency, Tc = kTs for some integer k. Slow FH is usually used with TDMA so that each time slot is transmitted on a carrier frequency according to a hopping scheme. Slow FH is used in GSM.

• HomeRF, Bluetooth, and the baseline IEEE 802.11. In Bluetooth, the full-duplex signal implements FH at up to 1,600 hops per second amongst 79 frequencies at 1-MHz intervals. FH is more commonly used in military systems to avoid jammers.

Comparison between DSSS -FHSS• Both DSSS and FHSS have no impact on the

performance in the AWGN channel, but they can improve the performance in the Rayleigh fading channel and in the case of narrowband interference.

• DSSS results in a reduced-power interference on the entire band all the time, while FHSS has a full power interferer on a narrowband channel over a chip period. Thus, coding with interleaving is needed for FHSS, and a collision in one or two frequencies can be recovered by coding.

• Implementation of FHSS is inexpensive.• FHSS is superior to DSSS in terms of immunity to

interference. In case of a strong interference in a segment of the band, the FHSS transmitter will use the remaining band effectively with a decrease in the throughput. FH leads to a whitening of the received signal and an averaging over all the frequencies. When different users use different FH sequences, the FH technique can also be used as a multiple access technique. Some spectral regions with steady interference can be omitted from the hopset, and this is a process called spectral notching.

• In contrast, a single interferer with sufficient power can stop communication of a whole DSSS system.

• Implementation of DSSS requires synchronization and tracking of the phase of the received signal, which is typically difficult in the presence of fast fading. Due to their complexity, DSSS systems consume more power than FSSS systems.

• Transmission time in DSSS is shorter than that in FHSS, since no frequency changing is necessary. DSSS achieves a higher processing gain and is more widely used in high rate systems.

• FH-CDMA is typically used in uplinks, and for military purposes. FH-CDMA typically supports a smaller number of users than DS-CDMA.

• the advantage of FH-CDMA over DS-CDMA is its robustness to the near-far effect: hopping can mitigate performance degradation caused by strong interferers.

• FHSS and FH-CDMA are used in some low-rate systems. For example, FH-CDMA is the core multiple-access technology in Bluetooth, and FHSS is employed in GSM.