Direct Sequence Spread Spectrum Block Diagram
The SS modulation techniques may be divided into two groups: averaging type systems and avoidance type systems. By averaging the interference over a long period, averaging systems reduce it. The DS-SS (Direct Sequence Spread Spectrum) system is an example of an averaging system.
By making the signal avoid interference over a large fraction of time, the avoidance type systems reduce the interference. Frequency Hopping (FH) systems, Time Hopping (TH) systems, Chirp systems, and hybrid modulation systems are examples of avoidance systems.
DIRECT SEQUENCE SPREAD SPECTRUM SYSTEMS
The most significant advantage of spread spectrum modulation is that it protects against interfering signals created outside, such as jamming signals. In practice, interference suppression may be achieved using the Direct Sequence Spread Spectrum (DS-SS) approach. A bandpass channel is used for the transmission of the information signal (eg. Satellite channel). In this case, the communication system employs coherent Binary Phase Shift Keying (BPSK).
When a PN sequence is used to modulate a phase shift keyed signal in a Direct Sequence Broadened Spectrum (DS-SS) system, the transmission bandwidth is spread instantly.
DS-BPSK Transmitter
The transmitter component of the Direct Sequence Spread Spectrum with coherent BPSK is shown in the figure. Two stages of modulation are used in the transmitter component. The NRZ encoder converts the incoming data sequence into an NRZ sequence b(t) at the first step. By applying these two sequences to the product modulator or multiplier, this sequence b(t) is utilized to modulate a broadband pseudo-noise sequence c(t). Both sequences are polar. m(t) = b(t) . c(t) is the product sequence. The spectrum of c(t) will be identical to that of c(t). At the second step, the modulated signal m(t) is utilized to modulate the local carrier for BPSK modulation. QPSK modulation can also be used.
Figure: DS-BPSK Transmitter
Hence, the second stage modulated output s(t) is a binary phase shift keyed Direct Sequence Spread (DS | BPSK) signal. Depending on the polarity of the data and PN sequences, the phase modulation ฯด(t) of the signal S(t) has one of the two values, 0 and ฯ as shown in Table.
Table -
Truth table for phase modulation ฯด(t), Radians
Waveforms
The figure illustrates the waveforms for the first stage of modulation.
The waveforms for the second stage of modulation for one period of the PN sequence are shown in Figure.
DS-BPSK Receiver
The receiver component of the DS-BPSK is shown in the figure.
Demodulation takes place in two stages in the receiver section. The received signal r(t) is initially subjected to coherent detection utilizing the locally produced carrier signal in the first step. This carrier signal is timed and frequency synchronized with the transmitter's carrier.
The output of the coherent detector is then de-spread in the second stage. It is multiplied by a locally produced PN sequence that is synchronized with the transmitter sequence. After despreading, the observed random signal v is integrated over a bit of time. This is utilized in decision-making since it gives an estimate of the original data sequence.
Important Observation
• In practice, the above-mentioned transmitter and receiver (DS-BPSK Transmitter and DS-BPSK Receiver) are used. Before phase modulation, spectrum spreading is conducted in the transmitter. In the receiver, phase demodulation is also done first, followed by despreading; however, the sequence of these two processes is reversed in the DS spread spectrum BPSK system model used for analysis.
• BPSK is performed first in the transmitter, followed by spectrum spreading. Similarly, at the receiver, spectrum despreading is performed first, followed by phase demodulation.
• Since spectrum spreading and BPSK are both linear processes, this is possible.
Advantages of DS-SS System
1. This system is the most effective at detecting and preventing deliberate interference (jamming).
2. For multipath signals, this system has a very high level of discrimination. As a result, the multipath interference is successfully reduced.
3. When compared to other systems, the DS-SS system outperforms them in the presence of noise.
Disadvantages of DS-SS system
1. The output rate of the PN code generator must be high. The length of such a series must be sufficient to ensure that it is genuinely random.
2. The acquisition time using the serial search method is too long. As a result, the DS-SS system is sluggish.
3. The varying distance between the transmitter and receiver affects synchronization.
4. The DS-SS signal is ineffective in the case of broadband interference.
Applications of DS-SS system
1. Anti-jamming application — protecting a jamming signal.
2. Signal transmission with low detectability - the signal is intentionally delivered at a very low power level. As a result, the signal is known as an LPI signal since it has a low probability of being intercepted (LPI).
3. Supporting numerous simultaneous signal transmissions on the same channel, such as with Code Division Multiple Access (CDMA) or spread spectrum multiple access (SSMA).
PERFORMANCE PARAMETERS OF DS-SS SYSTEM
A direct sequence spread spectrum system's important performance characteristics are 1) processing gain, 2) probability of error, and 3) jamming margin.
1) Processing Gain
The gain achieved by processing a spread spectrum signal over an unspread signal is referred to as the processing gain of a DS-SS system. It's also known as the spread spectrum signal's bandwidth divided by the unspread signal's bandwidth.
Thus,
Processing Gain (PG) = ๐ต๐๐๐๐ค๐๐๐กโ ๐๐ ๐ ๐๐๐๐๐ ๐ ๐๐๐๐๐/๐ต๐๐๐๐ค๐๐๐กโ ๐๐ ๐ข๐๐ ๐๐๐๐๐ ๐ ๐๐๐๐๐
The bandwidth of an unspread signal is determined by the bit rate of binary data entering the transmitter input. It is given by
Rb = 1/๐๐ ------------------- (1)
The chip rate of the PN sequence also refers to the spread spectrum signal's bandwidth. It is given by
Rc = 1/๐๐ ------------------ (2)
Therefore, Processing gain is given by
PG = ๐ ๐/๐ ๐ = (1/๐๐)/(1/๐๐) = ๐๐/๐๐
=> PG = ๐๐/๐๐ ----------------------- (3)
Also Tb = NTc. This can be rewritten as
N = ๐๐/๐๐ ------------------- (4)
N is the spread factor, which is the number of chips per information bit.
Both PG and N are equal. Hence
PG = N = ๐๐/๐๐ ------------------- (5)
Since it represents the advantage gained over the jammer by increasing the bandwidth of the transmitted signal, the Processing Gain (PG) is also known as the bandwidth expansion factor (Be).
2) Probability of Error
• The probability of error Pe for a coherent BPSK system can be determined as:
Where Eb is the energy per bit and ๐๐/2 is the power spectral density of white noise.
• The interference in a DS-SS BPSK system can be expressed as a wideband noise signal with a power spectral density of No/2. For the spread signal, we may write No as
No = JTc ------------------------ (7)
where J refers to the average interference power and Tc refers to chip duration or interval.
• On substituting the value of No in equation (6), the probability of error for the DS-SS-BPSK system can be calculated as:
3) Jamming Margin (Antijam characteristics)
• The bit energy to noise density ratio is represented as Eb/No. We may write No as JTC (No = JTc) for the DS-SS-BPSK system. The bit energy Eb is given by
Eb = PsTb -------------------------- (9)
Tb is the bit duration or interval, while Ps is the average signal power.
Thus ๐ธ๐/๐๐ can be written as
๐ธ๐/๐๐ = ๐๐ ๐๐/๐ฝ๐๐ = (๐๐ /๐ฝ) (๐๐/๐๐) ------------------- (10)
๐ฝ/๐๐ = ๐๐/๐๐/๐ธ๐/๐๐ = (๐๐บ/๐ธ๐/๐๐) ------------- (11)
Since we know that PG=๐๐/๐๐
The jamming margin is defined as the ratio J/Ps. Thus, the jamming margin is the ratio of average interference power J to average signal power Ps.
If the jamming margin and processing gain are both expressed in dB (decibels), the equation for the jamming margin is:
(Jamming margin)dB = (Processing gain)dB - 10log10(๐ธ๐/๐๐)๐๐๐ ----------------- (12)
where (Eb/No)min is the bit energy-to-noise density ratio required to support a given average probability of error.