Advantages, Disadvantages and Applications of Spread Spectrum

Advantages, Disadvantages and Applications of Spread Spectrum

Major Applications of Direct Sequence Spread Spectrum (DS-SS)

1. Anti-jamming application that protects a jamming signal.

2. Signal transmission with low detectability — the signal is intentionally delivered at a low power level. As a result, the signal is known as an LPI signal since it has a low risk of being intercepted.

3. Supporting a large number of simultaneous signal transmissions on the same channel, such as code division multiple access (CDMA) or Spread Spectrum Multiple Access (SSMA) (SSMA).

Major Applications of Frequency Hopping Spread Spectrum (FH-SS) System

1. For mobile communication, CDMA systems based on FH spread spectrum transmissions are particularly appealing.

2. Wi-wireless Fi's local area networks (WLAN) standard.

3. Bluetooth's Wireless Personal Area Network (WPAN) protocol.

Commercial Applications of Spread Spectrum Techniques

Spread spectrum signals are employed for the following purposes:

1) combating or mitigating the negative effects of jamming (Intentional interference). It can also be utilized for military purposes.

2) Allowing numerous users to send messages over the same channel bandwidth at the same time. Code Division Multiple Access (CDMA) or Spread Spectrum Multiple Access is a method of digital communication in which each user (transmitter-receiver pair) has a unique PN code for sending over a common channel bandwidth (SSMA). This is a common approach in digital wireless communications.

3) Minimizing inadvertent interference caused by other channel users.

4) self-interference suppression owing to multipath propagation.

5) Hiding a signal by transmitting it with low strength, making it impossible to detect in the presence of background noise for an accidental listener. It's also known as a signal with a low probability of intercept (LPI).

6) Maintaining communication privacy while additional listeners are present.

7) In radar and navigation, obtaining precise range (time delay) and range rate (velocity) readings.

Advantages and Disadvantages of Direct Sequence Spread Spectrum (DS-SS) System

The advantages of Direct Sequence Spread Spectrum are:

1. It is the most effective technology for combating deliberate interference (jamming).

2. It has a high level of anti-multipath signal discrimination. As a result, multipath reception interference is successfully reduced.

3. When compared to other systems, the DS-SS system outperforms them in the presence of noise.

Disadvantages of Direct Sequence Spread Spectrum are:

1. The output rate of the PN code generator must be high. The length of such a sequence must be sufficient to ensure that it is random.

2. The acquisition time using the serial search method is too long. As a result, the DS-SS system is slower.

Advantages and Disadvantages of Frequency Hopping Spread Spectrum (FH-SS) System

Some advantages of Frequency Hopping Spread Spectrum are:

1. The PG system has a larger processing gain than the DS-SS system.

2. The distance between two points has little effect on synchronization.

3. The acquisition time for the serial search system with FH-SS is shorter.

Some disadvantages of Frequency Hopping Spread Spectrum are:

1. The FH-SS system's bandwidth is excessive (in GHz)

2. Digital frequency synthesizers, which are complex and costly, are required.

CDMA - Digital Cellular System

CDMA - DIGITAL CELLULAR SYSTEM

The Digital cellular CDMA system is the main application of the spread spectrum technique. This CDMA digital cellular technology based on Direct Sequence (DS) spread spectrum will be discussed in depth here.

Qualcomm proposed and built this digital cellular communication technology. The Telecommunications Industry Association (TIA) has standardized and labeled it as Interim Standard 95 (IS-95) for usage in the 800 MHz and 1900 MHz frequency bands.

The forward link (or channel) between a base station and mobile receivers has a nominal bandwidth of 1.25 MHz. Signal transmission from mobile receivers to a base station takes place over a separate channel with a bandwidth of 1.25 MHz (reverse link or channel). DS Spread spectrum signals with a chip rate of 1.288 x 10⁶ chips per second (Mchips/s) are used in both the forward and reverse connections.

Forward link or channel

The Forward link or channel refers to the signal transmission from a base station to mobile receivers. The block diagram of the IS-95 forward link is shown in figure 1.

Source coding

A code-excited linear predictive (CELP) coder is used to code speech (source). It produces data at speeds of 9600, 4800, 2400, and 1200 bits per second. The data rate is determined by the user's speaking activity in 20ms frame intervals.

Channel coding

Rate = 1/2, constraint length K = 9 convolutional code is used to encode the data from the speech coder. The output symbols from the convolutional encoder are repeated for reduced speech activity. If the data rate is 4800 bits per second, the output symbols are repeated twice to keep the bit rate constant at 9600 bits per second.

                                                         Figure 1: IS-95 Forward Link

 

Block interleaver:

A block interleaver is used to transmit the encoded bits for each frame. It is important to minimize the effects of burst errors that may arise during channel transmission. The block interleaver's data bits arrive at a rate of 19.2kbits/s.

Symbol scrambler

Multiplication with the output of a long code (period N=2⁴²-1) generator scrambles the data bits from the block interleaver. The output of this generator is decimated by a factor of 64 to 19.2 kchips/s, even though it runs at a chip rate of 1.288M chips/s. On the forward and reverse links, the long code is used to uniquely identify a call from a mobile station.

Hadamard Sequence

A Hadamard (or Walsh) sequence of 64 characters is assigned to each channel user. Each base station is allotted 64 orthogonal Hadamard sequences. As a result, there are a total of 64 channels available.

A pilot signal is sent using a single Hadamard sequence. The channel properties, such as signal intensity and carrier phase offset, are measured using the pilot signal. For temporal synchronization, another Hadamard sequence is utilized. A different sequence might be used for paging (messaging). As a result, 61 channels are remaining to assign to different individuals. The data sequence is now multiplied by the Hadamard sequence that each user has been assigned.

Modulator

The resultant binary sequence is then multiplied by two PN sequences of length 215 and rate 1.2288 Mchips/s to spread it out. This technique generates in-phase signal components and quadrature. As a result, the binary data signal becomes a four-phase signal. After that, baseband spectral shaping filters are used to filter both the I and Q signals.

Various offsets of these PN sequences identify different base stations. The signals for all 64 channels are sent at the same time. Finally, a carrier wave is heterodyned using BPSK modulation and QPSK spreading. The CDMA signal is the total of the outputs.

Mobile receiver

A RAKE demodulator is utilized at the receiver to resolve the principal multipath signal components. Then, using the phase and signal strength estimations produced from the pilot signal, they are phase-aligned and weighted according to their signal strength. The Viterbi Soft decision decoder receives all of these components and combines them.

Reverse link or channel

The Reverse link or channel refers to the signal transfer from mobile transmitters to a base station. The block diagram of the IS-95 reverse link is shown in Figure.

Limitations

The signals transmitted from multiple mobile transmitters to the base station are asynchronous in the reverse connection. As a result, there is substantially greater user intervention. Furthermore, because mobile transmitters are often battery-powered, these transmissions are power constrained. To compensate for these two restrictions, we must create the reverse link.

                                                                Figure IS-95 Reverse link

 

Source coding

Data on the reverse link may be sent at rates of 9600, 4800, 2400, and 1200 bits per second. The data rate is determined by the user's speaking activity in 20ms frame intervals.

Channel coding

Rate = 1/3, constraint length K=9 convolutional code is used to encode the data from the speech coder. In a fading channel, this coder has a larger coding gain. This compensates for the constraints described above.

The output bits from the convolutional encoder are repeated two, four, or eight times for decreased speech activity.

Block interleaver

A block interleaver is used to pass the encoded bits for each frame. It is required to mitigate the consequences of burst mistakes. The 576 encoded bits are block-interleaved for each 20ms frame. The coded bit rate, on the other hand, is 28.2 kbits/s.

Hadamard sequence

An M=64 orthogonal signal set with 64-length Hadamard sequences is used to modulate the data. As a result, each of the 64 Hadamard sequences is mapped to a 6-bit block of data. At the modulator's output, this results in a bit (or chip) rate of 307.2 kbits/s.

Symbol scrambler

The time position of the transmitted code symbol repeats is randomized to prevent interference to other users. As a result, at lower speaking activity levels, subsequent bursts are not uniformly separated in time.

The output of the long code generator, which runs at a pace of 1.2288 Mchips/s, likewise spreads the signal. This is done for secrecy, scrambling, and spreading, as well as channelization (addressing).

Modulator

The multiplier's output binary sequence of 1.2288 Mchips/s is then multiplied by two PN sequences of length N=2¹⁵ at a rate of 1.2288 Mchips/s. This procedure generates in-phase signals and quadrature. Baseband spectral shaping filters are used to filter both the I and Q signals.

Before the baseband filter, the Q-channel signal is one-half PN chip time delayed from the I-channel signal. An offset QPSK signal emerges from the output of the two baseband filters. Finally, quadrature mixers receive the filtered signals. The CDMA signal is the total of the outputs.

Base station Receiver

To receive the signals of each active user in the cell, the base station allocates a different channel. Although the chips are sent as an offset QPSK signal, the base station receiver's demodulator uses non-coherent demodulation. In the demodulation procedure, a quick Hadamard transform is applied to decrease computing complexity. The demodulator's output is then passed into the Viterbi detector, which synthesizes the speech signal.

Synchronization in Spread Spectrum Systems

Synchronization in Spread Spectrum Systems

Need for Synchronization

Synchronization is the process of ensuring that the locally produced carrier at the receiver is in frequency and phase synchronism with the carrier at the transmitter. For successful functioning in spread spectrum communication systems, the transmitted and received PN codes must be perfectly matched.

(i) Both the carrier frequency and the PN clock can drift with time.

(ii) If the transmitter and receiver are moving relative to one other, as in mobile and satellite spread spectrum systems, the carrier and PN clocks will experience Doppler frequency shift.

As a result, the receiver's PN sequence must be synchronized with the transmitter's.

Synchronization steps:

The synchronization of the locally generated spreading signal with the incoming spread spectrum signal normally takes two phases. They are:

1) Acquisition: Bringing the two spreading signals into coarse alignment with one another implies the acquisition stage.

2) Tracking: After acquiring the received spread-spectrum signal, the second phase, tracking, is used to fine-tune the alignment.

The feedback loop is used in both acquisition and tracking.

Acquisition:

There are three types of acquisition schemes. They are

1) Serial search acquisition

2) Parallel search acquisition

3) Sequential search acquisition

1. Serial search acquisition:

A) DS Spread spectrum systems:

In Direct Sequence spread spectrum systems, the serial search scheme is shown in Figure 1.

Between the transmitter and the receiver, there is always some initial timing ambiguity. Assume the transmitter contains N chips and each chip has a T_c duration. It's important to linger for T_d=NT_c to verify synchronism at each time instant if initial synchronization is to take place in the face of additive noise and other interference. In(coarse) time steps of 1/2 T_c, we search throughout the time uncertainty interval.

Figure 1 Direct Sequence spread spectrum systems – Serial Search Acquisition

The incoming PN signal and the locally produced PN signal are linked. The output signal is compared to a predefined threshold at specified search intervals of NT_c (search dwell time). The locally produced code signal is advanced in time by 1/2T_c seconds if the output is less than the threshold. The procedure of correlation is carried out once more. These actions continue until a signal is detected or a threshold is reached. After then, it's presumed that the PN code was obtained.

As a result, assuming the initial misalignment between the two codes was n chips, the entire acquisition time is equal to

T_acq = 2nNT_c seconds

B) FH spread spectrum systems

The serial search strategy for frequency hopping spread spectrum systems is shown in Figure 2.

A mixer is followed by a bandpass filter (BPF) and a square-law envelope detector in this non-coherent matched filter. The frequency hopper is controlled by the PN code generator. When the local hopping matches that of the received signal, the acquisition is finished.

Let f_i be the frequency of the transmitter's frequency synthesizer. Assume that f_j is the frequency of the signal generated by the frequency synthesizer in the receiver's acquisition circuit. If f_i ≠ f_j, the output of BPF will only create a little voltage less than the threshold. If f_i = f_j at a later time during the search, a huge voltage above the threshold will be created at the BPF output. This means that local hopping is matched with the received signal.

Figure 2 Frequency hopping serial search acquisition

2. Parallel search acquisition

By running two or more correlators in parallel, the parallel search acquisition approach adds a level of parallelism to the process. They'll look for periods that don't overlap. The search time is lowered with this system, but the implementation is more complex and costly.

3. Sequential search acquisition

The dwell time at each delay in the search process is made variable using a correlator with a variable integration period whose (biased) output is compared to two thresholds in this method. As a result, the sequential search strategy produces a more efficient search by reducing the average search time.

Tracking

The initial search process is halted after the signal has been acquired, and fine synchronization and tracking may start. The PN code generator at the receiver is kept in sync with the incoming signal thanks to the tracking. Fine chip synchronization and carrier phase tracking are both included in tracking for coherent demodulation.

A) DS Spread spectrum system:

The Delay-locked loop (DLL) is a common tracking loop for a Direct sequence spectrum signal, as shown in Figure 3.

Figure 3: Delay-Locked Loop (DLL) for PN code tracking

The received DS spread spectrum signal is applied to two multipliers at the same time. The PN code for one of the multipliers is delayed by a fraction of the chip interval (δ). The identical PN code is sent to the other multiplier, but it is advanced by δ. Each multiplier's output is sent into a BPF centered on f₀.

Each BPF's output is envelope detected and then subtracted. The loop filter, that drives the voltage-controlled oscillator receives this difference signal. The PN code generator uses the VCO as a clock. If the synchronization isn't perfect, one correlator's filtered output will surpass the other. As a result, the VCO will be advanced or postponed properly. The two filtered correlator outputs will be equally displaced from the peak value at the equilibrium point. The output of the PN code generator will then be perfectly synchronized with the received signal supplied into the demodulator.

B) FH Spread spectrum system:

Figure 4 shows a common tracking method for FH spread spectrum communications.

The received signal and the receiver clock have a minor timing error, despite the initial acquisition is successful. The bandwidth of the BPF is on the order of 1/T_c, where T_c is the chip interval, and it is tuned to a single intermediate frequency. Its output is envelope detected before being multiplied by the clock signal to provide a three-level signal. The loop filter is activated by this.

Figure 4: Tracking loop for FH signals

Assume that the chip transitions from the locally generated sinusoidal waveform do not coincide with the incoming signal transitions. The loop filter's output will thus be either positive or negative, depending on whether the VCO is trailing or ahead of the input signal's time. The control signal for changing the VCO timing signal to drive the frequency synthesizer output to appropriate synchronization with the received signal will come from the error signal from the loop filter.