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Wednesday, 12 January 2022

Advantages, Disadvantages and Applications of Spread Spectrum

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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.

Friday, 7 January 2022

CDMA - Digital Cellular System

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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.

Monday, 3 January 2022

Synchronization in Spread Spectrum Systems

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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 Tc duration. It's important to linger for Td=NTc 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 Tc, 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 NTc (search dwell time). The locally produced code signal is advanced in time by 1/2Tc 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


Tacq = 2nNTc 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 fi be the frequency of the transmitter's frequency synthesizer. Assume that fj is the frequency of the signal generated by the frequency synthesizer in the receiver's acquisition circuit. If fi ≠ fj, the output of BPF will only create a little voltage less than the threshold. If fi = fj 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/Tc, where Tc 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.

Tuesday, 28 December 2021

Fast Frequency Hopping Spread Spectrum

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Fast Frequency Hopping Spread Spectrum Block Diagram:


Fast-hopped signals occur in the FH system when there are multiple hops per symbol. As a result, the hop rate Rh is an integer multiple of the MFSK symbol rate in fast-frequency hopping. During the transmission of a single symbol, the carrier frequency will hop or shift numerous times. As a result, each hop is a chip in a fast FH-MFSK system.


                                                     Figure 1: Fast FH-MFSK demodulator


In general, fast frequency hopping is used to overcome a smart jammer's technique, which entails two steps: measuring the spectral composition of the broadcast signal and returning the interfering signal to the same frequency range.


Before the jammer can execute these two operations, the transmitted signal must be jumped to a new carrier frequency.


Non-coherent detection is used to recover data at the receiver. The detecting process, on the other hand, differs from that utilized in a slow FH-MFSK receiver. A typical rapid FH-MFSK demodulator is seen in Figure 1.


The signal is first de-hopped using a PN generator that is identical to the one used in the transmitter. After that, a low pass filter with a bandwidth equal to the data bandwidth is used to filter the signal. A bank of 'M' envelope detectors is used to demodulate the filtered signal.


A clipping circuit and an accumulator follow each envelope detector. In the event of a deliberate jammer or other powerful unanticipated interference, the clipping circuit plays a crucial role. On a chip-by-chip basis, the demodulator does not make symbol judgments. The energy from the N chips is gathered and stored. After adding the energy from the Nth chip to the energy from the N-1 preceding ones, the demodulator chooses the symbol that corresponds to the accumulator with the most energy.

Friday, 24 December 2021

Slow Frequency Hopping Spread Spectrum

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Slow Frequency Hopping Spread Spectrum:


We have a slow-hopped signal in the FH system when the hopping is done at the symbol rate. As a result, the symbol rate Rs of the MFSK signal is an integer multiple of the hop rate Rh when using slow-frequency hopping, indicating that many symbols are transmitted on each frequency hop.


Transmitter:


The block diagram of a slow-frequency hopping FH-MFSK transmitter is shown in Figure 1.


The incoming binary data is first sent via an M-ary FSK modulator. A Mixer receives the resultant M-ary FSK modulated signal. The Mixer is made up of a multiplier and a bandpass filter (BPF).

Figure 1: FH-MFSK Transmitter


A digital frequency synthesizer provides the other input to the mixing block. A PN code generator controls the frequency synthesizer. As a result, a carrier created by the frequency synthesizer modulates the M-ary FSK modulated signal once again. The sum and difference frequencies are produced by the Mixer in two outputs. Only the sum frequency signal, which is the FH-MFSK signal, is selected by the bandpass filter that follows the mixer. Then the signal is transmitted.


• M symbols can be transmitted using the M-ary FSK system, where M=2K. The number of bits in the input binary data that make up one symbol is denoted by k.


• Each of these M symbols will be assigned a different frequency by the M-ary FSK modulator.


• The frequency hop is the output of the synthesizer at a specific point in time.


• The PN generator's output bits vary at random. As a result, the synthesizer's output frequency will fluctuate at random.


• To create the transmitted signal, each frequency hop is combined with the MFSK signal.


• If the number of consecutive bits at the PN generator's output is n, the total number of frequency hops is 2n.


• The sum of all frequency hops determines the entire bandwidth of the transmitted FH-MFSK signal. As a result, the transmitted FH-MFSK signal has an extremely wide bandwidth, on the order of a few GHz.


Receiver:


A block diagram of a slow-frequency hopping FH-MFSK receiver is shown in Figure 2.

Figure 2: FH-MFSK Receiver


• The received signal is sent into the Mixer as an input. The digital frequency synthesizer provides the mixer with the other input.


• A PN code generator powers the frequency synthesizer. The PN code generator at the transmitter is synced with this generator.


• As a result, the frequency hops generated at the synthesizer output will be the same as those produced at the transmitter.


• The sum and difference frequencies are produced by the mixer in two outputs. Only the difference frequency, which is the MFSK signal, is selected by the bandpass filter. As a result, the frequency hopping is eliminated by the mixer.


• After then, the MFSK signal is sent into a non-coherent MFSK detector. For non-coherent MFSK detection, a bank of M non-coherent matching filters is utilized. 


Each matching filter corresponds to one of the MFSK signal's tones. By selecting the greatest filter output, an approximation of the original symbol sent may be derived.


• For an FH/MFSK system,


(i) The chip rate, Rc = max (Rh, Rs) ------------------- (1)

where Rh is the hop rate and Rs is the symbol rate


(ii) The transmission of several symbols per hop characterizes a slow FH/MFSK system. As a result, in a sluggish FH/MFSK system, each symbol is referred to as a chip.


(iii) We can relate all rates as

Rc = Rs = 𝑅𝑏/𝑘 ≥ Rh ----------------- (2)

where k = log2𝑀


(iv) Processing gain, PG = Bandwidth of Spread signal / Bandwidth of unspread signal

Let fs be the symbol frequency and 2n be the number of frequency hops

Then, Processing gain, PG = 2𝑛𝑓𝑠/𝑓𝑠 = 2n -------------- (3)


(v) Probability of error, Pe = ½ 𝑒𝑟𝑏𝑅𝑐/2 ---------------- (4)

Thursday, 16 December 2021

Frequency Hopping Spread Spectrum Systems

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FREQUENCY-HOPPING SPREAD SPECTRUM SYSTEMS (FH-SS)


The use of a PN sequence to modulate a phase shift keyed signal allows immediate spreading of the transmission bandwidth in Direct sequence spread spectrum systems (DS-SS). A different technique that can be used is the frequency hopping spread spectrum (FH-SS) system. The transmitted signal's spectrum is spread sequentially in FH-SS by randomly hopping the data modulated carrier from one frequency to the next.


Thus, a Frequency-hopped Spread Spectrum (FH-SS) system is a type of spread spectrum in which the carrier hops from one frequency to another at random.


Basic Principle


The available channel bandwidth in an FH-SS communication system is split into a large number of contiguous frequency slots. The transmitted signal occupies one or more of the available frequency slots at each signalling period. The frequency slots in each signalling interval are chosen pseudorandomly from a PN generator's output. Figure 1 shows a specific FH pattern in the time-frequency plane.

Figure 1: Example of a Frequency - Hopped (FH) Pattern


Reason for employing M-ary FSK modulation


M-ary frequency shift keying is a popular modulation scheme for FH systems (MFSK). The combination is simply referred to as FH/MFSK. Although PSK modulation performs better than FSK in the AWGN channel, maintaining phase coherence in 


Although PSK modulation performs better than FSK in the AWGN channel, maintaining phase coherence in


(i) The synthesis of frequencies employed in the hopping pattern 

(ii) the signal's propagation through the channel as it hops from one frequency to another over a large bandwidth.


With FH spread spectrum transmissions, FSK modulation with non-coherent detection is typically used.


Types of Frequency hopping


We consider the rate at which the hops occur since frequency hopping does not cover the complete spread spectrum instantaneously. Frequency-hopping may be broadly classified into two types (both of which are technology-independent). They are namely:


1) Slow-frequency hopping

2) Fast-frequency hopping


Slow-frequency hopping:


We have a slow-hopped signal in the FH system if the hopping is performed at the symbol rate. As a result, with slow-frequency hopping, the MFSK signal's symbol rate Rs is an integer multiple of the hop rate Rh, implying that various symbols are transmitted per each frequency hop.


Frequency hopping example:


An example of frequency hopping is shown in Figure 2.

Figure 2: Example of Frequency Hopping


• The input binary sequence data rate is: Rb=150bits/s

• The modulation is 8-ary FSK.

• Then the symbol rate is Rs=𝑅𝑏/𝑘 = 150/log28 = 50 bits/s

• The symbol interval is Ts = 1/𝑅𝑠 = 1/50 = 20ms

• The frequency is hopped once per symbol. Thus the hopping rate is given as Rh=50hops/s.

• The abscissa (x-axis) of the figure's time-bandwidth plane represents time, while the ordinate (y-axis) denotes hopping bandwidth.

• There is an 8-ary FSK symbol-to-tone mapping available. The data band's non-fixed centre frequency is designated as f0.

• The tone separation is Δf = 1/𝑇𝑠 =1/20𝑚𝑠= 50Hz.

• At the top, there is a normal binary data sequence. The bits are grouped three at a time to generate symbols since the modulation is 8-ary FSK.

• According to symbol-to-tone assignment, a single-sideband tone (offset from f0) would be transmitted.

• f0 hops to a new position in the hopping bandwidth for each new symbol. f0+25Hz assignment is done for the first symbol in the data sequence 011. f0 is shown with a dashed line and the symbol tone f0+25Hz is shown with a solid line in the figure.

• Likewise, f0 - 125Hz assignment is done for the second symbol 110. For the third symbol 001, f0 + 125Hz assignment is done. The centre frequency f0 hops to a new position for each symbol.


Frequency hopping with diversity:


Robustness is the capacity of a sent signal to withstand channel impairments such as noise, jamming, fading, and so on in communication. A signal containing many duplicate copies, each delivered on a different frequency, has a better chance of surviving than a single signal of the same type.


Multiple broadcasts of the same signal at various frequencies that are spaced apart in time are referred to as diversity. The higher the signal's variety, the more resistant it is to random interference.


We may extend the frequency hopping example illustrated in Figure 2 to show the beneficial effect of diversity. A chip repeat factor of N=4 is used to introduce frequency hopping diversity. The effect of diversity is seen in Figure 3.

Figure 3: Frequency hopping with diversity (N =4)


• There are now four columns for each 20ms symbol interval, corresponding to the four distinct chips to be transmitted for each symbol.

• Each symbol is now transmitted four times. The centre frequency f0 is hopped to a new part of the hopping band for each transmission.

• The chip interval is Tc = 𝑇𝑠/𝑁 = 20𝑚𝑠/4 = 5ms.

• The hopping rate is Rh= 𝑅𝑏/log28 . N = 150 x 4/3 = 200 hops/s.

• To obtain orthogonality, the spacing between frequency tones must also change. Hence the tone separation is Δf = 1/𝑇𝑠 .N =4/20𝑚𝑠= 200Hz.

• Thus, the resulting transmissions provide a more robust signal than those that do not have such diversity.


Fast-frequency hopping:


If there are multiple hops per symbol in the FH system, we have a fast-hopped signal. As a result, the hop rate Rh in fast-frequency hopping is an integer multiple of the MFSK symbol rate. During the transmission of a single symbol, the carrier frequency will vary or hop numerous times. Thus, each hop in a fast FH-MFSK system is a chip.


Advantages of the FH-SS system:


1. The PG ( processing gain) is higher than the DS-SS system's processing gain.

2. The distance between two points does not affect synchronisation.

3. The acquisition time for a serial search system using FH-SS is reduced.


Disadvantages of the FH-SS system:


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

2. Digital frequency synthesisers, which are complex and expensive, are required.


Applications of FHSS system:


1) For mobile communication, CDMA systems based on FH spread spectrum signals are particularly attractive.

2) The Wi-Fi standard for wireless local area networks (WLAN).

3) The Bluetooth WPAN (Wireless Personal Area Network) standard


Fast hopping Versus Slow hopping:


The table compares the performance of fast hopping and slow hopping systems.


SI No.

Slow frequency hopping

Fast frequency hopping

1.

For each frequency hop, more than one symbol is transmitted.

To transmit one symbol, multiple frequency hops are needed.

2.

Chip rate and symbol rate are equal.

The chip rate is greater than the symbol rate.

3.

The symbol rate is larger than the hop rate.

 

Hop rate is bigger than Symbol rate.

 

4.

One or more symbols are transmitted using the same carrier frequency.

In different hops, one symbol is transmitted over multiple carriers

5.

If the carrier frequency in one hop is known, a jammer can detect this signal.

Since one symbol is transmitted using multiple carrier frequencies, a jammer will be unable to detect it.

 

The following two examples can be used to compare slow and fast hopping performance:


1) Figure 4 shows the chip as part of an FH-MFSK system.

 

Figure 4: Chip in the context of an FM-MFSK System


• A fast frequency hopping example is shown in Figure 4 (a). The data symbol rate is 30 symbols per second, and frequency hopping is 60 hops per second. The waveform s(t) over one symbol time (1/30s) is shown in the figure. A new frequency hop is responsible for the waveform shift in (the middle of) s(t).

• A slow frequency hopping example is shown in Figure 4 (b). Although the data symbol rate remains at 30 symbols per second, the frequency hopping rate has been lowered to 10 hops per second. The waveform s(t) is shown across three symbols (1/10s).


2) The comparison for a binary FSK system is shown in Figure 5.


• For a binary FSK system, Figure (a) shows an example of rapid frequency hopping. N=4 is the diversity. Per bit, four chips are transmitted. The hop duration is the chip duration.

 

Figure 5: Comparison for a binary FSK system


• Figure (b) shows an example of a binary FSK system of slow frequency hopping. In this case, three bits are transferred throughout the time of a single hop. In this case, the bit duration is the chip duration.