Study On Frequency Hopping Spread Spectrum Computer Science Essay

Spread spectrum (SS) is a wireless modulation technique in which the bandwidth (BW) and the power of the original information/data signal is augmented by spreading it and thus providing it immunity against electrical noise, multipath distortion, and makes jamming and interference more difficult. There are two types of SS; Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). This report discusses only FHSS. In FHSS, the data signal is hopped from one frequency to another after a specific interval of time. FHSS makes use of randomly generated chipping/spreading code to select the frequency sequence on which the signal hopes. BPSK is used in conjunction with FHSS spreader to convert data signal from digital to analog signal. Sender and receiver are in synchronization with respect to each other for carrier frequencies sequence. There are many advantages and applications of FHSS in wireless communication. The basic operation using Matlab, advantages and application of FHSS are discussed in this report.

Introduction

Spread spectrum (SS) is a wireless modulation technique in which the bandwidth (BW) and the power of the original information/data signal is increased by spreading it and thus making it resistant to electrical noise, diffusion, multipath distortion and making jamming and interference more difficult.

In SS the digital or analog data signal is transmitted through an analog signal by first performing BPSK operation (in case of digital data signal).

Typically there are two types of SS. The first type developed was Frequency Hopping Spread Spectrum (FHSS). It was developed for military and intelligence purposes. In FHSS the data signal is hopped on different frequencies for a particular period of time. The most recent type is Direct Sequence Spread Spectrum (DSSS). In DSSS the data signal is spread onto a wide bandwidth signal by making it a high power signal. In this report only FHSS is discussed.

Frequency Hopping Spread Spectrum

Frequency hopping spread spectrum is a Spread Spectrum modulation technique in which the data signal is transmitted over different frequencies. The data signal is hopped from one frequency to another frequency for a specific interval of time.

In FHSS modulation technique a perfect synchronization is there between transmitter and receiver.

FHSS Operation

In FHSS different frequency channels are used and the data signal is hopped from one frequency channel to the other frequency channel after a specific interval of time. The data signal remains on a specific channel for a particular period of time (300 ms, IEEE standard) [3], and then hopped to the next channel.

The sequence of the frequency channels is created through spreading/chipping code which is generated by pseudorandom number generator. At the receiver end, same chipping/ spreading code is used to demodulate the signal. Hence there is a one to one synchronization at both ends.

For i bits of chipping code there are at most 2i frequency channel, among which the data signal can be hopped. After a particular period of time the signal is hopped from that frequency channel to the other channel.

Figure 1: Frequency Channel Selection (i=2) [2]

Figure 2: Frequency Channel Assignment [2]

FHSS Modulation (Transmitter end)

At transmission end, first the digital data signal d(n) is converted to the analog data signal d(t) using BPSK modulation technique [2]. It is convenient to take data signal values as -1 and 1 instead of 0 and 1.

(1)

t = bit duration

fo = frequency of data signal

aj = value of jth bit of data signal

= frequency separation

Then the signal is passed through the FHSS spreader. In FHSS spreader a PN generator is there to produce chipping/spreading code. Hopping frequencies pattern is directly a function of chipping/spreading code. Then frequency table is created and then frequency channels are selected. In FHSS spreader, the data signal d(t) is multiplied with a particular channel frequency. The BW of the frequency channels are the same as that of the data signal.

Figure 3: FHSS Transmitter [2]

After multiplying the BPSK modulated signal with c(t) = cos2Ï€fit, the overall signal becomes;

(2)

(3)

By using the identity

Equation (2) becomes;

}+}] (4)

Then, this signal is passed through the band pass filter to filter out the negative sign frequency. The overall signal becomes, [2]

Sd(t) } (5)

This signal is transmitted into the air.

The following figure shows the Matlab results for the FHSS modulation.

Figure 4: FHSS Modulation for Digital Input

The spectrum analysis of the input data signal, carrier signal and the FHSS modulated signal is shown in the figure below:

Figure 5: Spectrum analysis of signals

FHSS Receiver

At the receiver side first the signal is passed through the FHSS despreader. For despreader the same chipping/spreading code is used as at the transmitter side. The operation is described through the following equations;

s(t) (6)

(7)

By using the identity

(8)

[2]. At this step the positive frequency component is stopped by the band pass filter and only the other component is passed through the filter. And hence the original data signal is recovered. The following figure explains the operation at receiver side.

Figure 6: FHSS Receiver

FHSS Advantages:

It is quite difficult to intercept the FHSS modulated signal [1]. A third party can only intercept the receiver if they have the same chipping/spreading code as at the transmitting side.

With FHSS modulation, the same frequency channel can be shared among different users with little interference. This is an efficient use of BW.

FHSS modulated signals appear as noise to unknown receiver, because the signal is hopped between different frequencies.

Through FHSS modulation the signal has immunity power against the diffusion, electrical noise and multipath distortion.

FHSS modulation makes the jamming more difficult.

FHSS is highly robust modulation technology.

FHSS Applications:

FHSS modulation technique is used for military purposes. Cryptographic algorithms are used to generate the chipping/spreading code, which is shared between the sender and receiver [1].

FHSS modulation is used in wireless LANs (WLAN). According to IEEE standards, FHSS works almost over 97 frequencies. The range of frequencies is from 2.402GHz to 2.480GHz [6].

FHSS modulation is used in Global Positioning System (GPS). GPS has three segments; control segment (CS), space segment (SS) and user segment (US). There is wireless communication between control CS and SS and then SS and US. And in this wireless communication FHSS modulation is used [6].

Figure 7: GPS System

FHSS is used in IS-95. It is a digital cellular radio system using CDMA technique for communication of voice

FHSS modulation is also used in Bluetooth technologies.

Conclusion

FHSS is a Spread Spectrum modulation technique in which the data signal is hopped from one frequency to the other frequency after a specific interval of time, by making the signal resistant to noise, diffusion, multipath distortion, jamming and interception and makes an efficient use of frequency channels. Now a days FHSS is being used in many wireless applications like WLANs, Bluetooth, GPS and military purposes. Much advancement has also being done with this technology.

References

[1] “Frequency Hopping Spread Spectrum” online at <Frequency-hopping spread spectrum – Wikipedia, the free encyclopedia.htm>

[2] William Stallings, Data and Computer Communications, 8th Edition.

[3] Sorin M. SCHWARTZ, “Frequency Hopping Spread Spectrum (FHSS) vs. Direct Sequence Spread Spectrum (DSSS) in Broadband Wireless Access (BWA) and Wireless LAN (WLAN)”

[4] “FHSS” online at <What is FHSS – A Word Definition From the Webopedia Computer Dictionary.htm>

[5] “Frequency-Hopping Spread Spectrum” online at <What is frequency-hopping spread spectrum – Definition from Whatis_com.htm>

[6] ir .J. Meel ” Spread Spectrum (SS)Applications”

[7] Chip (CDMA)” online at <Chip (CDMA) – Wikipedia, the free encyclopedia.htm>

[8] “What is Frequency Hopping Spread Spectrum” online at < What is frequency hopping spread spectrum – Networking & Security.htm”