SLOW FREQUENCY HOPPING SPREAD SPECTRUM ANIL SHARMA 11003973 RE1E01A13 B-TECH (ECE)

Abstract: FHSS is a radio transmission process where user information is sent on a radio channel that regularly changes frequency according to a predetermined code. The FHSS technique is useful for suppressing interference, making interception difficult, accommodating fading and multipath channels, and providing a multiple-access capability.

Introduction:

Spread-spectrum techniques are methods by which energy generated in a particular bandwidth is deliberately spread in the frequency domain resulting in a signal with wider bandwidth. Spread-spectrum telecommunications is a signal structuring technique that employs direct sequence, frequency hopping or a hybrid of these , which can be used for multiple access and or multiple functions. This technique decreases the potential interference to other receivers while achieving privacy. Frequency - hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channel , using a pseudorandom sequence known to the both transmitter and receiver. Spread-spectrum signals are highly resistant to narrowband interference.

Frequency Hopping :The type of spread spectrum in which the carrier hops randomly from one frequency to another is called a frequency hopping spread spectrum (FHSS) Frequency hopping was the first used for military electronic counter measures,because the transmitted signal that

uses frequency hopping is difficult to detect and monitor.

In an FHSS system the signal frequency is constant for specified time duration, referred to as a hop period 𝑇ℎ. The hop period is the time spent in transmitting a signal in a particular frequency slot of bandwidth 𝐵 ≪ 𝑊 , where 𝑊 and 𝐵 are spread and symbol bandwidth , respectively . The sequence of carrier frequencies is called the frequency hopping pattern.

The set of 𝐿 possible carrier frequencies {𝑓1, . ,𝑓𝐿} is called the hopset. The rate at which the carrier frequency changes is called the hop rate . The carrier frequencies are changed periodically. Hopping occurs over a frequency band called hopping band that includes 𝐿 frequency channels.

Hopping is typically done in a pseudo-random manner. Each frequency channel is defined as a spectral region that includes a single carrier frequency of the hopset as its center frequency and has a bandwidth 𝐵 large enough to include most of the power in a signal pulse with a specific carrier frequency. Fig. 1 illustrates the frequency channels associated with a particular frequency hopping pattern.

a)Transmitter

b) Receiver

Fig.shows the block diagram of FH Transmitter ,which involves frequency modulation followed by mixing. First the incoming binary data are applied to an M-ary FSK modulator. The resulting modulated wave and output from frequency synthesizer are then applied to a mixer that consists of a multiplier followed by band pass fliter .The filter is designed to select sum frequency component resulting from multiplication process as the transmiited signal.

In particular, successive k bit segments of PN sequence drive frequency synthesizer ,which enable the carrier frequency to hop over 2^k distinct values .On a single hop ,the bandwidth of transmitted signal is same as that resulting from use of conventional MFSK with alphabet of M = 2^K orthogonal signals. However for complete range of 2^k frequency hops , the transmiited FH signal occupies much larger bandwidth . In the Receiver ,the frequency hopping is first removed by mixing the received signal

with the output of local synthesizer that is synchronously controlled in same manner as that in transmitter. The resulting output is than band pass filtered and subsequently processed by non coherent M ary FSK detector.

There are two kinds of frequency hopping:*Fast Hopping:

*Slow Hopping:

Slow Frequency Hopping: It is one in which symbol rate of MFSK Signal is integer multiple of hop rate. That is several symbols are transmitted on each frequency hop .several user bits per frequency (SFH). In this case one or more data bits are transmitted within one hop. An advantage is that coherent data detection is possible. Often, systems using slow hopping also employ (burst) error control coding to restore loss of (multiple) bits in one hop. Fast Frequency Hopping: It is one in which hop rate of MFSK Signal is integer multiple of symbol rate. That is carrier frequency will change several times during the transmission of one symbol. several frequencies per user bit (FFH) One data bit is divided over multiple hops. In fast hopping, coherent signal detection is difficult, and seldom used. Mostly, FSK or MFSK modulation is used. Obviously the slow frequency hopping and fast frequency hopping are converse of each other.

Slow frequency Hopping overview: An individual FH/ MFSK tone of shortest duration is referred to as a chip.The chip rate Rc for an FH/ MFSK system is defined by Rc = max(Hope rate, symbol rate) A slow FH/MFSK signal is characterized by having multiple symbols transmiited per hop. Hence ,each symbol of slow FH/MFSK siganl is a chip. Corresponding ,in a slow FH/MFSK system , the bit rate of incoming binary data ,the symbol rate of the MFSK signal and chip rate and hop rate are related by

At each hop, MFSK tones are separated in frequency by integer multiple of chip rate

ensuring their orthogonality.

Fig: a)frequency variation from one complete period of PN sequence b) Variation of dehopped frequency with time

Description of Slow Frequency Hopping:

First, let us consider the case where 𝑇𝑐 > 𝑇𝑠, which is called slow frequency hopping. A slow FH/MFSK signal is characterized by having multiple symbols transmitted per hop. Hence, each symbol of a slow FH/MFSK signal is a chip. We impose the constraint that 𝑇𝑐 = 𝑁𝑇𝑠 for slow frequency hopping.

The operation of a slow FHSS system with 4-FSK modulation (𝑀 = 4), 6 hop

frequencies(𝐿 = 6), and 4 symbols per hop(𝑇𝑐 = 4𝑇𝑠, 𝑁 = 4)

Fig:Time frequency plot for slow frequency hopping

Fig: Time frequency plot after dispreading

Power spectral density of Slow Frequency Hopping :

To obtain the power spectral density (PSD) of the 𝑀-ary FHSS signal, we model the phases 𝜙𝑘 and 𝜙𝑙 as independent identically distributed random variables uniformly distributed on [0,2𝜋). The hopping frequencies 𝜔𝑙 are modeled as independent identically distributed random variables taking values from the set {𝜔𝑐0,𝑐1,…,𝜔𝑐𝐿−1} with equal probabilities. FSK symbol frequencies 𝜔𝑘 are independent identically distributed random variables taking values from the set {𝜔𝑠0,𝑠1,…,𝜔𝑠𝑀−1} with equal probabilities.

Merits:i) Less affected by Near-Far problem

ii) Better for avoiding jamming

iii) Less affected by multi-access interference

Demerits:i) Needs FEC ii) Frequency acquisition may be difficult

References:

1) Communication system by haykin 4 edition 2) wireless communication by rapport.

3) Design and simulation of frequency hopping technique in matlab research paper P.olsovsky, p.podhoransky

4)Improvement in the Spread Spectrum System in DSSS, FHSS, AND CDMA