PRJ 014: Tracking of FHSS Signals

PRESENTATION BY IAN ODHIAMBO OTIENOREG. NO: F17/1409/2011

FHSS• The information signal is transmitted at multiple carrier frequencies

• The sequence of the carriers is pseudorandom in nature• The choice of carriers is controlled by a code• The interference problem is avoided by moving to a new frequency

channel• The bandwidth of the system is the entire range of the carriers averaged

over time• Two implementations

• Slow Frequency Hopping (SFH)• Fast Frequency Hopping (FFH)

Spread Spectrum Characteristics

• Signal must occupy much larger bandwidth than needed to transmit information signal

• Spreading is accomplished using a pseudo random sequence called a code. The sequence is independent of the information signal

• Receiving the signal requires correlation between the received spread signal with a synchronised replica of the code

Spread Spectrum Uses and Advantages

• Spread Spectrum found early application in military the advantages are military oriented but still useful for civilian applications

• Preventive measure against jamming and interference• Concealing a signal (LPI- Low Probability of Intercept)• Providing means to target a message towards a recipient despite presence of

other listeners• Allows transmitters to radiate higher total powers while staying within flux

density limits set by regulators

Synchronisation

• Consists of twin tasks• Acquire the signal• Maintain the acquisition

• Since spread spectrum uses a PN sequence to prevent interference, the receiver needs to know the position of the sequence it needs to be on to recover the transmitted signal

• Ideal situation requires• Receiver know exact time instance transmitter began sending the signal• The delay between transmitter and receiver• The Frequency of transmitter and receiver be exactly in sync

Acquisition

• Before tracking occurs, the signal must be brought into coarse alignment. This process is called acquisition.

• There are some general strategies• Both systems pick a channel, stay on it until they hear from each other, start hopping• Receiver waits on a channel, as soon as it detects transmission, starts hopping• Transmitter waits on a channel, receiver keeps hopping, transmitter gives receiver

enough time to detect transmission and hop before beginning to hop• Both receiver and transmitter hop through sequence with receiver spending longer

on each hop listening for transmitter, as soon as transmission is detected they hop in sync

Acquisition Methods

• In general the Acquisition methods are • Parallel Search• Serial Search

• Parallel search is faster than serial search, implementation is more complex

Parallel Search Serial Search

What is Tracking

• In FHSS the term tracking refers to Code tracking• Bringing the code at the receiver into fine alignment with code at transmitter• Maintaining the code at transmitter and receiver in sync

• The tracking system corrects for clock timing instabilities and carrier frequency offsets

• With transmitter and receiver locally generated codes in sync, hopping and de-hopping operations occur in sync and transmission of signal is successful

FHSS System Overview

Tracking Method• The tracking scheme is an adaptation of the Early-late gate method.

• This method relies on the fact that if either signal is out of sync, a control signal is generated, if synchronism is achieved, the control signal returns to its mean

Implementation of Tracking Loop

Matched filter energy detector model

Delayed transmitter hopping pattern

Model waveforms for delayed transmitter code

Advanced transmitter hopping pattern

Model waveforms for advanced transmitter code

Code Sequences brought into sync receiver lagging

Code Sequences brought into sync receiver leading

Transmitter code clock frequency is 10Hz behind

Transmitter code clock frequency is +10Hz ahead

Conclusion

• The speed of the tracking system is reliant on the VCC sensitivity (40Hz/V) and bandwidth of Lowpass filter.

• Tracker allows higher signal energy to be received. Raises accuracy of recovering data

• Without tracker, clocks controlling PN sequence will go out of sync in presence of carrier frequency offsets