Novel Low Cost Synchronisation Network For Spread Spectrulyi Systelvis

Abstract

Spread Spectrum systems are found in many flavours, used in many applications and have

existed since the early days of radio communications. The properties of spread spectrum do

however place restrictions on the design, and often make the implementation expensive and

complex. When using spread spectrum to provide a basic communications infrastructure,

many factors need to be considered. These indude supplying the appropriate technology

at the right cost. To achieve this a trade-off against performance is often required.

One of the more difficult aspects of Spread Spectrum design is the synchronisation of the

spreading waveform. The primary characteristic of pseudonoise sequence synchronisation is

the need for two levels of synchronisation namely acquisition {course synchronisation) and

tracking (fine synchronisation). In these networks (the term network is used to describe

a circuit or system throughout the thesis.) a decision is required to switch between the

two synchronisation modes. The two layer structure of the typical pseudonoise sequence

5,Ynchronisation network can increase the overall cost of spread spectrum systems. The

objective of the research was therefore to find solutions to reduce the overall cost and

complexity of the synchronisation network. The synchronisation structure should perform

acquisition and tracking in a single structure, and thereby be low cost.

To achieve the primary objective of this dissertation a. mi.xture of theory, simulations and

practical implementation was used. The basis of the investigation was a time-variant spectral

evaluation of pseud~noise sequences. It is shown that by multiplying a differentiated

pseudonoise sequence with another pseudonoise sequence, useful information is obtained

that can form the basis of a synchronisation network.

Using the conclusions drawn from this investigation, a low cost synchronisation network

wa.S proposed. The network was simple in design and through simulation was shown to

achieve the accurate synchronisation in one structure. The network was also implemented

and was shown to work. However, the network was parameter sensitive and not robust .

. -. second structure based on the same principles was proposed. This network was shown.

through simulation and practical results, to be effective, low cost and achieved the primary

objective.

The low-cost synchronisation network is highly non-linear and therefore the use of linear

techniques to analyse the network is not possible. The non-linear equation for the synchronisation

network was derived and phase-plane techniques were applied. The chaotic

nature of the network was clearly illustrated. Using phase-plane plots the effect of various

parameters on the synchronisation of spreading sequences was illustrated

One important result of this analysis was how the network evaluates the, code-phase -

frequency grid. This grid is a representation of the tracking space for spread spectrum

synchronisation networks. The traditional methods used to perform course acquisition

evaluate the grid uniformally and deterministically. The novel low cost synchronisation

network evaluates this grid in a random fashion.

The low cost spread spectrum synchronisation network was compared to the methods discussed

in the literature. Based on a number of assumptions, locking times were compared

and it was shown that the novel synchronisation network's locking times were of the same

order.

One of the short-comings of the basic network is that it is sensitive to data modulation. This

is a common problem with networks not using energy detectors. The problem was solved

using a mixture of correlation and digital circuitry. The detection circuitry altered the loop

conditions and with that improved the noise performance of the low cost synchronisation

network. The effect of noise and carrier tracking on the novel low cost synchronisation

network was also investigated. The network was evaluated down to an input signal-to-noise

ratio of -18 dB, where it still achieved lock. It is important to realize that due to the

structure of the synchronisation network it is more suited to low-noise spread spectrum

applications.

While implementing the network;·a novel low cost square-wave differentiator was designed.

This signed edge detector has a number of advantages over standard differentiators. It

is wide-band, with the only limiting factors being the speed of the logic gates and the

bandwidth of the multiplier. It is also shown to perform better at lower signal-to-noise ratios.

Other practical aspects that were investigated in the search for a low cost synchronisation

network, was the synchronous oscillator (a form of an injection-locked oscillator with a

constant output amplitude).

The novel low cost spread spectrum synchronisation network achieved its objective of offering

a single solution to acquisition and tracking without the need for an intermediate

decision stage. The network was shown to operate at an input signal-to-noise ratio of -18

dB (in half the simulation bandwidth) and was shown to deal effectively with data modulation.

The network is also low cost and easy to implement. It is important to realize that due

to the structure of the synchronisation network, it is more suited to higher signal-to-noise

ratio applications.