1. Field of the Invention
The field of the invention is that of digital transmission, in particular to mobiles. To be more precise, the invention concerns a threshold detector for digital transmission system receivers.
2. Description of the Prior Art
There are essentially three types of multiple access digital transmission systems: FDMA, TDMA, CDMA.
Frequency division multiple access (FDMA) was the first system to be used. It entails separating calls to be transmitted by allocating each call a specific frequency band which can easily be separated from others by filtering at the receiver.
This system is little used nowadays because it requires a receiver for each transmission channel used, so that a central station must have a large number of receivers to enable simultaneous: conversation with a large number of distributed stations.
Time-division multiple access (TDMA) is a system which entails time-sharing all of the transmission channel: to prevent information overlapping only one station transmits at a time and, when it transmits, it uses all of the channel.
TDMA systems pose difficult equalization problems if the transmission channel is disturbed by echoes or by jamming.
Finally, the code division multiple access system (CDMA) uses spread spectrum techniques.
One spectrum spreading technique, known as direct sequence spectrum spreading, entails transmitting a signal s(t) obtained by multiplying a digital data signal d(t) by a spreading code g(t). The signal d(t) is characterized by its bit frequency. The spreading code g(t) is a pseudo-random signal characterized by its chip frequency, which is greater than the bit frequency by a known factor G which is called the spreading gain or the band expansion factor. The following equations apply: ##EQU2## where: T.sub.b is the reciprocal of the bit frequency;
T.sub.c is the reciprocal of the chip frequency; PA1 [] is the integer part. PA1 the length of said first and second series of samples; PA1 a value representative of the rate of variation of said transmission channel; PA1 the length of a data burst; PA1 the spreading factor or bandwidth expansion factor of a direct sequence code division multiple access signal. PA1 equalizer activation control; PA1 decision weighting in an equalizer for a time division multiple access signal; PA1 control of activation of despreading means of a code division multiple access signal; PA1 signal transmit power control.
Decoding at the receiver is effected by combining the signal received with a local replica of g(t) synchronized to the transmission. One prior art device of this type is known to those skilled in the art as a "rake receiver".
Threshold detectors are used in several of these systems, in many applications. One instance is for acquisition of synchronization in DS-CDMA (direct sequence CDMA) systems, and another is for optimization of the estimated impulse response of the transmission channel in TDMA and CDMA systems.
Generally speaking, prior art type threshold detectors calculate the correlation between a transmitted data series and a reference data series and then compare the result of this correlation with a predetermined threshold.
The major drawback of these detectors is that the predetermined threshold is fixed, once and for all, whereas the correlation result cannot be analyzed reliably without an instantaneous knowledge of the signal propagation conditions.
Thus, in the case of synchronizing a CDMA system, the correlation result comprises a wanted part, corresponding to a given propagation path length, and an unwanted part corresponding to the additive noise on the channel and to the other propagation path lengths of the channel.
It is therefore maximum when the maximum propagation path length has been detected. It is therefore necessary to choose a threshold such that the decision variable is above the threshold for the maximum propagation path length. However, setting this threshold requires a knowledge of the maximum propagation path length. This is a random variable whose instantaneous values are not known, only its statistical probability being known.
The threshold used is therefore generally a compromise, leading to two types of error. Either it is too low (at a given time) and generates false alarms (threshold exceeded although synchronization has not been acquired), or it is too high and does not detect situations which are in fact acceptable.
Thresholds are also used in many other fields which presume a known signal to noise ratio. This is the case in channel estimate optimizing systems, for example, such as those described in French patent application no 92 11886. This cannot be known in advance.
An object of the invention is to overcome the drawbacks of the prior art.
To be more precise, an object of the invention is to provide a threshold detector which is not affected by variations on the transmission channel of a digital transmission system.
In other words, an object of the invention is to provide a threshold detector which is independent of the signal to noise ratio.
Another object of the invention is to provide a threshold detector which, in the case of multipath signal propagation, makes positive use of the different propagation path lengths.
A specific object of the invention is to provide a synchronization device for a direct sequence code distribution multiple access spread spectrum system using a decision threshold independent of the signal to noise ratio.
Another specific object off the invention is to provide a device for optimizing an estimate of the impulse response of the transmission channel allowing for the value of the signal to noise ratio.
Another object of the invention is to provide a device for estimating the value of the signal to noise ratio.