Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
1998-06-15
2001-06-05
Pham, Chi (Department: 2631)
Pulse or digital communications
Equalizers
Automatic
C455S307000, C703S007000
Reexamination Certificate
active
06243415
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process of multisensor equalization allowing the demodulation of a digital message of serial digital modulation type in the presence of multiple propagation paths and interfering sources, also called jammers in respect of modulations formed from frames comprising learning sequences and information symbol sequences. It is based on the techniques of antenna processing and therefore requires the use of an array containing several sensors.
2. Discussion of the Background
For numerous applications in digital radio communications, transmission between the transmitter and the receiver occurs along several propagation paths. Since the delay time between the various paths may be greater than the symbol duration, equalization becomes necessary in order to compensate for the inter-symbol interference (ISI) thus generated.
This phenomenon occurs in particular in the HF range, where the multiple propagation paths arising from the reflections off the various ionospheric layers, may be 5 ms apart, i.e. several times the symbol duration (for modulations whose bandwidth is typically of the order of 3 kHz). It also occurs in other frequency ranges in respect of very high speed communications, of the GSM type (270 kbits/s, i.e. a symbol duration of 3.7 &mgr;s), in, an urban or mountainous setting, where the various paths stemming from reflections off various obstacles (buildings, mountains, etc.) may be separated by 10 or even 20 &mgr;s.
The invention may therefore be applied in particular to the high-frequency (HF) range which is of particular interest to radio communications since it allows long-distance communications on account of the phenomena of reflection off the various layers of the ionosphere, or else in respect of GSM-type applications. The invention may also lead to an increase in the capacity of cellular radio communication systems by allowing the implementation of Space Division Multiple Access (SDMA) techniques which consist in allowing several users who are sufficiently far apart spatially to use the same frequency at the same time.
In many systems currently in service, adaptation to the conditions of propagation is made possible by inserting learning sequences, which are known to the receiver, into the waveform. Various solutions are then possible for carrying out adaptive equalization of the useful signal received, the two most common being:
equalization by a Viterbi algorithm, requiring prior estimation of the propagation channel using the learning sequence. This equalization has the advantage of minimizing the probability of error with regard to the complete sequence of information signals, but it becomes very expensive when the duration of the impulse response of the channel is much greater than the symbol duration Ts. This is because the number of states which the Viterbi algorithm must process is equal to M
L
, where M is the size of the alphabet of the modulation and L the length of the impulse response of the channel in terms of number of symbol periods. This solution is used for GSM-type applications where the Viterbi algorithm typically contains 32 states (L=5 and M=2).
In the HF range, the number of states becomes too large for the Viterbi algorithm to be usable (typically, M is equal to 4 or 8, and L is equal to 12, corresponding to an impulse response stretching over 5 ms) and the second solution using a DFE equalizer, standing for “Decision Feedback Equalizer”, is often used.
This second solution consists in using the learning sequences to optimize a MSE (Mean Square Error) criterion. The equalizer attempts to provide the decision facility, adapted to the modulation, with a signal devoid of ISI, or in which the ISI has been greatly reduced. For this purpose, the DFE equalizer uses transverse filters and auto-adaptive recursive filters, which are adapted by an algorithm of the recursive least squares type (preferably to a gradient algorithm for reasons of speed of convergence) or are calculated directly from an estimate of the transmission channel—see in this respect the article by P. K. Shukla and L. F. Turner, “Channel-estimation-based adaptive DFE for fading multipath radio channels”, Proc. of 1989 International Conference on Communications, ICC'89[1]. In the learning sequences, the known symbols are used to adapt the various coefficients. The tracking of the channel variations outside of the known sequences is ensured by using the symbols as and when decided as replica.
In the HF range, the various propagation paths are usually affected by flat “fading”. When this “fading” is large, the performance of the DFE equalizer is degraded.
On the other hand, when jamming is present, these techniques rapidly become ineffective and special anti-jamming techniques are necessary, such as error-correcting coding, the removal of jamming by notch filtering or the use of frequency-hopping links. These techniques, used in numerous operational systems, are nevertheless limited when the interference is strong and occupies the whole of the useful signal band. Under these conditions, higher-performance anti-jamming means should be used, based on the use of antenna filtering techniques.
Antenna filtering techniques, which appeared in the early 1960s and are described in particular in an article by P. W. Howells, “Explorations in fixed and adaptative resolution at GE and SURC”, IEEE Trans-Ant-Prop, Vol. AP-24, No. 5, pp 575-584, September 1976 [2], an exhaustive overview of which is presented in a thesis by P. Chevalier, “Antenne adaptative: d'une structure linéaire à une structure non linéaire de Voltera [Adaptative antenna: from a linear structure to a nonlinear Voltera structure]”, June 1991 [3], aim to combine the signals received by the various component sensors of the antenna, in such a way as to optimize the response of the latter to the useful-signal and jammers scenario.
The choice of sensors and of their arrangement is an important parameter and has a major influence on performance. Three types of possibilities may be envisaged:
the sensors are identical and arranged at various points in space, discrimination between the useful signal and the jammers being effected via the direction of arrival,
the sensors are arranged at one point in space (co-localized antenna) and possess different radiation patterns. Discrimination may then be effected according to polarization and direction of arrival,
the above two possibilities can be combined: several co-localized antennas may be arranged at various points in space.
However, since the propagation and jamming conditions may alter over time, it is necessary to be able to adapt the antenna in real time to these variations through the use of a particular antenna filtering technique: the adaptative antenna. An adaptative antenna is an antenna which detects the sources of interference automatically, while constructing holes in its radiation pattern in their direction, while simultaneously improving the reception of the useful source, without a priori knowledge about the interference and on the basis of minimum information about the useful signal. Moreover, on account of the tracking ability of the algorithms used, an adaptative antenna is capable of responding automatically to a changing environment.
Adaptative antennas are characterized by the way in which they discriminate between the useful signal and the jammers, that is to say by the nature of the information about the useful signal which they exploit. This discrimination can be effected in five different ways according to [3]:
based on direction of arrival,
based on modulation,
based on time, for example, for frequency-hopping links,
based on power
blindly (for example, the higher-order source separation methods).
Until very recently, transmission systems were still envisaged as operating independently of the adaptative antenna and single-sensor adaptative equalization techniques, this leading to sub-optimal performance.
Thus, the
Pipon Francois
Pirez Didier
Vila Pierre
"Thomson-CSF"
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Pham Chi
Phu Phuong
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