Receiver module and receiver formed from several cascaded...

Details Receiver module and receiver formed from several cascaded... Receiver module and receiver formed from several cascaded...

2000-03-30

2004-01-13

Pham, Chi (Department: 2631)

Pulse or digital communications

Receivers

Angle modulation

C375S152000

Reexamination Certificate

active

06678338

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a receiver module and to a receiver formed from several cascaded modules.
The invention has a general application in digital communications and in particular in wireless local area networks (WLAN), wireless local subscriber loops (WLL), mobile telephony, electronic funds transfer, integrated home systems, communications in transportation vehicles, cable television and multimedia services on cabled networks, etc.
PRIOR ART
The invention relates to the spread spectrum technique. It is known that this technique consists of modulating a digital symbol by a pseudorandom sequence known to the user and the emission of said modulated symbol. Each sequence is formed from N elements known as chips, whose duration is the Nth of the duration of a symbol. This leads to a signal, whose spectrum is spread over a range N times wider than that of the original signal. On reception, a demodulation takes place by correlating the signal received with the sequence used on emission, which makes it possible to reconstitute the starting symbol.
This technique has numerous advantages:
discretion, because the power of the emitted signal is constant and distributed in a band N times wider, the spectral power density being reduced by a factor N;
immunity to (spurious or intended) narrow band emissions, the correlation operation at the receiver leading to the spread spectrum of these emissions;
difficulty of interception (for conventional signal-to-noise ratios), because demodulation requires the knowledge of the sequence used on emission;
resistance to multiple paths which, under certain conditions, give rise to selective frequency fading and therefore only partly affect the emitted signal;
possibility of code division multiple access (CDMA), several spread spectrum links by direct sequence being able to share the same frequency band using orthogonal spread codes.
However, this technique suffers from a disadvantage constituted by its limited spectral efficiency. This means the ratio between the binary data rate and the width of the occupied band. If each data symbol contains m bits, the binary data rate is equal to m times the symbol rate, i.e. mDs. With regards to the occupied band, it is equal to double the chip frequency, i.e. 2N times the symbol rate, i.e. 2NDs. Thus, finally, there is a spectral efficiency equal to the ratio
mDs
2
⁢
NDs
,
i.e.
m
2
⁢
N
.
Consideration could be given to an increase in the spectral efficiency by decreasing N, but this would be in opposition to the qualities inherent in the spread and would in particular prejudice the immunity of transmissions to interference. Consideration could also be given to increasing the symbol rate, but the interference phenomenon between symbols would be aggravated.
Another solution consists of increasing m, the number of binary data per symbol, which leads to the use of complex modulations such as phase shift keying (PSK) with several phase states, which is a phase modulation (or coding) or the so-called “M-ary Orthogonal Keying” (MOK) or order M orthogonal modulation.
A description of these modulations appears in two general works:
Andrew J. VITERBI : “CDMA-Principles of Spread Spectrum Communication” Addison-Wesley Wireless Communication Series, 1975,
John G. PROAKIS : “Digital Communications McGraw-Hill International Editions, 3rd edition, 1995.
Firstly with respect to phase modulation, it is pointed out that this is more usually a binary modulation or BPSK or quaternary modulation or QPSK. In the first case it is possible to encode symbols with one bit (m=1) and in the second case symbols with two bits (m=2).
These modulations are more usually implemented in differential form (DBPSK, DQPSK) ensuring a good robustness in difficult channels, because no phase recovery loop is necessary. This differential form is also very suitable for processing the multiplicity of propagation paths.
On reception, a differential demodulator carries out the multiplication between the signal to be demodulated and its version delayed by a symbol period. In the case of quaternary modulation use is made of two signal channels, one channel processing the component of the signal in phase with a carrier and another channel which processes the component in quadrature with the carrier.
In the case of MOK modulation, it constitutes a technique in which with each symbol to be emitted is associated a signal taken from among a group of orthogonal signals. These signals can be spread codes of a same family of orthogonal codes. In this case, the modulation also implements the spread. However, these signals may also not be perfectly orthogonal and in this case the performance characteristics are less satisfactory.
If a symbol is constituted by m bits, there are two m possible configurations for the symbols. The number M of available codes must therefore be at least equal to M, with M=2
m
If the length of the codes is N, it is known that it is possible to find N orthogonal codes.
Thus, we obtain M=N and the number of bits per symbol is consequently limited to log
2
N. A known MOK receiver is illustrated in the attached
FIG. 1
, where it is possible to see a bank of matched filters
10
1
,
10
2
,. . . . ,
10
M
, followed by the same number of samplers 12
1
,
12
2
, . . . . ,
12
M
, circuits
14
1
,
14
2
, . . . . ,
14
M
for determining the energy (or amplitude) of the sampled signal, a circuit
16
for determining the highest energy (or highest amplitude) signal and which delivers the number of the channel corresponding to said signal and finally a circuit
18
which, on the basis of the number of said channel, restores the corresponding code, i.e. the transmitted symbol S.
The MOK technique has a variant called MBOK (“M-ary Bi-Orthogonal Keying”) consisting of adding to the set of orthogonal signals used in a MOK modulation their opposites in order to constitute a set of 2M signals, which are obviously not all orthogonal to one another. Demodulation uses M correlators, each adapted to M orthogonal codes, but also requires sign recovery means.
If, for increasing the spectral efficiency, there is an-increase by one unit of the number m of bits in each symbol, the number M of available codes doubles, which multiplies by 2 by the number of channels of the receiver. Thus, the complexity increases much more rapidly than the spectral efficiency, so that this technique has certain limits.
MOK and MBOK modulations are used in certain digital communications systems, in conjunction with a coherent reception structure, which requires the knowledge of the phase of the carrier. The emission of a preamble, prior to the emission of the useful data, is a standard process permitting the estimation of said phase. However, in channels subject to fading and/or multiple path, the phase of the Carrier undergoes variations, which can be very fast and which the reception system must detect and compensate. This is generally obtained by the periodic emission of preambles, which then occupy the channel and lead to a reduction in the useful data rate. In accordance with this diagram, the durations of the preamble and the useful data packet must be less than the channel coherence time (time during which the channel is considered to be stationary). Moreover, there is an increase in the complexity of the reception structure.
Therefore the expert prefers to use non-coherent demodulation diagrams or diagrams which are differentially coherent, which do not require the knowledge of phase information. These techniques avoid the need for long preambles, for phase estimators and for phase derotators, at the cost of a slight sensitivity loss. Moreover, non-coherent demodulation very significantly simplifies the processing of the multiplicity of propagation paths, because each path has inter alia its own phase (and therefore would not require its own phase estimator in a coherent diagram).
French patent application 98 11564 filed on Sep. 16, 1998 by the present applicant proposes a mixed modulation-demodulation digital trans

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