Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-06-21
2004-11-02
Fan, Chieh M. (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S346000, C375S354000
Reexamination Certificate
active
06813308
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a CDMA (code division multiple access) receiver with parallel interference suppression and optimized synchronization.
More generally, the invention relates to direct sequence spread spectrum (DSSS) digital transmission.
The invention has applications in radiocommunications systems with mobiles, in wireless local area networks (WLAN), in wireless local loops (WLL), in cable television and online multimedia services, in integrated home systems and electronic funds transfer, etc.
PRIOR ART
Direct sequence spread spectrum consists of modulating each symbol of a digital signal by a binary pseudorandom sequence. Such a sequence consists of N pulses or chips, whose duration Tc is equal to Ts/N. The modulated signal has a spectrum spread over a range N times wider than that of the original signal. On reception, demodulation consists of correlating the signal with the sequence used on transmission making it possible to once again find the information linked with the starting symbol.
This procedure has numerous advantages:
discretion, because the spectral power density of the signal is reduced by a factor N;
immunity against deliberate or parasitic, narrow band transmissions, because the correlation operation carried out at the receiver leads to the spectral spread of such transmissions;
difficult interception, because demodulation requires the knowledge of the sequence used on transmission;
resistance to multiple paths which, under certain conditions, give rise to selective frequency fading and consequently partly affect the transmitted signal;
possible multiple access by the allocation of different sequences to different users.
The direct sequence spread spectrum modulation method has been extensively described in the specialist literature and reference can e.g. be made to the following:
“CDMA Principles of Spread Spectrum Communication”, by Andrew J. VITERBI, Addison-Wesley Wireless Communication Series;
“Spread Spectrum Communications”, by Marvin K. SIMON et al., vol. I, 1983, Computer Science Press;
“Spread Spectrum Systems”, by R. C. DIXON, John WILEY and Sons.
The attached
FIG. 1
illustrates the principle of a spread spectrum signal receiver. The receiver shown receives a signal r(t) and comprises a first circuit
10
, referred to hereinafter as the correlation means, and which can be a matched filter or a sliding correlator, a circuit
12
for recovering a symbol clock signal, which makes it possible to synchronize the receiver means, optionally a processing circuit
14
able to perform various supplementary processing operations, such as a delayed multiplication, a channel estimation, etc. and finally a circuit
16
able to take a decision on the value of the transmitted symbol.
The first circuit of said receiver (correlation means
10
), no matter whether it is a sliding correlator or a matched filter, plays an important part which can be defined with the aid of
FIGS. 2 and 3
.
A sliding correlator (
FIG. 2
) diagrammatically comprises a pseudo-random sequence generator
20
and a multiplier
22
receiving the input signal r(t) and the sequence delivered by the generator
20
, an adder
24
, a circuit
26
connected to the output of the adder
24
and relooped thereon and effecting a time lag. The sliding correlator output is connected to an undersampler
28
. The circuits
20
,
26
,
28
are controlled by a symbol clock signal Hs.
The matched filter (
FIG. 3
) is generally a digital filter
30
, whose coefficients are matched to the sequence used. This filter receives the input signal r(t) and delivers a filtered signal applied to an undersampler
28
, which is controlled by the symbol clock signal Hs.
Viewed from the output of the undersampler
28
, said two architectures are equivalent. However, viewed from the input of the undersampler
28
, they are different, because they do not deliver the same signal, as is revealed by
FIGS. 4
,
5
and
6
.
FIG. 4
shows the output Sf of the matched digital filter of
FIG. 3
, as a function of the rank n of the samples.
FIG. 5
shows the output Sc of the sliding correlator when the local replica of the transmitted sequence is aligned with the transmitted sequence.
FIG. 6
shows the output Sc of said sliding correlator when the local replica of the sequence is not aligned with the transmitted sequence. The correlation peak carrying the information on the symbol is marked P in
FIGS. 4 and 5
.
These drawings show that the sliding correlator needs an information linked with the timing of the symbols, so-called symbol clock signal and designated Hs to enable the local replica of the sequence to be aligned with the sequence modulating the symbols received, otherwise the demodulation of the symbols is impossible (case of FIG.
6
). The matched filter does not require this information. Thus, what firstly differentiates a sliding correlator structure and a matched filter structure is that the former needs an external synchronization information.
A matched filter makes it possible to recover the symbol clock, e.g. by a recursive detection of the correlation peak on a window of N points (FIG.
4
). It is also possible to recover the symbol clock with the aid of a sliding correlator, but this is more complex. There is a need for a stepwise modification of the phase of the local replica of the sequence until the output of the sliding correlator corresponds to an energy maximum and consequently to a correlation peak (case of FIG.
5
).
Although both these structures make it possible to find the symbol clock again, they do not do so at the same speed. The symbol clock recovery operation lasts a maximum of N symbol periods, i.e. NTs with a sliding correlator, whereas it only requires a single symbol period Ts with a matched filter.
Thus, the advantage of the matched filter is obvious with respect to the symbol clock signal acquisition speed. Its disadvantage is its operational complexity, because its installation in the form of a finite pulse response digital filter (operating at the speed of the chips multiplied by the number of samples) requires N multiplications and N additions for each sample. Thus, its structural complexity is linked with its operating complexity.
The sliding correlator only performs one multiplication and one addition for each new sample. Thus, although it is relatively poorly adapted to the clock recovery, it is very advantageous from the operating complexity standpoint.
The CDMA method can be of two types. If the different transmitters of users do not have a common time reference the system is said to be asynchronous, because the starts of the symbols of each user arrive at the receiver at different times. It is also possible to proceed in such a way that the starts of the symbols received from the different users (modulo period Ts of a symbol). The system is then said to be synchronous. Asynchronous systems have the major advantage of not requiring an external synchronization signal, unlike in synchronous systems, but this is to the detriment of more serious constraints with respect to the spread sequence properties.
Thus, in an asynchronous CDMA system, the sequences have random relative phases at the reception level. Thus, a good separation of the signals assumes that the intercorrelations between sequences are small, no matter what the relative phases between the sequences.
If g
i
(t) and g
k
(t) are used for designating two pseudorandom sequences allocated to users i and k, it is possible to define a coefficient &mgr;
i,k
translating the correlation between these two sequences. This coefficient is equal to the mean, on the duration Ts of a symbol, of the product of the sequences, i.e.:
μ
i
,
k
=
1
Ts
∫
0
Ts
g
i
(
t
)
g
k
(
t
)
ⅆ
t
.
This coefficient represents an autocorrelation if i=k and an inter-correlation if i≠k.
The signal at the output of the correlator corresponding to the user of rank k (i.e. following the undersampler
28
of
FIG. 2
) can be written, as a function of its coefficient:
A
k
d
k
Lattard Didier
Leveque Sébastien
Ouvry Laurent
Varreau Didier
Commissariat A l'Engerie Atomique
Fan Chieh M.
Thelen Reid & Priest LLP
LandOfFree
CDMA receiver with parallel interference suppression and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with CDMA receiver with parallel interference suppression and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and CDMA receiver with parallel interference suppression and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3363849