Broad band digital radio receiver for multicarrier signal

Details Broad band digital radio receiver for multicarrier signal Broad band digital radio receiver for multicarrier signal

2000-05-19

2004-02-10

Chin, Stephen (Department: 2634)

Pulse or digital communications

Systems using alternating or pulsating current

Plural channels for transmission of a single pulse train

C375S346000, C375S350000, C375S316000

Reexamination Certificate

active

06690735

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of the technique concerning professional telecommunication systems, and more in particular, to a broad band digital radio receiver for multicarrier signals.
The use of the radiofrequency spectrum in telecommunication is governed by international standards assigning specific frequency bands to given services, either public or private. Inside these bands services are generally organized in order to exploit the band occupation at best, for instance, subdividing the same into a plurality of contiguous channels. A number of examples are available on this matter. A first example is represented by telephone radio links, where thousands of telephone channels are multiplexed among them, either in frequency or in time, in order to give a result contiguous within a microwave band. A second example is the Paneuropean telephone system, hereinafter referred to through the acronym GSM (Groupe Special Mobile), based on the time division use of even 174 carriers, 200 KHz spaced among them, modulated according to a GMSK scheme (Gaussian Minimum Shift Keying), and individually transmitted within a 35 MHz band, positioned around 900 MHz (EGSM). Reference to the GSM system is purposely made since, being the same as an essentially digital system, it results a preferred field of application according to the subject invention. The digital receiver definition means that it is designed to receive signals for which the parameter, or parameters, characterizing the modulated carriers, assume a discrete number of values; in the GSM, as in the most modern telecommunication systems, the carriers are modulated in an orthogonal way, starting from a modulating signal consisting of bursts of information or synchronization bits.
A problem arising in the modern transceivers is in fact that of the conversion of the reception analog signal into a digital format, from which the original burst has to be obtained through appropriate processing with the DSP techniques (Digital Signal Processing). The classical implementation scheme of radio receivers operating in the field of the present invention foresees at least an intermediate frequency conversion stage, followed by a demodulator and an analog-to-digital converter (A/D) of the demodulated signal. The reasons inducing to the intermediate frequency conversion of the signal received are multiple, among which the main one is undoubtedly that of an improved and more easy selectivity of the receiver. Of course, the conversion to filter the signal falling in a so-called “image” band at radio frequency is a speculation compared to one versus the frequency f
oI
of the local oscillator governing the intermediate frequency converter. Such a filtering is generally very complex, due to the close distance usually present among adjacent radio channels. A second problem arising, is the conversion speed of the A/D converter, since it depends on the bandwidth of the signal to be processed. The above mentioned speed corresponds to the sampling frequency f
s
of a sampler of the analog signal preceding the A/D converter. The frequency f
s
must be equal to at least the double of the maximum frequency included in the BW band of the signal to be converted, as defined by the Nyquist proposition, which represents a non negligible burden in the case of broad band signals, just like multicarrier ones.
2. Background Art
In order to double the band to be processed by the A/D converter, a functional diagram is shown in
FIG. 1
of a multicarrier receiver, simplified for description sake, to the case of only two carriers representing two adjacent channels in a comprising BW band. The receiver of
FIG. 1
is enabled to halve the sampling frequency f
s
and can be obtained through the sole application of the conventional knowledge of the skilled in the art.
Referring first to
FIG. 1
, a radiofrequency stage including a low-noise amplifier RFAMP for a RF input signal consisting of two carriers having f
c1
and f
c2
frequency, respectively, is orthogonally modulated by the information conveyed by the relevant channels CH
1
and CH
2
associated to the same. The signal coming out from RFAMP is equally shared over two branches leading to the input of two relevant band pass filters PBAND
1
and PBAND
2
having width BW/2, sharing the whole RF band. The signals coming out from the filters reach two first inputs of relevant mixers MIX
1
and MIX
2
, the second inputs of which are reached by two sinusoidal signals of local oscillator, respectively, having f
oI1
=f
c1
−BW/4 and f
oI2
=f
c2
−vBW/4 frequencies. Thanks to the particular values of f
oI1
and f
oI2
, the two channels CH
1
and CH
2
are included in the 0 to BW/2 band. The signals are filtered by two low-pass filters, not shown in the figure, eliminating the 2f
ol1
and 2f
ol2
components reaching two A/D blocks operating at f
s
=BW frequency. The digital signals coming out from the A/D blocks reach two DDC blocks representing some numeric demodulators in quadrature. For the detail of these blocks, reference will be made to the description of the following figures. The in phase component I
1
and the in quadrature component Q
1
of the demodulated signal concerning channel CH
1
are present at the two outputs of block DDC
1
likewise the in phase component I
2
and the in quadrature component Q
2
of the demodulated signal concerning channel CH
2
are present at the two outputs of block DDC
2
. The above-mentioned components are sent to a detector block, not shown in the figure, giving back the starting information. The diagram of
FIG. 1
can be extended to a receiver for more than two channels, simply adding as much DDC blocks as are the new channels.
As it can be noticed from the previous description, in the receiver of
FIG. 1
the A/D converters operate at halved speed compared to those used in the receivers mentioned above. However, this advantage versus the background art is soon made vain by the cost of the two high selectivity, radiofrequency filters PBAND
1
and PBAND
2
and by the need to equip two local oscillators.
OBJECTS OF THE INVENTION
Object of the present invention is to overcome the drawbacks of the background art and of the receiver of
FIG. 1
, and to indicate a method for the implementation of a broad band radio receiver for multicarrier signals with orthogonal modulation.
SUMMARY OF THE INVENTION
The above object is solved by the present invention which is addressed to a method for the implementation of a broad band receiver for a signal (z
1
(t)) consisting of a plurality of equispaced carriers, orthogonally modulated by information conveyed by relevant channels in order to carry out a radiofrequency multicarrier signal, the method comprising in sequence the following steps:
(a) direct demodulation (DEMI/Q) of the radiofrequency filtered signal (z
1
′(t)) by multiplication of the signal for two local phase quadrature carriers (cos&ohgr;
0
t, −sin&ohgr;
0
t), whose frequency corresponds to a central value of the radiofrequency multicarrier signal spectrum, obtaining a demodulated signal having a relevant spectrum in the lower half of the base band (BW) where pairs of channels (CH
4
, CH
5
; . . . ; CH
1
, CH
8
) in symmetric positions at the two sides of the local phase quadrature carriers are superimposed;
(b) broad band filtering in base band in phase component (z
2
′(t)) and in quadrature component (z
3
′(t)) that correspond to the demodulated signal for the suppression of additional components outside an interest band;
(c) sampling of the components broad band filtered in base band (z
2
′(t)), making use of a sampling frequency equal to the bandwidth (BW) of the multicarrier signal, and subsequent analog-to-digital conversion (A/D) of the sampled components, obtaining first digital in phase and quadrature components;
(d) digital demodulation (DEM
4
, DEM
5
; . . . ; DEM
1
, DEM
8
) of the first digital components by multiplication for pairs of relevant phas

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