Pulse or digital communications – Transmitters
Reexamination Certificate
2000-02-25
2003-08-26
Vo, Don N. (Department: 2631)
Pulse or digital communications
Transmitters
C375S267000
Reexamination Certificate
active
06611565
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the technique field concerning professional telecommunication systems, and more particularly a broad band transmitter for a signal consisting of a plurality of digitally modulated carriers.
The use of the radio frequency spectrum in telecommunications is governed by international standards which assign specific frequency bands to given services, both public and private. Inside these bands, services are generally organized in order to take advantage of the band occupation to the best extent, for instance, dividing the same into a plurality of contiguous channels. We have number of examples on this matter. A first example is represented by telephone radio links, where thousands of telephonic channels are multiplexed among them, in frequency or in time, and the multiplexed signals are used to modulate the relevant carriers of a same number of radio channels, arranged in order to result contiguous within a microwave band. A second example is given by the Pan-European communication system, hereinafter defined with the acronym GSM (Global System Mobile), based on the time share use of as much as 124 carriers, 200 KHz spaced among them, digitally modulated according to a GMSK scheme (Gaussian Minimum Shift Keying), and individually transmitted within a 35 MHz band (Extended GSM) placed around 900 MHz. The reference to the GSM system is desired since, being the same an essentially digital system, it is the field of preferential application of the transmitter according to the subject invention. As it is already known, by digital modulation we mean a modulation scheme where the parameter, or the parameters, characterising the modulated carriers assume only a discrete number of values; in the GSM, like in the most advanced telecommunication systems, the carriers are orthogonally phase modulated starting from a modulating signal consisting of bursts of information bits.
TECHNICAL PROBLEM
In any type of transmitter for digital signals, in addition to the usual filtering of the image band generated by the radio frequency converter and of the residual of local oscillator, it is necessary first to filter the replicas of the base band spectrum caused by the conversion of the digital signal to the analogue form, the sole possible for radio transmission.
FIGS. 1 and 2
show what described above. In particular, in
FIG. 1
we can notice that the sampling frequency fs is higher than the double of the useful band BW of the signal to sample, as defined by the Nyquist criterion to avoid spectral superimposition in the sampled signal.
In the case a multicarrier transmitter is implemented, according to the architecture that can be assumed, the above filtering can result more or less expensive. In fact, if one wants to construct a multicarrier signal of the digital type, it should be useful to sum up in a digital way the largest possible number of modulated carriers in order to avail of the speed allowed by the digital section performing such construction to the maximum extent, compatibly with the maximum operation speed of the digital-to-analogue converter. However, this operation method would involve a considerable shortening of the distances existing between the lower edge of the base band and the continuous, on one side, and the upper edge and the fs/2 frequency, on the other side. The above mentioned distances are indicated with &Dgr;F in FIG.
3
and have the following expression in case of simmetric allocation of BW in the Nyquist band:
Δ
F
=
(
f
s
2
-
BW
)
2
.
(
1
)
The approaching of the useful spectrum to the continuous would complicate the radio frequency filtering to eliminate the residue of local oscillator and the image band (see FIG.
2
), while the approaching to fs/2 would complicate the reconstruction filtering for the elimination of undesired spectral replicas (see FIG.
1
). There is therefore a compromise between the choice of the sampling frequency fs and the bandwidth of the multicarrier signal in the first Nyquist area. Concerning the sampling frequency, it corresponds to that of a clock signal used by the digital section. Said frequency shall necessarily be higher than that resulting from the choice of two samples to represent the modulated numeric phase carrier placed at the upper edge of the broad band spectrum, since it is necessary to maintain said filtration margins. The maximum value of the sampling frequency should be at present 40 MHz approximately, limit imposed by the technology of the marketable components, while concerning the maximum band width of the useful signal, this would depend on the margin one wants to leave to simplify the above mentioned filtering. At 40 MHz frequency no limit would be imposed by the digital/analogue converter, which can easily reach a speed more than double.
It is now assumed the project of a broad band transmitter for a digital multicarrier signal, to the purpose of highlighting the difficulties encountered in a similar implementation, difficulties that up to now have discouraged this type of realization approach. In the postulated transmitter we assume:
sampling frequency 34.6 MHz;
number of channels 16;
a spacing between channels 600 kHz, corresponding to a GSM cluster size equal to 3.
With these assumptions it results that the band of the useful signal BW occupies 10 MHz approx., to be allocated in a first Nyquist area, 17.3 MHz wide. Considering to position the intermediate frequency IF at the centre of the first Nyquist area, that is: IF=8.65 MHz, we obtain that the distances &Dgr;F between the edges of the useful spectrum and the edges of the first Nyquist area have a value of 3.65 MHz; the margins destined to filtering are therefore very narrow.
FIG. 4
shows the GSM 11.21 specifications relevant to the emission of spurious signals (for systems operating in the GSM band). They foresee that each spurious signal emitted by the transmitter lays under −36 dBm in the whole frequency spectrum up to 1 GHz, except for the reception band, where it is necessary to observe −98 dBm. For frequencies higher than one GHz the specifications impose to emit no more than −30 dBm, except for the bands destined to the 1800 MHz DCS service (Digital Cellular System).
Assuming to employ a local oscillator power P
ol
equal to 10 dBm, to have an isolation between the local oscillator and the radio frequency in the balanced mixer that generates the frequency convertion of ISO
—
ol
—
rf
=30 dB, and that the gain of the whole transmission chain G
tot
is 50 dB, we obtain that at the output, without filtering, the residue of local oscillator Res
ol
is equal to:
Res
ol
=P
ol
−Iso
ol
+G
tot
=10−30+50=30 dBm. (2)
In the case the residue of local oscillator falls in transmission band, it is necessary to increase the 30 dBm to −36 dBm, that is, a radio frequency band pass filter must be employed, which at a distance &Dgr;F=3.65 MHz from the edges of the band attenuates 66 dB. To obtain this, it is necessary to use two identical Chebyshev filters with 6 resonators; a similar filtering results very expensive.
In addition to the disadvantage of an expensive radio frequency filtering, the use of a low IF could involve a second disadvantage represented by the fact that conversion products generated by the non linearity of the mixer could fall in the useful band of the signal. The mixer in fact, besides generating undesired signals at the frequencies:
f
OL
±f
can
(3)
produces spurious signals at frequencies:
N·f
OL
±M·f
can
(4)
for all the combinations of M and N integer positive and negative. The width of the spurious signals decrease as M and N increase. Out of these spurious products, those, which can highly disturb the radio frequency signal, are those of lower rank, because they have higher width and, as we will see now, they can fall in the useful band.
Let's consider the case N=1 and M=2, that is
f
1MD1.2
=f
OL
±2
Badá Anna Marina
Politi Marco
Siemens Information & Communication Network S.p.A.
Vo Don N.
LandOfFree
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