Pulse or digital communications – Transmitters
Reexamination Certificate
1999-04-01
2001-09-11
Chin, Stephen (Department: 2634)
Pulse or digital communications
Transmitters
C375S300000, C375S303000, C375S240000
Reexamination Certificate
active
06289056
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention concerns mobile telephones. It consists in a broadband multicarrier modulator that can be used in a base transceiver station or in a mobile telephone. In the latter case, the technique developed for the base transceiver station is used with advantage even though the modulator is not normally a multicarrier modulator. The invention enables a relatively large number of mobile telephones to be connected to the same base transceiver station. Generally speaking, the invention is beneficial if a plurality of modulator circuits must be able to modulate a plurality of carriers simultaneously. The invention also consists in a method of programming the multicarrier modulator.
2. Description of the prior art
The invention will be described in the context of a GSM type application but is not limited to that mode of use. Other protocols are equally feasible. The GSM has been chosen because it is sufficiently comprehensive and representative of the problems solved by the invention.
FIGS. 1
to
5
show particular features of use of a base transceiver station in a GSM network. The operating constraints of the mobile telephones themselves will be deduced therefrom.
FIG. 1
shows geographical domains. The domains correspond to a district within a town, to a number of streets, for example. The contour of the domains is not necessarily as precise as the hexagons shown. In reality the contour of the domain corresponds to a physical limit beyond which signals transmitted at given frequencies from base transceiver stations within the domain are no longer received. Accordingly, a base transceiver station BTS in a first domain transmits signals on carriers F
1
to F
8
. Other base transceiver stations in adjoining domains transmit on carriers F
9
to F
16
, F
17
to F
24
, F
25
to F
32
, and so on. A further domain adjacent the domains adjacent the first domain could also use the frequencies F
1
through F
8
of the first domain to broadcast messages to mobile telephones therein. With this system there is little risk of signals getting mixed up.
FIG. 2
shows for one frequency F
1
how a time frame (of 4.615 milliseconds duration) is used by eight users. The frame is divided into eight time windows. The users use the frequency F
1
to transmit their signals in turn. The time windows have a duration limited to 577 microseconds. Another frequency F
2
is used to broadcast messages relating to eight other users, and so on up to frequency F
8
.
Thus in practice
64
(8×8) users can be connected at the same time to the same base transceiver station in one domain. The time distribution of the windows within the frame is known as time-division multiple access (TDMA).
The carrier frequencies are switched at the end of each 4.615 millisecond frame to avoid problems associated with poor quality of propagation of the signal transmitted by a base transceiver station to a mobile on a given carrier (for example carrier F
1
). Users who were previously using frequency F
1
can use another frequency F
2
. By circular permutation those previously using frequency F
2
use frequency F
3
, and so on.
Eight frequencies from
64
have been retained for a domain in order to solve interference problems that could nevertheless arise between two non-adjoining domains, like those linked by the arrow in FIG.
1
. Eight other frequencies from the 64 frequencies are used for the next frame. Those eight other frequencies are necessarily the same as the frequencies of the preceding frame. The principle is substantially the same but immunity to mixing of signals is increased.
FIG. 3
shows the amplitude of the signals transmitted in each of the bands and in each time window, depending on their use. It shows a modulation signal level for a user U
1
. Nothing is being transmitted for a user U
2
who is silent. Another user U
3
is far away from the base transceiver station. During an initialization window the base transceiver station recognizes that it is receiving from the user U
3
at a low level. It then transmits at a higher level toward that user so that the user can receive their messages directly. The user then transmits a signal whose level corresponds to the effects of the distance from the base transceiver station. With a dynamic range of 30 dB, steps of
1
or
2
dB are adopted in practice, with the result that the level is quantified in terms of 15 or 30 values.
Moreover, although there is no signal being sent to user U
2
, there is a signal being sent to user U
3
. However, it is not feasible to increase the transmit level too sharply on passing from the time window of user U
2
to that of user U
3
. In spectral terms, too sharp an increase will be interpreted as the presence of high-level interference frequencies. As shown here, the signal for user U
3
is boosted using a substantially bell-shaped envelope to reduce the distortion that occurs in this situation. This neutralizes transmit periods at the beginning and end of the window. This is not a problem in practice because these periods are used for synchronization bits, in any case for bits with no message content.
FIG. 4
shows another constraint on the use of the modulators in the base transceiver stations. Frequencies F
1
, F
4
, F
14
, etc are used in a first frame t1. The bandwidth at each frequency is in practice 200 kHz. Under the GSM standard, what happens in one band must not have effects above a given level in an adjacent band (at a nearby frequency). In practice the standard requires that the signal transmitted in band F
1
be attenuated at least 60 dB above 600 kHz, and therefore here in band F
4
. The bottom of
FIG. 4
shows the spectrum of a filter required for a given frequency plan corresponding to a choice of eight frequencies from the 64 frequencies at the time of a given frame t1.
The main problem addressed by the invention is broadband multicarrier transmission. In particular, the invention seeks to generate a multiplicity of carriers which are modulated (F
1
, F
4
, F
14
) with high spectral purity in a given frequency band to feed a final transmit modulator and an amplifier. The problem is obviously particularly severe if the traffic level in the domain is high: if 64 mobile telephones are actually making calls in the domain at a given time.
In the base transceiver stations used at present a narrowband filtering technique is used to produce independent signals on each carrier. Each signal is therefore modulated, filtered and transposed in the analog domain before individual amplification and combination prior to transmission. In a few cases weak signals are combined before overall broadband power amplification.
The problem to be solved is that of finding a method and an architecture for a low cost multicarrier modulator receiving at its input various modulation signals (speech signals) and producing at its output an analytical signal representing the sum of the modulated carriers in the baseband or at an intermediate frequency. Each carrier must be present in the aforementioned sum with the spectral purity required by the constraints of the transmit interface. A posterior analog filtering of each channel is not possible in a broadband radio transmission system. To be satisfactory in applications where the same frequency is re-used the multicarrier modulator must also be able to receive a plurality of modulation signals to be superposed on the same carrier and to control the phase and amplitude of the carrier accordingly.
The usual problem encountered in digital methods is that of the number of operations effected, especially multiply and accumulate (MAC) operations on analytical numbers (complex numbers). The problem encountered with one carrier is obviously intensified if there is more than one carrier. The invention seeks to process simultaneously 64 carriers in a 25 MHz band with a sampling rate around 100 MHz, for example.
A prior art method of reducing digital MAC calculations transforms some of the operations into tabulations. In this case tables must be
Alcatel
Chin Stephen
Rupert Paul N
Sughrue Mion Zinn Macpeak & Seas, PLLC
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