Multiplex communications – Communication over free space – Repeater
Reexamination Certificate
2000-04-18
2004-05-04
Ho, Duc (Department: 2665)
Multiplex communications
Communication over free space
Repeater
C370S324000, C370S528000, C455S103000
Reexamination Certificate
active
06731615
ABSTRACT:
The field of the invention is that of radio communications. To be more precise, the present invention relates to a multipoint-to-point TDMA transmission system using a particular burst structure enabling the same transmission system to transmit different types of information, for example voice and data.
BACKGROUND OF THE INVENTION
In a time-division multiple access (TDMA) system, each user employs a given frequency during a given time slot, the other time slots being reserved for other users. The signal transmitted by a user in the allocated time slot is referred to as a burst.
In the remainder of this description each transmitted burst is considered to include:
a guard time containing no signal at the start and/or the end of the burst, and
information symbols obtained by modulating the transmitted signal.
In the remainder of this description:
the expression “unit burst” refers to the signal transmitted in the shortest time slot allocated to a user, referred to as a “unit time slot”, and
the term “superburst” refers to a burst whose length is a multiple of that of the unit burst allocated to a single user, corresponding to a plurality of consecutive unit bursts, referred to as “adjacent bursts”.
In the case of voice transmission, characterized by exactly the same regular bit rate for each user, each user is regularly allocated a unit time slot. The duration of the time slot is a constant of the transmission system and is matched to the characteristics of the voice transmission service in terms of bit rate, delay, or required error rate. Adaptation is required if the same transmission system must simultaneously support different types of services each having its own characteristics. Different types of information, such as voice and data, are conventionally transmitted by allocating users respective numbers of unit time slots according to their requirements.
This approach can be extended to increase bit rate, as described in U.S. Pat. No. 5,566,172, which discloses a method in which a plurality of consecutive unit time slots are grouped together and allocated to a single user who can transmit a superburst in that time slot. The grouping substitutes information symbols for redundant sequences, such as the guard times between two superburst information symbol sequences. This reduces redundancy within the superburst and optimizes the quantity of information transmitted in the superburst.
If the communications system includes an entity for amplifying the received signal before retransmitting it, for example a satellite, the inherent characteristics of the amplifier of that entity generate interference at sudden transitions in signal amplitude. Such transitions are observed each time that a guard time, in which there is no signal and which therefore has a negligible signal amplitude, intersects a flow of information having a non-negligible signal amplitude. The interference is caused by the fact that the amplifier introduces a modification to the phase of the output signal which is a function of the amplitude of the input signal. If the amplitude of the amplifier input signal varies suddenly, the phase of the amplified signal at the amplifier output varies by a large amount.
FIG. 1
shows an example illustrating the above problem.
Four transmitters
1
,
2
,
3
and
4
connected to respective transmit antennas
11
,
21
,
31
and
41
communicate with a base transceiver station
6
via communications channels and a satellite
5
. The transmitters
1
,
2
and
3
are of the same type and transmit voice type information. These three transmitters transmit in turn on the same carrier frequency F
1
during unit time slots. In each triplet of consecutive time slots, the transmitter
1
is allocated the first time slot, the transmitter
2
is allocated the second time slot and the transmitter
3
is allocated the third time slot. The transmitter
4
transmits data type information on a carrier frequency F
2
different from the carrier frequency F
1
. The transmitter
4
is the only one to transmit on the frequency F
2
and transmits a series of consecutive superbursts occupying a time slot three times longer than a unit time slot. It therefore offers a transmission bit rate three times that of the voice type information transmitters
1
,
2
and
3
.
The amplifier connected directly to the receive antenna of the satellite
5
simultaneously amplifies the signal contained in the bursts which are transmitted by different transmitters on different carriers and which reach the amplifier synchronously. If all the bursts are the same length, the guard times coincide exactly and the interference has no harmful effects. However, when bursts and superbursts coexist, as in the example to which
FIG. 1
relates, there are times at which the guard time of a burst received on a given frequency corresponds to one or more information symbols in a superburst received on another frequency.
FIG. 2
shows the time and frequency spreading of the information received at the satellite
5
in the case of the transmission system shown in FIG.
1
. To be more precise,
FIG. 2
represents a succession of bursts received in parallel at the frequencies F
1
and F
2
and plotted on two axes having the same time origin. The bursts B
1
, B
2
and B
3
received on the carrier frequency F
1
are juxtaposed unit bursts from the transmitters
1
,
2
and
3
, respectively. The bursts B
4
received on the carrier frequency F
2
are superbursts three times the length of the unit burst from the transmitter
4
.
There is a guard time at the start and at the end of each transmitted unit burst or superburst. The guard times
12
,
22
,
32
and
42
are start of burst guard times transmitted by the transmitters
1
,
2
,
3
and
4
, respectively. The guard times
13
,
23
,
33
and
43
are end of burst guard times transmitted by the transmitters
1
,
2
,
3
and
4
, respectively.
Each burst includes a training sequence directly after the start guard time and directly before the end guard time. These training sequences are shown in
FIG. 2
but are not identified by any reference symbols.
Each of the bursts B
1
, B
2
, B
3
and B
4
also includes respective information symbols
14
,
24
,
34
and
44
corresponding to the payload information transmitted by the user to whom the burst is allocated.
The start of burst guard time
12
transmitted by the transmitter
1
is received at the same time as the start of burst guard time
42
transmitted by the transmitter
4
.
The end of burst guard time
13
transmitted by the transmitter
1
and the start of burst guard time
22
transmitted by the transmitter
2
are adjacent and received at the same time as the information symbols
441
of the burst transmitted by the transmitter
4
.
The end of burst guard time
23
transmitted by the transmitter
2
and the start of burst guard time
32
transmitted by the transmitter
3
are adjacent and received at the same time as the information symbols
442
of the burst transmitted by the transmitter
4
.
The end of burst guard time
33
transmitted by the transmitter
3
is received at the same time as the end of burst guard time
43
transmitted by the transmitter
4
.
Interference generated when the amplifier of the satellite
5
receives the guard times modifies the phase of the signal conveyed in the superburst at the corresponding locations
42
,
441
,
442
and
43
. The locations
42
and
43
are guard times and interference here therefore has no harmful effect. The locations
441
and
442
containing information symbols are sensitive to the interference, however, which gives rise to a signal demodulation problem. Because the phase of the information symbols contained in each burst is estimated, and used to demodulate the signal, any such phase modification causes many demodulation errors in the information symbols at the locations
441
and
442
.
OBJECTS AND SUMMARY OF THE INVENTION
Because the superbursts are used for data services, which generally have stricter error rate requirements than voice services, an object of the
Bousquet Jacques
Durnez Francois
Gerrier Christophe
Alcatel
Ho Duc
Sughrue & Mion, PLLC
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