Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1997-01-28
2001-08-14
Ton, Dang (Department: 2661)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
Reexamination Certificate
active
06275486
ABSTRACT:
TECHNICAL FIELD
This invention relates to radio communication networks and more particularly, but not exclusively, to radio communication networks using multiple access techniques.
BACKGROUND TO THE INVENTION
In radio communications systems such as GSM digital mobile radio protocol, the communications channel hops from one frequency band to another according to a specified routine. The system overcomes the effects of fading, scattering and other transmission problems on a particular channel by swapping channels and providing an average of the signal strength of the channels available, which will provide a sufficient signal. Obstacles in a signal path, such as buildings in built-up areas and hills in rural areas, act as signal scatterers and can cause signalling problems. These scattered signals interact and their resultant signal at a receiving antenna is subject to deep and rapid fading and the signal envelope often follows a Rayleigh distribution over short distances, especially in heavily cluttered regions.
A receiver moving through this spatially varying field experiences a fading rate which is proportional to its speed and the frequency of the transmission. Since the various components arrive from different directions, there is also a Doppler spread in the received spectrum. If the channel allocation was static, then as the subscriber, for example, moved to an urban environment where signal reflections affected the particular frequency in which the channel was operating more than other frequencies, then the channel which was previously best then becomes poor. In fact such movement may produce a break in communications.
In fixed radio applications, the problems of fading still exist but are not so rapid; in a fixed system, the best channel would be likely to stay the best signal for a period of time. Frequently, the fading follows a Rayleigh distribution.
In radio communications, signals are transmitted at a particular frequency or in a frequency band. The signals may be modulated in a variety of fashions using techniques such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), and a multitude of other techniques. Nevertheless there are a finite number of available individual communications channels for separate sets of parties to communicate with each other.
A radio communications system of the TDMA-TDD type is designed so that a multiple frame is made up of a plurality of frames each divided into a plurality of time slots; each base station selects an idle time slot of a multiple frame for transmission of the control-channel signal to send control-channel information to the associated receiver at intervals of a multiple frame period.
FIG. 1
is a timing chart showing the relationship of the transmission and reception of signals between an base station (BS) and an outstation (OS).
In
FIG. 1
, a block of up-link signal time slots and a block of down-link signal time slots have four slots respectively. The time slots of each frame are divided into a block of down-link signal (for communication from the base station to the out station) slots
10
(down-link signal slot block
10
) and a block of up-link signal (for communication from the out station to the base station) slots
20
(up-link signal slot block
20
), and the aforementioned slot for transmission of the control-channel signal directed to the out station (which slot will be referred to as the down-link control-channel slot, hereinafter) is selected from the down-link signal slots of the block of a frame (for example, a time slot
4
in
FIG. 1
is selected).
The transmission of the control-channel signal from the mobile station to the base station is carried out at one (which will be referred to as the up-link signal slots of each frame having a corresponding positional relationship with the above down-link control-channel slot. e.g., located as shifted by a half frame from the down-link control-channel slot. For example, when the time slot
4
in
FIG. 1
is used as the down-link control-channel signal slot, a time slot
8
shifted by a half frame from the time slot
4
is used as the up-link control-channel slot. The remaining slots (time slots
1
,
2
,
3
,
5
,
6
and
7
in
FIG. 1
) of the up and down-link signal slot blocks of each frame other than the up and down-link control-channel signal slots are used as slots for communication of data information between the base station and out station.
Each base station transmits the control-channel signal at intervals of the multiple frame period with use of a signal carrier of an identical frequency commonly used by the other base stations and also with use of the down-link control-channel slot of the specific frame selected by its own base station. With respect to the frames of each multiple frame other than the specific frame, ones of the down-link signal slots located to correspond to the down-link signal slots located to correspond to the down-link control-channel slot, e.g. located as shifted by one frame are not effectively used. Each base station assigns specific up-and down-link traffic-channel slots of each frame to each of the out stations under the jurisdiction of the base station and assigns a frequency to one selected from a plurality of predetermined channels. Accordingly, each out station communicates with the base station and another out station via the base station at intervals of each of the frames of the multiple frame with use of the traffic channel slots specified by the base station.
A disadvantage of employing such schemes, however, is that the numbers of time slots for actual transmission of data are reduced by the presence of these control-channel slots which represent large overheads, and inevitably reduce system capacity. These control-channel slot overheads detract from the gains in efficiency achieved by the use of adaptive techniques. Where such training sequences are employed over an asymmetrical channel only an approximation of the forward channel characteristics can be determined, further reducing the optimisation that can be achieved.
Where training sequences have not been employed, systems have tended to rely on each transmitter analysing the characteristics of received signals transmitted from the other end of the circuit. However during data transmission, the majority of information tends to flow in one direction, e.g. during transmission of a large data file. Where transmission time is long, the channel conditions may change sufficiently that the characteristics of the transmitted signal are no longer optimal. However, as the majority of information flows in one direction only, the transmitter does not receive information relating to required changes in signal characteristics.
OBJECT OF THE INVENTION
Accordingly the present invention seeks to ameliorate the above disadvantages.
SUMMARY OF INVENTION
According to a first aspect of the present invention, there is provided a communications network including at least two stations for transmitting and receiving; wherein at least one of the stations is capable of sending a control signal to the other station after receiving a signal transmitted over said network from said other station after analysis of the transmitted signal, which control signal is transmitted as a data packet independent of the other data and overhead signals. The data packet is transmitted independently of the rate and type of the other data and overhead signals, although could take, for example a data slot normally reserved for a TDMA transmission. The control signal could also be transmitted as an overhead on an infrequent basis. In this fashion, the control signal is transmitted by way of a data packet whereby no specific system overheads are required, thereby providing increased system capacity.
According to a further aspect of the present invention, there is provided a method of communicating over a communications network including at least two stations; said method comprising the steps of:
1) transmitting a signal from a first station of the communications network to a s
Burr Alister Graham
Edwards Keith Russell
Pearce David Andrew James
Tozer Timothy Conrad
Lee Mann Smith McWilliams Sweeney & Ohlson
Nortel Networks Limited
Ton Dang
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