Method and system for compensating signal propagation delay

Details Method and system for compensating signal propagation delay Method and system for compensating signal propagation delay

2000-03-29

2004-05-25

Sam, Phirin (Department: 2661)

Multiplex communications

Communication techniques for information carried in plural...

Combining or distributing information via time channels

C370S332000, C370S519000, C370S517000

Reexamination Certificate

active

06741614

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a method and system for compensating for signal propagation delay between network nodes in a data communications network.
More particularly, the present invention relates to a method and system for compensating for signal propagation delay which can be integrated with a method or system for controlling network node transmit power levels, and which is of particular use in a data communications network where accurate power control and time synchronisation is required.
BACKGROUND OF THE INVENTION
Data networks can be classified in many ways, but for the purpose of the present invention, it is useful to classify them by their means of accessing the medium over which data is communicated. The relevant classifications are broadcast and non-broadcast.
An existing type of data network is Ethernet. Ethernet uses broadcast medium access. All network nodes sharing the network medium hear all traffic being passed over the medium. Traffic is directed to individual network nodes via physical layer addresses that are attached to the data packets being sent over the medium. When multiple network nodes attempt to transmit data simultaneously, there is the possibility for contention among the nodes for access to the medium.
A modification to the broadcast network is the broadcast network with hidden terminals. In this network, all terminals share the same medium, however it cannot be guaranteed that all terminals can hear each other. All that can be guaranteed is that all terminals can hear the central network node, referred to herein as the access point. For this reason, it is not enough for each terminal simply to monitor the channel in order to detect contentions. Feedback on success or failure of network contention must also be communicated back to the network terminals by the access point.
Point-to-point full duplex connections, in contrast to the above, do not require contention for the network medium. This is because only two network elements, at each end of the network medium, share access to the medium.
Point-to-multipoint networks in which several network nodes share access to a network medium can simulate point-to-point connections. In such a network a fixed time slot is assigned to each node of the network, and the transmissions of each node are restricted to its particular assigned time slot. An example of such an arrangement of the prior art would be a Time Division Multiple Access (TDMA) network. A cellular wireless network, in which there is a central access point and multiple subscriber terminals sharing access to the channel using TDMA medium access, is an example of such a network. In such networks, since the various subscriber terminals are separated from the central access point by an unknown distance, there is a bounded but unknown elapsed time of propagation as the signal passes between the subscriber terminal and the central access point. In this situation, a guard time equal to twice the maximum propagation time over the cell radius must separate each time slot, since the network has no knowledge from where in space each burst transmitted in a time slot will originate. The problem will be illustrated further with reference to
FIGS. 1 and 2
.
Within
FIG. 1
, a central access point
2
provides access to a wide area network (not shown) for a number of subscriber terminals
4
. The subscriber terminals may be scattered throughout the access point cell coverage area. The cell coverage area may further be split into sectors
3
and
5
wherein each sector is covered by a different frequency. Now consider one sector containing subscriber terminals
6
,
8
, and
10
, each respectively further from the access point than the former. With reference to
FIG. 2
, it is apparent that each particular upstream time slot
20
must have allotted an amount of time corresponding to the sum of a guard time
21
and a burst time
23
. The first time slot
20
follows immediately from the downstream burst
25
, allowing for RF turnaround time
27
. As the access point does not know how far away the terminal which has been allotted that particular transmission slot is, the guard time must be provided to allow for the maximum signal propagation delay across the cell. For example, if time slot
20
has been allotted to terminal
6
, then the burst
14
from terminal
6
begins to arrive only a little after the start of the guard time as shown by arrow
302
. However, if the time slot
20
is allotted to terminal
10
, then the signal propagation delay from terminal to the access point causes the upstream burst
18
from terminal
10
to begin to arrive at the end of the guard time as shown by arrow
304
. Where the subsequent time slot is then allotted to a different terminal (e.g. terminal
6
), there is the possibility that signals from the first time slot (i.e. from terminal
10
) and the subsequent time slot (i.e. from terminal
6
) could arrive at the access point concurrently, thus corrupting each signal. In order to avoid this, guard times (
22
) must be provided for each time slot. This clearly reduces overall transmission efficiency, as a significant portion of each upstream time slot must be vacant.
SUMMARY OF THE INVENTION
The present invention overcomes this problem by providing a method and system which compensates for the differences in signal propagation delay by causing each subscriber terminal to artificially simulate being at the same distance from the access point as every other subscriber terminal. This eliminates the need for guard times between each subsequent slot, as the propagation delays are simply forced to be the same for every subsequent transmission and hence there is no possibility of bursts transmitted in different time slots arriving at the access point at the same time. Only one guard time is required at the start of the very first upstream time slot to allow for the very first propagation delay. Subsequent time slots then do not require guard times as the delay is always the same. The removal of the requirement for guard times means that channel efficiency is improved.
According to the present invention, there is provided a method of compensating for signal propagation delay in a data communications network comprising a central control node and one or more remote subscriber nodes, comprising the steps of:
a) measuring the respective signal propagation delays for each subscriber node; and
b) using the respective signal propagation delays in each remote subscriber terminal to artificially simulate that each remote subscriber terminal is at the same distance from the central control node as every other remote subscriber terminal.
Each remote subscriber terminal may artificially simulate being at a maximum allowable distance corresponding to the maximum signal propagation delay from the central control node.
The data traffic on the network can be regulated per unit time frame by the central control node. In this case, the measuring step a) further comprises the steps of: designating a registration time slot per time frame in which the remote subscriber nodes may each first transmit a first transmission; transmitting said first transmission from the remote subscriber nodes at the start of the registration time slot; measuring one or more respective time period from the start of the registration time slot to the receipt at the central control node of each first transmission from each remote subscriber node, each time period corresponding to a respective propagation delay for transmission of signals from one of the remote subscriber nodes to the central control node; and indicating to each respective remote subscriber node the respective measured time period for that node; wherein each remote subscriber node then uses its respective measured time period to compensate for signal propagation delay when transmitting subsequent data traffic to the central control node.
The designating step may further comprise the steps of: defining said registration time slot for a present time frame at said central control node; and

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