METHOD FOR SCHEDULING SIMULTANEOUSLY TERMINALS IN A COMMON FREQUENCY BAND, AND CORRESPONDING BASE
STATION.
BACKGROUND OF THE INVENTION
The present invention relates to a method for scheduling simultaneously terminals belonging to two radio communication systems in a common frequency bond.
In common radio communication system, when a new standard starts operations, a new frequency band is chosen so that the operations of a legacy system and the operation of a new system are well separated.
This presents the disadvantage not to optimize the use of the different frequency bands. The frequency bands of both new and legacy system are dimensioned taking into account the full capacity of both networks although at the beginning of bperations of the new system the legacy system still has a high number of users while after some months the use of the legacy system decrease for the benefit of the new system. This results in a non optimal usage of the frequency band for the new system at the beginning of its operation and a non optimal usage of the legacy system at the end of its operation.
A particular object of the present invention is to provide a better usage of the frequency spectrum in case a new system is intended in the short or middle term to replace the legacy system.
Another object of the invention is to provide a corresponding base station of the new system providing backward compatibility to the legacy system. SUMMARY OF THE INVENTION

These objects, and others that appear below, are achieved by a method for scheduling simultaneously terminals belonging to two radio communication systems in a common frequency band according to claim 1, and a base station according to claim 8.
According to the present invention, two radio communication systems (e.g. a legacy system and a new system) share the same frequency band, one system, herein called first system, being aware of the operation of the other system, herein called second system. The first system is able to detect the amount of resources needed by the second system and is able to allocate dynamically resources to users belonging to the first and to the second systems depending on the number of users of both systems and the resource they require, the resource sharing being operated in the frequency and also preferably in the time domain.
The method according to the present invention presents the advantage to limit the necessary frequency spectrum in case of upgrade from a legacy to a new system exploiting the dynamic resource allocation between the two systems.
Another advantage of the present invention consists in allowing a smooth migration strategy for the operator without the need to replace all legacy terminals when introducing new and more efficient communication systems in existing frequency bands. The introduction of new systems using the same frequency band as a legacy system can be made completely transparent to the subscribers.
Further advantageous features of the invention are defined in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS
Other characteristics and advantages of the invention will appear on reading the following description of a preferred embodiment given by

way of non-limiting illustrations, and from the accompanying drawings, in which:
- Figure 1 shows two systems cooperatively operating in the same frequency band according to the present invention;
- Figure 2 shows a first embodiment of a frame structure of a first system coexisting with a second system in the same frequency band;
- Figure 3 shows a second embodiment of a frame structure of a first system coexisting with a second system in the same frequency band;
- Figure 4 shov« a base station according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 shows two systems cooperatively operating in the same frequency band according to the present invention. In this simplified representation, the first system, which is preferably a new system, comprises a terminal 12 and a base station 13 and the second system, which is preferably a legacy system, comprises a terminal 11 and a base station 14. According to the present invention, the base station 13 on the first system is able to perform a cooperative resource sharing between terminals 11,12 of both first and second systems in the same frequency band. Both systems are using OFDM/OFDMA techniques on the air interface for exchanging information so that the frame structure of both systems comprise a time and frequency extension.
The resource sharing is, according to the present invention, performed in the frequency domain by base station 13, and preferably additionally also in the time domain.
The scheduler performing resource allocation in the first system is aware of both first and second system requirements. Preferably, the allocation of resources for the second system is performed dynamically, on an first system frame duration basis, according to predefined criteria (e.g. number

of users of the first and second systems, traffic requirement of the first and second systems.
Figure 2 shows a frame structure of a new system coexisting with a legacy system in the same frequency band, the bandwidth of the new system being larger than the bandwidth of the legacy system.
In this embodiment the bandwidth of the new system is larger than the bandwidth of the legacy system. This means that the second system's frame extension is as follow: the frequency extension is Y' sub carriers where Y' and forwarded to scheduler 41. Scheduler 41 performs a resource sharing according to the present invention as described in Fig 2 and 3 and sends grants to the terminals of both systems over transmitter 42,

This way, both systems can coexist in the same frequency band using dynamic assignment on a frame by frame basis. The base station is then able to support both first and second systems terminals transparently.
Such an invention could be envisaged in the field of 802.16e evolution, standard as 802.16m being able to be smoothly introduced to replace slowly the 802.16e standard.

CLAIMS
1/ Method for scheduling simultaneously terminals belonging to two radio communication systems in a common frequency band, a base station of said first communication system having knowledge of said second radio communication system, both systems using an OFDM resource allocation mechanism on the air interface, an OFDM frame structure being predefined for said first system, said method being characterised in that it comprises the steps of:
- detecting in said first system the amount of resources needed for said second system for a predefined time period;
- determining a first part of said OFDM frame reserved for said first system and a second part of said OFDM frame reserved for said second system for said predefined time period, said first and second part of said OFDM frame being separated in the frequency domain and taking into account the resources needed for said second system.
2/ Method according to claim 1, wherein said first and second part of the OFDM frame are also separated in the time domain.

3/ Method according to claim 1, wherein said predefined time period corresponds to the OFDM frame duration in said first system.
4/ Method according to claim 1, wherein said frame duration in said second system is shorter than said frame duration in said first system.
5/ Method according to claim 1, wherein said first and second systems are working in Time Division Duplex mode.
6/ Method according to claim 1, wherein said frame duration in said second system is longer than said frame duration in said first system, downlink resources being allocated to a terminal belonging to said second system during the same time duration as the downlink frame part of said first system, uplink resources being allocated to said second terminal during the same time duration as the uplink frame part in said first system, the frequency and time extension of the resources allocated to said first system and to second system during the downlink frame part duration and the uplink frame part duration of the first system being disjoint.
II Method according to claim 1, wherein the frame structure of said first system comprises a field containing information on said first and second part of said OFDM frame.
8/ Base station adopted to be used in a first radio communication system and adapted to schedule in a common frequency band terminals belonging to said first and a second radio communication system both using an OFDM mechanism on the air interface, said base station comprising:
- means for detecting in said first system the amount of resources needed for said second system for a predefined time period;

- means for determining a first part of said OFDM frame reserved for said first system and a second part of said OFDM frame reserved for said second system for said predefined time period, said first and second part of said OFDM frame being separated in the frequency domain and taking into account the resources needed for said second system.