This invention relates to a wireless communication system in which one radio access technology operates in conjunction with a second radio access technology. The invention has particular, but not exclusive, relevance to the co-existence of a GSM public land mobile network (PLMN) and an LTE or LTE-advanced PLMN.

Although LTE offers a number of advantages over older mobile communication standards, there is a need to maintain GSM coverage for voice calls and also for legacy telematics systems which utilise the GSM communication network. An LTE signal can have a system bandwidth of 1.4, 3, 5, 10, 15 or 20MHz, and is centred at a frequency that is a multiple of 100kHz. In the time domain an LTE signal is divided into contiguous radio frames of 10ms duration. As shown in Figure 4, each radio frame is made up of 10 sub-frames, each subframe of 2 resource blocks (RBs), which in turn are made up of a number of symbols. So, in the time domain a resource block is 0.5ms in duration. In the frequency domain a resource block is 180kHz wide, corresponding to 12 subcarriers, each separated by 15kHz. There are 6, 15, 25, 50, 75, and 100 resource blocks in the 1.4, 3, 5, 10, 15 or 20MHz bandwidth systems respectively. Control data is scheduled across the system bandwidth of the LTE signal.

A GSM signal has a nominal bandwidth of 200kHz centred at a frequency that is a multiple of 200kHz.

The present invention provides a new system in which GSM signals are embedded within the bandwidth of LTE signals. In this new system, a hybrid base station includes both an LTE eNodeB having an associated LTE frequency spectrum and a GSM base station in which the LTE eNodeB is arranged to clear space in the associated LTE frequency spectrum for the GSM signals. This can be achieved by inhibiting, within the physical layer of the LTE eNodeB, transmissions of LTE signals that would be in the RBs within the frequency range to be used for transmission of GSM signals. Optionally, the system can be enhanced by adding a datalink from the Basestation Subsystem (BSS) of the GSM cell to the eNodeB that carries timely information about the GSM traffic. Various embodiments of the invention will now be described, by way of example only, with reference to the attached figures in which: Figure 1 schematically shows a wireless communication system according to an embodiment of the present invention;

Figure 2 schematically shows components of a hybrid LTE/GSM node forming part of the wireless communication system illustrated in Figure 1; Figure 3 schematically shows physical layer processes performed by an eNodeB forming part of the hybrid LTE/GSM node illustrated in Figure 2; and

Figure 4 shows a radio frame for an LTE communication system having a bandwidth of 3MHz.

Figure 1 shows a wireless communication system including both an evolved packet core (EPC) 1 of a long term evolution (LTE) wireless communication system and a mobile switching centre (MSC) 3 of a GSM wireless communication system. A conventional enhanced node B (eNodeB or eNB) 5 provides an access point to the LTE wireless communication system and a base station sub-system 7 provides an access point to the GSM wireless communication system. In addition, wireless communication apparatus 9 according to the present invention, hereafter referred to as a GiLTE node 9, provides an access point to the LTE wireless communication system and an access point to the GSM wireless communication system. In this way, an access terminal for the LTE wireless communication system, hereafter referred to as UE 11, can communicate with the EPC 1 via either the eNodeB 5 or the GiLTE node 9, and an access terminal for the GSM wireless communication system, hereafter referred to as an MS 13 can communicate with the MSC 3 via either the BSS 7 or the GiLTE node 9. The EPC 1 provides for the LTE 11 to communicate with remote network devices via the Internet 15, and the MSC 3 provides for the MS 13 to communicate with remote telecommunication devices via a Public Switched Telephone Network (PSTN) 17. Figure 2 shows the main components of the GiLTE node 9. As shown, the GiLTE node 9 includes a modified eNodeB 21 and a BSS 23, which are both connected to backhaul 25 for communication with the EPC 1 and MSC 3 respectively. Further, the modified eNodeB 21 and the BSS 23 are both connected to a radio transceiver 27 for communication with a UE 11 and a MS 13 respectively. In particular, the radio transceiver 27 communicates with UEs 11 and MSs 13 using radio signals within a frequency band. In this embodiment, this frequency band has 3MHz of paired spectrum from 1876.7 MHz to 1880.0 MHz for the downlink and 1781.7 MHz to 1785 MHz for the uplink, which is within the DECT

Guardband in the UK. At present, the licensing of radio transmissions in this frequency band is technology agnostic. The invention can be utilised in DECT Guardbands for other countries, or at radio frequencies outside of the DECT Guardband in a country. Within the frequency band, it is desired to send both LTE signals, allowing for data connectivity, and GSM signals, to take account of legacy mobile phones and telematics systems. According to this embodiment of the invention, a 3MHz bandwidth GiLTE cell is provided together with a single GSM cell at a frequency within the frequency band over which the LTE signals are transmitted. This advantageously allows for greater data throughput in comparison with using a 1.4 MHz LTE bandwidth cell with the rest of the bandwidth being used by GSM carriers. There are, however, challenges to having the GSM cell operating at a frequency within the system bandwidth of an LTE cell.

The purpose of the changes to the modified eNodeB 21 is to leave space in the LTE system bandwidth for the GSM signals. A GSM signal has a nominal bandwidth of 200kHz centred at a frequency that is a multiple of 200kHz. An LTE signal has a system bandwidth of 1.4, 3, 5, 10, 15 or 20MHz, and centred at a frequency that is a multiple of 100kHz. To allow one or more GSM signals to be transmitted within the LTE spectrum, the LTE signal must be suppressed across the band(s) occupied by GSM signals. To create space for one or more GSM signals, no LTE signal is transmitted on RBs with specified indices. (Note that as LTE and GSM have the same duplex spacing in relevant bands, the index of the RBs carrying the downlink and uplink GSM signals will be the same). This is achieved by modifying the eNodeB's physical layer and scheduler, as described below.

For the downlink, the Physical Layer is modified so that LTE signals that would be in the RBs selected to carry GSM signals are not transmitted. The effect of this change on LTE performance is mitigated by:

• changes to the eNodeB's RRM function, specifically the downlink scheduler,

• careful placement of the GSM signals within the LTE signal by a new O&M function.

For the uplink, the RRM functions are modified to ensure that no UE is commanded to transmit in the RBs selected to carry GSM signals. As with the downlink, the effect of this on LTE system performance is mitigated by the placement of the GSM signals. As will be described in more detail hereafter, the modification of the Physical Layer that inhibits the downlink signals in any of the RBs that are specified for use of a GSM signal involves the addition of a notch filter function, which may be invoked continuously or intermittently. Intermittent operation is appropriate where the GSM BSS provides timely information on the utilisation of the GSM signals, such as occupied timeslots for traffic channels.

The modification of the downlink scheduler is to prevent the scheduling of Physical Downlink Shared Channels (PDSCHs) for transmission over RBs being used by the GSM signals. Further, in the uplink UEs are not allocated RBs being used for GSM signals for the transmission of Physical Uplink Shared Channels (PUSCHs) or Sounding Reference Signals (SRSs).

The new O&M function informs both GSM BSS and LTE eNodeB of the location of GSM signal(s) and which of these are designated as the beacon (CO). In locating the GSM signals, and hence the LTE RBs that are assigned to carry it, the O&M function ensures the following.

• The overlap between the GSM allocation and the RBs used for PRACH is minimised.

• The overlap between the GSM allocation and the RBs used for PCFICH and PHICH is minimised, for example by careful selection of the Physical Layer Cell Identity.

• ARFCN and EARFCN are selected to maximise the minimum separation of GSM bandedge to the edge of the closest RBs above and below the GSM gap that carry LTE.

For the GSM cell to overlay a single LTE cell we need the duplex separation to be the same. Examples of such bands include the following.

• GSM 450 with E-UTRA Band 31.

• GSM 710 with E-UTRA Band 12. · T-GSM 810 with E-UTRA Band 27.

• GSM 850 with E-UTRA Band 5.

• P-GSM 900 with E-UTRA Band 8.

• E-GSM 900 with E-UTRA Band 8. • R-GSM 900 with E-UTRA Band 8.

• DCS 1800 with E-UTRA Band 3.

• PCS 1900 with E-UTRA Band 2.

• ER GSM 900 with E-UTRA Band 8. Note that in some cases the FDD LTE band does not exactly match the GSM band so some ARFCN may not be supported.

The GSM cells are on a 200kHz raster. The LTE cells are on a 100kHz raster. This means that the centre of two adjacent RBs may lie on a raster frequency, or may be displaced from the raster frequency by 20kHz or 40kHz. So, provided there is a free choice of the ARFCN, the nominal 200kHz GSM channel may be located in one of the five positions. This gives a minimum transition bandwidth of 40kHz, rather than the 80kHz that would be available if the GSM cell is centralised.

If only a single ARFCN is supported, then there is by definition no GSM frequency hopping. If more than one ARFCN is supported, then hopping in the usual manner is acceptable. In the basic scheme there is no LTE frequency hopping.

In the basic scheme all LTE transmissions in the selected RBs are suppressed by the notch filter function. The eNodeB is modified to alleviate the effect of this on system performance in a channel dependent fashion. Examples of this follow.

• PDSCH - by modifying the scheduler not to use these RBs. · PDCCH - by using a lower coding rate, if necessary.

• PHICH - by minimising the overlap with the suppressed RBs.

• PCFICH - by minimising the overlap with the suppressed RBs.

• PBCH - by minimising the overlap with the RBs.

• PSS and SSS - the synchronisation signals are essential to operation of the modified eNodeB 21, and accordingly the notch filter function (and therefore the GSM signals) is positioned not to affect these.

• CSRS - given that the CSRS is typically used for demodulation of local resource elements, as none are being sent the suppression of the CSRS by the notch filter is acceptable, although there may be an impact on pathloss estimation and accordingly power control.

Given the importance of the PBCH to correct operation, avoiding use of these RBs for GSM is preferable. The RBs occupied by PCFICH and PHICH are a function of a number of system parameters. The example given in Table 1 below is for:

. 3MHz bandwidth (ie 15 RBs)

• 7 symbols per slot

• 12 subcarriers per UE

• Normal cyclic prefix

• PHICH group scaling of one half (ie only a small number of PHICH groups) and for physical layer cell IDs of 0 to 10.

This shows that it is possible to find neighbouring RBs, marked "X", that could be used for GSM that avoid those RBs used for PBCH (the central 7 RBs in this example) and those used for PCFICH ( C) and PHICH (H).

Table 1

The situation is likely to improve at higher LTE system bandwidths because although the number of PHICH groups will increase the PCFICH size remains the same.

The location of the GSM RBs in the UL is the same as their location in the DL. So, if the GSM RBs are pushed to the bandedge, then they will overlap with the location of the PUCCH. This is undesirable.

A larger PUCCH region than we need for LTE may be defined and indexing may be used to avoid the outer RBs, but as the PUCCH region is at both ends of the band this might be prohibitively inefficient. A better solution might be to move the GSM RBs away from the bandedge (as is seen to be necessary in any case from the discussion on avoiding PCFICH and PHICH). The SRS would need to be set so that it avoids the GSM RBs.

As discussed above, a notch filter function is provided in the Physical Layer of the modified eNodeB 21 to make space in the LTE system bandwidth for the GSM signals by ensuring that no LTE signals are transmitted on RBs with specified indices. Figure 3 shows an implementation of the notch filter function which employs a frequency domain notch filter to zero the inputs to the IFFT used in the OFDM modulator corresponding to frequencies used by the GSM cell. Alternatively, the notch filter function could be implemented using a time domain filter following the IFFT or modification of the physical channel so that signals in the suppressed region are not created. Modifications and Further Embodiments

The PHICH and PCFICH could be transmitted in the GSM RBs. The LTE signal will be seen as interference to the GSM signals, but have a low duty cycle and are subject to beating. They will overlap with the training sequence about 7% of the time. The GSM signal will corrupt the LTE signal. Where we need to transmit GSM CO, this is always present. By clearing 3 RBs there would nominally be space for 2 GSM chanels. The present invention is equally applicable to LTE-advanced, in which addition traffic bearing RBs are provided. Further, it will be appreciated that the invention can be applied to other communication system having radio access technologies in which a first radio access technology communicates wirelessly using a first carrier at a first frequency and having a first bandwidth and a second radio access technology communicates wirelessly using a set of carriers at respective different frequencies within a second frequency band, wherein the second frequency band has a second bandwidth larger than the first bandwidth and the first frequency is within the second frequency band.