TITLE

METHOD OF TRANSMISSION IN A COMMUNICATIONS NETWORK

FIELD OF THE INVENTION

The invention generally relates to a method of trans ^¬ mission in a communications network. More particu ^¬ larly, the invention relates to radio resource man ^¬ agement in a macro wireless network cell including one or more femto cells accessible via femto access points or home Node Bs (HNBs) .

BACKGROUND OF THE INVENTION

A femto access point is a small base station designed for use in a home or enterprise (business) environ ^¬ ment. It provides access to a femto cell, which is located in a macro cell of a radio access network, where access to the macro cell is provided by a regu- lar base station, Node B or eNode B.

A femto cell allows mobile network operators to ex ^¬ tend network coverage indoors, especially where ac ^¬ cess would otherwise be limited or unavailable. Femto access points can be used to provide access to femto cells in any standardized radio network technology, for example GSM, WCDMA, CDMA2000, TD-SCDMA, WiMAX and LTE . A femto access point is also known as a home base station (HBS) in GSM, a home Node B (HNB) in UMTS and a home eNode B (HeNB) in LTE .

Femto cells are very attractive to network operators, as they can provide improvements to both network cov ^¬ erage and capacity, especially indoors. This can re ^¬ duce both capital expenditure and operating expense. There may also be opportunity for new services to be provided by the operator. Network subscribers may benefit from improved coverage and better voice qual ^¬ ity and battery life, as well as operator-dependent benefits, for example discounted calls from home.

A subscriber station or user equipment (UE) located in the vicinity of a HBS or HNB, yet connected to or camping on a macro base station (MBS) or macro Node B operating on a first frequency fl may be unaware of the presence of the HNB operating on a second fre ^¬ quency f2. As a means of encouraging handover of the UE from the macro Node B on fl to the femto HNB on f2 (where handover from a macro to a femto cell is known as capture) the HNB may transmit a low power downlink signal, known as a beacon on fl in order to trigger inter-frequency measurements and synchronization in the UE to lead to capture of the UE by the HNB.

However, there are problems associated with using the beacon for macro to femto cell capture. Firstly, the beacon triggers unnecessary inter-frequency measure ^¬ ments in nearby UEs that are not permitted to connect to the HNB (for example because they are non Closed

Subscriber Group (CSG) UEs) . Secondly, the beacon causes interference to the nearby non-CSG UEs. For example, at a beacon power of 5 dBm, the macro UE average downlink throughput is reduced by approximately 10% compared to when the beacon is absent.

The invention seeks to provide a solution to at least some of the problems out lined above.

SUMMARY OF THE INVENTION

Accordingly, the invention provides a method of transmission in a communications network. The method includes monitoring a reception parameter of the network on a first frequency at a network node, wherein the network node is configured to exchange data with the network on a second frequency, triggering switching on of a signal at the network node when a change in the reception parameter on the first frequency is detected by the network node, and transmitting the signal from the network node to the network on the first frequency.

The signal can be a noise signal or beacon signal, which is transmitted from the network node to encourage handover of a subscriber station operating on the first frequency to the network node operating on the second frequency. A reception parameter of the net ^¬ work is monitored on a first frequency by the network node, which can exchange data with the network on a second frequency (although data exchange on the sec ^¬ ond frequency may not be taking place when the recep ^¬ tion parameter is being monitored on the first fre- quency) . If a change in the reception parameter is detected on the first frequency by the network node, a signal is triggered to be switched on at the net ^¬ work node and transmitted to the network on the first frequency .

When the change in reception parameter is detected, this may indicate to the network node that a sub ^¬ scriber station subscribed to the network node is in the vicinity. The invention thus provides that the signal is only switched on and transmitted to the network if subscriber stations are nearby. In this way, unnecessary inter frequency measurements of sub ^¬ scriber stations are avoided, as well as associated problems such as battery drain on subscriber stations. Furthermore, since the signal is only switched on when a change in reception parameter is detected, this reduces the power consumption of the network node and reduces interference on the first frequency .

In one embodiment of the invention, switching on of the signal comprises gradually increasing a power level of the signal. When the signal is switched on, the power level of the signal may be gradually in ^¬ creased from zero. Alternatively, the power level may be gradually increased from a non-zero value.

The power level of the signal may also be increased in proportion to a received power level on the first frequency, for example according to a distance be ^¬ tween the network node and a subscriber station. The change in the reception parameter detected by the network node may be the exceeding a predetermined threshold. Alternatively, the change may be a first noise level to a second noise level, an increase in uplink transmission power or a change in pathloss function, for example.

Preferably, the method may further include handing over an exchange of data between a subscriber station and the network on the first frequency to the second frequency if the subscriber station reacts to the signal transmitted from the network node.

Advantageously, transmissions on the first frequency and the second frequency may be switched off until a receiver threshold is exceeded on the first fre ^¬ quency. In this way, power is saved by only allowing transmissions when a receiver threshold is exceeded.

The invention also provides a network node for ex ^¬ changing data with a communications network. The net ^¬ work node includes a transceiver configured to moni ^¬ tor a reception parameter of the network on a first frequency and configured to exchange data with the network on a second frequency. A detector is pro ^¬ vided, which is configured to detect a change in the reception parameter on the first frequency. A switch is configured to trigger the transceiver to transmit a signal on the first frequency when the change in the reception parameter on the first frequency is de ^¬ tected by the detector. Since the signal is only switched on and transmitted when a change in the re ^¬ ception parameter is detected, this avoids unneces ^¬ sary inter frequency measurements and accordingly saves power in the network node and in nearby sub ^¬ scriber stations. In addition, interference is re ^¬ duced on the first frequency. In one embodiment of the invention, the network node is a home base station, such as a home Node B, for providing access to a femto cell of the network.

The switch may be configured to gradually increase a power level of the signal. The power level of the signal may be increased from zero or from a non-zero value. The switch may be further configured to gradu ^¬ ally increase the power level of the signal in pro ^¬ portion to a received power level received by the transceiver on the first frequency. This could be dependent on a distance between the network node and a nearby subscriber station, for example.

The change in the reception parameter may be the ex- ceeding a predetermined threshold, a change from a first noise level to a second noise level, an in ^¬ crease in uplink transmission power or a change in pathloss function, for example. In one embodiment of the invention, the transceiver is configured to exchange data with a subscriber sta ^¬ tion on the second frequency if the signal transmit ^¬ ted from the network node is reacted upon by the sub ^¬ scriber station. The signal can be used to trigger handover from the subscriber station from operating on the first frequency to operating on the second frequency and exchanging data with the network node. However, since the signal is only transmitted from the network node if a change in reception parameter is detected by the network node, unnecessary inter frequency measurements at the subscriber station are avoided, which saves power and increases the battery life of the subscriber station.

In one embodiment of the invention, the switch is configured to switch off transmissions on the first frequency and the second frequency when a receiver threshold in the transceiver is below a predetermined level and to switch on the transmissions on the first frequency and the second frequency when the threshold is equal to or exceeds the predetermined level. This provides the advantage that power consumption at the network node is reduced.

The invention further provides a communications net ^¬ work, which has a network node for exchanging data with a network. The network node includes a trans ^¬ ceiver configured to monitor a reception parameter of the network on a first frequency and to exchange data with the network on a second frequency. A detector is configured to detect a change in the reception pa ^¬ rameter on the first frequency. A switch is configured to trigger the transceiver to transmit a signal on the first frequency when the change in the recep ^¬ tion parameter on the first frequency is detected by the detector. Since the signal is only switched on and transmitted when the change in reception parame ^¬ ter is detected, this provides the advantage of re- ducing unnecessary inter frequency measurements, re ^¬ ducing interference on the first frequency and reduc ^¬ ing power consumption in the network. In one embodiment of the invention, the network node is a home base station, such as a home Node B, which is configured to provide access to a femto cell of the network. The signal is then transmitted to the network from the network node to encourage handover of a subscriber station from operating in a macro cell of the network on the first frequency to operat ^¬ ing in the femto cell on the second frequency. Advantageously, the switch may be configured to gradually increase a power level of the signal. The switch may be configured to increase the power level of the signal from zero or, alternatively to increase the power level of the signal from a non-zero value. Further, the switch may be configured to gradually increase a power level of the signal in proportion to a received power level received by the transceiver on the first frequency. The change in the reception parameter may be the ex ^¬ ceeding a predetermined threshold, a change from a first noise level to a second noise level, an in ^¬ crease in uplink transmission power, or a change in pathloss function, for example.

The network may further include a subscriber station. The transceiver of the network node may then be configured to exchange data with the subscriber station on the second frequency if the signal transmitted from the network node is reacted to by the subscriber station. This provides the advantage of a reduced power consumption and increased battery life in the subscriber station, due to avoidance of unnecessary interference measurements. Preferably, the switch is configured to switch off transmissions on the first frequency and the second frequency at the network node when a receiver thresh- old in the transceiver is below a predetermined level and the network node is not exchanging data with a subscriber unit on the second frequency. The switch can then be further configured to switch on the transmissions at the network node on the first fre- quency and the second frequency when the threshold is equal to or exceeds the predetermined level. In this way, power consumption at the network node is reduced . The invention will now be described, by way of example only, with reference to specific embodiments, and to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a simplified schematic diagram of a wire ^¬ less communications network according to an embodi ^¬ ment of the invention;

Figure 2 is a simplified schematic diagram of a net ^¬ work node according to an embodiment of the inven ^¬ tion; Figure 3 is a graph of noise rise as a function of pathloss at the network node when no subscriber sta ^¬ tion is in the vicinity of the network node; Figure 4 is a simplified schematic diagram of noise rise as a function of pathloss at the network node when a subscriber station in the closed subscriber group of the network node is in the vicinity of the network node;

Figure 5 is a flow diagram illustrating a method according to an embodiment of the invention;

Figure 6 is a message flow diagram illustrating a method according to an embodiment of the invention; and

Figure 7 is a simplified schematic diagram of noise rise threshold for a beacon signal "on" decision as a function of pathloss between a home Node B and a macro Node B.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Figure 1 schematically shows a communications network according to an embodiment of the invention. The network includes a macro cell CI, which operates on a frequency fl and is accessible via a macro Node B MNB . A femto cell C2 operating on a frequency f2 is located within the macro cell CI and is accessible via a home base station or home Node B HNB . The home Node B HNB may be located in a home environment, on a business premises or similar.

Subscriber stations (also known as user equipment (UEs)) UE1, UE2 and UE3 may access the network and transmit and receive data on the frequency fl via the macro Node B MNB . The subscriber station UE1 is in the closed subscriber group (CSG) of the home Node B HNB and may also use the home Node B HNB to access the network and transmit and receive data on the fre- quency f2. The subscriber station UE1 is therefore located closer to the home Node B HNB than the other subscriber stations UE2 and UE3.

The home Node B HNB is shown m more detail m Figure 2. It includes a transceiver T configured to exchange data with the network on the frequency f2 so that the home Node B operates on the frequency f2. In other words, the transceiver T of the home Node B HNB can receive uplink data from the UE UE1 (and any other UEs in its CSG) and transmit downlink data to the UE UE1 on the frequency f2. However, the home Node B HNB may also communicate with the network on the fre ^¬ quency fl of the macro cell CI. In particular, the transceiver T is configured to transmit a beacon sig ^¬ nal to the network on the frequency fl, which facili ^¬ tates capture (handover) of UEs in the CSG of the home Node B HNB from the macro cell CI to the femto cell C2. A detector D is coupled to the transceiver T and is configured to monitor and detect a change in a recep ^¬ tion parameter on the frequency fl. The change in reception parameter occurs when a UE in the CSG of the home Node B HNB, such as the UE UE1, comes into the vicinity of the home Node B HNB while exchanging data with the macro Node B MNB. A switch S is coupled to the detector D and the transceiver T and is configured to trigger the transceiver T to transmit the beacon signal on the frequency fl only when the change in the reception parameter on the frequency fl is detected by the detector D.

In order for the home Node B HNB to be able to moni ^¬ tor reception parameters, for example UL noise (rise over thermal noise (RoT) ) on a different frequency carrier fl, compared to the frequency carrier f2 it operates on, the transceiver T may include an addi ^¬ tional UL receiver. Alternatively, the transceiver T may include a single UL receiver, switching the UL frequency from time to time between f2 and f1. How ^¬ ever, this implementation cannot be realised if a UE is already connected to the home Node B HNB.

In one embodiment, the change in reception parameter detected at the home Node B HNB by the detector D is an uplink noise rise.

In the network illustrated in Figure 1, the UE UE1 in the CSG of the home Node B is located closer to the home Node B HNB than the other UEs UE2 and UE3, in practice most likely in the same room of a building. If the terminal UE1 also remains connected to the macro cell CI on the frequency fl when it is in the vicinity of the home Node B HNB, it causes a signifi ^¬ cantly higher UL noise rise compared to the other terminals UE2 and UE3, which are likely to be located in different rooms or different buildings to the ter ^¬ minal UE1, or outdoors.

This embodiment of the invention will now be de ^¬ scribed with reference to the graphs shown in Figures 3 and 4, and the flow diagram shown in Figure 5. The UL noise level in the network is monitored by the home Node B HNB at the detector D in step 100 and the UL noise level is then used by the switch S to con ^¬ trol the TX power of the beacon signal transmitted by the transceiver T. In its simplest form the beacon power is controlled by the switch S in an on/off fashion; i.e., the switch S switches the beacon sig ^¬ nal on if the UL noise level equals or exceeds a cer ^¬ tain predetermined threshold (step 101) and the bea- con signal remains off if the UL noise level remains below that predetermined threshold.

This is illustrated by the graphs of noise rise as a function of pathloss between the home Node B HNB and the macro Node B MNB in Figures 3 and 4. As shown in

Figure 3, the noise rise detected by the detector D (step 101 of Figure 5) in the absence of a UE active in the macro cell CI in the vicinity of the home Node B HNB is relatively low. In this case, the noise rise is below the predetermined threshold (step 101) and the beacon signal remains switched off (step 102), which avoids generating unnecessary interference in the macro cell CI. If the UE UE1, which is active in the macro cell CI on the frequency fl and is in the CSG of the home Node B HNB, moves into the close vicinity of the home Node B HNB, the noise rise detected by the detector D on the frequency fl (step 101) is significantly higher, as shown in Figure 4. This means that the predetermined noise rise threshold is exceeded, which indicates that a user of the femto cell C2 is likely to be in the close vicinity of the home Node B HNB. The switch S is then triggered to switch on the bea- con signal (step 103), which is then transmitted by the transceiver T on the frequency fl (step 104) .

If the UE UE1 reacts to the beacon signal, handover of the UE1 from the macro cell CI on the frequency fl to the femto cell C2 then takes place. The UE UE1 then accesses the network via the home Node B HNB, instead of the macro Node B MNB, and starts operating on the frequency f2 instead of f1.

One embodiment is illustrated by the message flow diagram in Figure 6. The home Node B HNB transmits signals to the network on the frequency f2 (e.g. Pi ^¬ lot, SIB, CSG-ID, etc.) and performs RX UL measure- ments on the frequencies fl and f2. An evaluation of the RX UL measurements is performed at the home Node B HNB on f1. If the detector D detects an active UE UE1 in the vicinity, then the HNB transmits the bea ^¬ con signal (for example a noise signal) on the fre- quency fl, for example with increasing power, to attract the UE UE1 and trigger the UE UE1 to perform measurements on alternative frequencies to fl (in ad ^¬ dition to measurements on fl) . A timer is started at the home Node B HNB and on expiry of the timer the home Node B stops transmission of the beacon signal.

The UE UE1 detects a worse SNR and interference, as well as more noise, and reduces its RX quality and data rate. The UE UE1 sends a measurement report for the femto cell C2 on the frequency f2 to the macro Node B MNB. In Case 1, the UE may not be allowed by the macro Node B MNB to enter the femto cell C2 but the home Node B is not notified to stop transmission of the beacon (noise) signal. In Case 2, the macro Node B MNB performs a measurement evaluation based on the measurement report received from the UE UEl. If, based on the measurement evaluation, the macro node B MNB determines that a handover is required, the macro Node B sends a handover request to the UE UEl, re ^¬ questing the UE UEl to handover to cell C2 and the home Node B HNB on the frequency f2. If the macro Node B MNB determines that handover is not required, it triggers the UE UEl to perform a new network measurement ( s ) .

In one embodiment, instead of having a fixed on/off threshold related to UL noise rise or UL energy, the threshold can be a function of the pathloss PL from the strongest macro base station, in this case the macro Node B MNB, using the NLM, for example:

Threshold = f (PL) (1), where f(PL) = y - PL + S (2)

Where the parameters γ and δ may be predicted from Monte Carlo simulations, for example, or configured by the network. An illustration of the pathloss dependent decision threshold above which the beacon signal is switched on at the home Node B is shown in Figure 7, which is a graph of noise rise at the home Node B as a func ^¬ tion of pathloss from the macro Node B MNB to the home Node B HNB. Below the calculated threshold, the beacon signal remains switched off and when the threshold is reached or exceeded, the switch S is triggered to switch on the beacon signal, which is then transmitted by the transceiver T on the fre ^¬ quency f1.

In one embodiment, the 'off state is implemented as a predetermined constant very low power beacon sig ^¬ nal transmission, which ensures a minimum capture effect of UEs by the home Node B. The ' on ' state is then implemented by increasing the beacon TX power from the predetermined constant very low power.

In addition to using the NLM to define the on/off threshold, in one embodiment the power of the beacon signal in the on and off states is set based on the NLM measurements.

When moving from the on state to the off state (or vice versa) , in one embodiment of the invention the power of the beacon signal is be gradually ramped up (or ramped down, respectively) . This avoids a sud ^¬ den loss of service of the macro cell CI to the UE UE1.

In one embodiment, the beacon signal in the on state is switched off after a pre-configured time out pe ^¬ riod. In the case that a non-member UE UE2 or UE3 is close to the home Node B HNB and thus generates significant noise rise, this off-threshold is useful to avoid spamming the UE UE2, UE3 that is not in the CSG of the home Node B HNB and will therefore anyway not handover to the HBS .

In one embodiment, the beacon signal state is changed from on to off based on a successful capture of the UE UE1 by the home Node B HNB, rather than based on a reduction in UL noise rise. Alterna ^¬ tively, the beacon signal state is changed from on to off based on a combination of a successful cap ^¬ ture of the UE UE1 by the home Node B HNB and on a reduction in UL noise rise.

In one embodiment, hysteresis is added to a decision threshold triggering switching on of the beacon signal .

In one embodiment, the decision threshold is fine tuned during the operation of the home Node B HNB based on collected statistics such as capture rate, the number of access attempts by non-CSG UEs, long term noise rise statistics, and a sudden increase of the noise rise above the current level.

In one embodiment, change of the state of the beacon signal is triggered by the user, for example by mak- ing the switch S a push button switch.

In one embodiment, the macro cell CI operates on more than one frequency, for example fla and fib. The de ^¬ tector D in the home Node B HNB monitors both the frequencies fla and fib. This can be done by sweep ^¬ ing the frequencies fla and fib one at a time. If a change in reception parameter is detected by the de ^¬ tector D on one or both of the frequencies fla and fib, for example a change in noise rise, the switch S is triggered to switch on the beacon signal at the home node B HNB for transmission by the transceiver T on the frequency (either fla or fib or both fla and fib) where the change in reception parameter has been detected. Handover of the UE1 may then take place from the macro node B MNB operating on fla and/or fib to the home Node B operating on f2. The macro cell CI may also operate on more than two frequencies, in which case the beacon signal is always triggered to be switched on and transmitted on the frequency or frequencies on which the change in reception parame ^¬ ter has been detected. One of the operating frequencies of the macro cell CI may be f2, the frequency of operation of the femto cell C2 (although in this case the beacon signal should not be a noise signal but should rather contain information such as synchronization channel, pilot, broadcast signalling etc.) .

In general, the various embodiments of the UEs UE1, UE2 and UE3 can include, but are not limited to, cellular telephones, personal digital assistants (PDAs) having wireless communication capabilities, portable computers having wireless communication ca ^¬ pabilities, image capture devices such as digital cameras having wireless communication capabilities, gaming devices having wireless communication capabilities, music storage and playback appliances hav ^¬ ing wireless communication capabilities, Internet appliances permitting wireless Internet access and browsing, as well as portable units or terminals that incorporate combinations of such functions.

Although the invention has been described hereinabove with reference to specific embodiments, it is not limited to these embodiments and no doubt further al ^¬ ternatives will occur to the skilled person that lie within the scope of the invention as claimed.

For example, while the exemplary embodiments have been described above in the context of the WCDMA sys ^¬ tem, it should be appreciated that the exemplary em ^¬ bodiments of this invention are not limited for use with only this particular type of wireless communica- tion system.

It should be noted that the terms "connected, " "cou ^¬ pled, " or any variant thereof, mean any connection or coupling, either direct or indirect, between two or more elements, and may encompass the presence of one or more intermediate elements between two elements that are "connected" or "coupled" together. The cou ^¬ pling or connection between the elements can be physical, logical, or a combination thereof. As em- ployed herein two elements may be considered to be

"connected" or "coupled" together by the use of one or more wires, cables and/or printed electrical con ^¬ nections, as well as by the use of electromagnetic energy, such as electromagnetic energy having wave- lengths in the radio frequency region, the microwave region and the optical (both visible and invisible) region, as several non-limiting and non-exhaustive examples . Further, some of the features of the various non- limiting and exemplary embodiments of this invention may be used to advantage without the corresponding use of other features. As such, the foregoing de ^¬ scription should be considered as merely illustrative of the principles, teachings and exemplary embodi ^¬ ments of this invention, and not in limitation thereof . LIST OF ABBREVIATIONS

3GPP 3 ^rd Generation Partnership Project

CSG Closed Subscriber Group

FAP Femto Access Point

HBS Home Base Station

HNB Home NodeB

NLM Network Listening Module

UE User Equipment

UL Uplink

RoT Rise over thermal noise

DL Downlink

TX Transmit

RX Receive