WO2008063820A1

WO2008063820A1 - Frequency reuse in communication systems

Info

Publication number: WO2008063820A1
Application number: PCT/US2007/082591
Authority: WO
Inventors: Remy Pintenet; David Grandblaise; Guillaume Vivier
Original assignee: Motorola, Inc.
Priority date: 2006-11-20
Filing date: 2007-10-26
Publication date: 2008-05-29
Also published as: GB2445599A; GB0622965D0; GB2445599B

Abstract

A first communication unit comprises a signature generator which generates a signal by spreading a first signature using a first spread spectrum code A transmitter transmits the signal in a first frequency interval at a first transmit power level The first frequency interval is a frequency interval assigned to the first communication unit for transmissions A second communication unit comprises a receiver which receives the signal in the first frequency interval A signature receiver generates a received signature by de-spreading the signal using the first spread spectrum code and a comparison processor determines if the received signature corresponds to the first signature If so, a transmitter is allowed to transmit in the first frequency interval and otherwise no transmissions are made by the second communication unit in the first frequency interval

Description

FREQUENCY REUSE IN COMMUNICATION SYSTEMS

Field of the invention

The invention relates to frequency reuse in communication systems and in particular, but not exclusively, to frequency reuse in hybrid communication systems comprising sub-communication systems operating in accordance with different radio standards.

Background of the Invention

Wireless communication systems are becoming increasingly ubiquitous and are continuously being developed to provide improved coverage and services. Currently, the trend is towards integrating different communication systems and standards to provide a more flexible and enhanced seamless user experience. Furthermore, there is an ever increasing desire to optimally exploit the scarce frequency resource. Specifically, because of the digital convergence and the over-increasing demand in terms of data rate, the wireless systems will require more and more bandwidth to support the emerging services

Recent measurements campaigns on the spectrum occupancy as a function of time, space and frequency indicate that great parts of the spectrum, although dedicated to some service, are not fully utilised. For example, the available resource may not be used for significant periods of time or in specific geographical areas. Accordingly, spectrum reuse opportunities exist both in time and space. In particular (as advocated by the Federal Communications Commission (FCC) in the United States of America) , spectrum reuse can be used by a secondary wireless system in situations where the primary system that has been allocated the spectrum allows this. Typically, the primary system will still require an element of control allowing it to dynamically regulate the spectrum use by the secondary system (s) .

At a given location and a given time, such spectrum reuse raises the following issues:

• How can the primary system efficiently inform the secondary systems that its assigned spectrum can be temporally reused?

• How can the secondary system reliably detect that the primary system' s originally assigned spectrum can be temporally reused?

• How can the primary system control the interference created by the second system (s) (which may for example use different radio access technologies) ?

• How can the secondary system be informed of the interferences it creates to the primary one?

Traditionally, resource allocation has been achieved by a centralised approach where a central resource controller allocates resource to individual communication units and systems. However, such an approach has a number of disadvantages. For example, the approach results in a substantial communication overhead and wasted resource in order to communicate all required control messages between the central controller and the remote terminals. Also, in order to achieve efficient resource usage, the central controller becomes complex and the system is sensitive to any failure of the central resource controller. In addition, the central controller requires knowledge of the possible parameter choices for all users which is unrealistic in practice (e.g. how many access technologies can be supported in parallel by the individual user, how many frequency bands it can decode in parallel, which modes are supported etc.) .

A centralised resource allocation system is particularly unsuitable for temporary spectrum reuse between systems as it requires that the systems are coordinated which is often impractical. In order to alleviate such problems, it has been proposed to use cognitive radio systems which can automatically detect whether a given frequency band is available for a temporary reuse. If so, the systems may proceed to use the frequency band without any further interaction with the primary system.

However, detecting whether a frequency band is available is not a trivial task. Different approaches have been proposed depending on the extent of knowledge of the characteristics of the signal and/or the noise/interference which is available to the individual units. However, conventional approaches tend to be suboptimal and have a number of disadvantages.

For example, H. L. Van Trees. Detection, Estimation, and Modulation Theory, Part I. Wiley, 1967 considers the use of a matched filter for signal level detection. This approach is optimal when there is a full knowledge about the signal structure but this information is not available in practice resulting in a relatively unreliable detection.

H. Urkowitz, Energy detection of unknown deterministic signals. Proceeding of the IEEE, 55 (4) : 523-531, Apr. 1967 discloses an energy detection method. However, the accuracy of the detection is dependent on the estimation of noise variance which is difficult to estimate and accordingly leads to suboptimal results.

Hence, an improved system would be advantageous and in particular a system allowing improved resource reuse between systems, increased flexibility, reduced complexity, improved reliability, reduced communication overhead and/or improved performance would be advantageous .

Summary of the Invention

Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

According to a first aspect of the invention there is provided a communication system comprising: a first communication unit comprising: means for generating a signal by spreading a first signature using a first spread spectrum code, a first transmitter for transmitting the signal in a first frequency interval at a first transmit power level, the first frequency interval being assigned to the first communication unit for transmissions; and a second communication unit comprising: a second transmitter; a receiver for receiving the signal in the first frequency interval; means for generating a received signature by de-spreading the signal using the first spread spectrum code; and means for determining if the received signature corresponds to the first signature; and wherein the transmitter is arranged to transmit in the first frequency interval only if the received signature corresponds to the first signature.

The invention may allow improved and/or facilitated frequency reuse. In particular, a reliable detection and autonomous decision whether to temporarily use a frequency interval assigned to another communication unit or system can be achieved for individual communication units .

Specifically, the invention may allow or facilitate e.g. that a frequency interval assigned/allocated/reserved to one communication system comprising the first communication unit is automatically reused by a second communication system comprising the second communication unit if e.g. the interference is sufficiently low (thereby allowing the signature to be correctly decoded) .

The invention may allow or facilitate a reliable and/or low complexity approach for secondary users to detect whether a first frequency interval of a primary communication system is available for reuse while providing the primary communication system with efficient means of controlling the reuse. Thus, the invention may allow an efficient way of a primary communication system indicating that a first frequency interval is available for reuse while automatically controlling that such frequency reuse is limited to acceptable levels.

According to an optional feature, the first communication system comprises a transmit power controller arranged to dynamically modify the first transmit power.

This may allow a primary communication system including the first communication unit to dynamically control the frequency reuse by the second communication unit(s) while allowing the individual second communication unit(s) to autonomously decide whether to reuse the first frequency interval .

The transmit power controller may modify the first transmit power in response to radio environment characteristics, e.g. for the first or second communication unit. For example, the first transmit power may be adjusted depending on a measured, predicted or desired interference level for e.g. the first or second communication unit.

In accordance with an optional feature of the invention, the second communication unit further comprises means for determining a predicted interference level resulting from transmissions in the first frequency interval by the second transmitter, and means for determining a likelihood of correctly receiving a transmitted signature in the presence of an additional interference corresponding to the interference level, and wherein the transmitter is arranged to determine whether to transmit in the first frequency interval in response to the likelihood.

This may allow improved frequency reuse and may in particular allow a communication unit to take into consideration the likely effect of the reuse before deciding whether to proceed. The likelihood may be a binary likelihood indication which indicates whether the signature can be decoded correctly or not.

According to another aspect of the invention, there is provided a communication unit comprising: a transmitter; a receiver for receiving a signal in a first frequency interval, the signal comprising a first signature spread using a first spread spectrum code and the first frequency interval being assigned to another communication unit for transmissions, means for generating a received signature by de-spreading the signal using the first spread spectrum code; and means for determining if the received signature corresponds to the first signature; and wherein the transmitter is arranged to transmit in the first frequency interval only if the received signature corresponds to the first signature.

According to another aspect of the invention, there is provided a method of frequency reuse in a communication system, the method comprising: a first communication unit performing the steps of : generating a signal by spreading a first signature using a first spread spectrum code, transmitting the signal in a first frequency interval at a first transmit power level, the first frequency interval being assigned to the first communication unit for transmissions; and a second communication unit performing the steps of: receiving the signal in the first frequency interval; generating a received signature by de-spreading the signal using the first spread spectrum code; determining if the received signature corresponds to the first signature; and transmitting in the first frequency interval only if the received signature corresponds to the first signature.

These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment (s) described hereinafter.

Brief Description of the Drawings

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

FIG. 1 illustrates an example of a communication system in accordance with some embodiments of the invention;

FIG. 2 illustrates an example of a communication unit in accordance with some embodiments of the invention;

FIG. 3 illustrates an example of a communication unit in accordance with some embodiments of the invention; and

FIG. 4 illustrates an example of a method of frequency reuse in accordance with some embodiments of the invention; Detailed Description of Some Embodiments of the Invention

The following description focuses on embodiments of the invention applicable to frequency reuse between (sub) communication systems using different radio technologies. However, it will be appreciated that the invention is not limited to this application but may be applied to many other hybrid or non-hybrid communication systems.

FIG. 1 illustrates an example of a communication system in accordance with some embodiments of the invention. The following description will focus on frequency reuse between (sub) communication systems. Thus, in the example, a hybrid communication system comprising a plurality of (sub) communication systems uses frequency reuse between the systems to achieve a more efficient usage of the available spectrum. The (sub) communication systems forming the hybrid communication system may be completely independent of each other with no other interaction than the described approach for allowing the frequency reuse.

A system for reusing a frequency allocated to a primary communication system by a secondary communication system will be described in the following. In the example, the primary communication system is a third generation cellular communication system, such as UMTS. However, it will be appreciated that in other embodiments, the primary communication system may be another type of communication system including for example a content broadcast communication system such as a terrestrial radio or TV transmission system. In the example of FIG. 1, the communication system comprises a primary base station 101 which is a base station of the primary cellular communication system. The cellular communication system operates in a frequency band which has been assigned to it by the appropriate authorities. Thus, the primary communication system has the full right to use the frequency band for communications within the cellular communication system. The primary base station 101 transmits (and possibly receives) in a first frequency band in a given geographical region corresponding to a cell of the cellular communication system. Thus, the primary base station supports communication services in the first frequency band. The primary base station 101 is furthermore coupled to a network 103 which provides interfacing to other base stations and networks and implements the management and control functions which are required for operating a cellular communication system.

It will be appreciated, that although FIG. 1 for brevity and clarity illustrates only a single base station 101 the cellular communication system will typically comprise a large number of base stations of which some or all may include the same functionality for frequency reuse as described for the primary base station 101.

It will also be appreciated that in other embodiments, the primary communication system to which the frequency band is assigned may not be a cellular communication system but may use completely different radio technologies and standards and may provide completely different communication services. For example, the primary communication system may be a television broadcast system with one or more transmitters generating broadcast television transmissions.

The primary communication system furthermore comprises a number of communication units which can communicate with the primary base station over the air interface of the cellular communication system. For brevity and clarity, FIG. 1 illustrates only a single primary cellular communication unit communicating with the primary base station 101.

Thus, a cellular communication system may use conventional techniques to provide a cellular communication service to a plurality of communication units. However, in addition, the cellular communication system of FIG. 1 also provides functionality for allowing the available air interface resource to be reused by other communication systems. For example, in areas or at times where the loading is particularly low for the cellular communication system, one or more other communication systems may temporarily be allowed to transmit in the frequency band assigned to the cellular communication system. The approach described in the following allows for other communication systems to automatically determine whether they are allowed to transmit in the frequency band and furthermore provides the cellular communication system with efficient means for controlling the frequency reuse by the other communication systems.

FIG. 1 illustrates a secondary communication unit 107 which may monitor a transmitted spread spectrum signature signal to determine whether it is allowed to reuse some or all of the resource of the first frequency band. The secondary communication unit 107 is in the specific example part of a secondary communication system which is completely independent of the primary communication system except for monitoring a signature signal transmitted by the primary communication system to indicate whether temporary frequency reuse is allowable.

Specifically, in the system the primary communication system continuously transmits a signal which contains a predetermined signature (e.g. a known data sequence). The signature is spread by a spread spectrum code such that the signal covers a relatively large frequency band with resulting low interference levels. Thus, spread spectrum techniques are used to ensure that the signature can be transmitted without interfering substantially with other services using the frequency band and especially that it does not interfere with other communications in the primary communication system.

Furthermore, any secondary user which is seeking to reuse the frequency band will attempt to decode the signature. If a secondary communication unit is able to decode the signature, it is allowed to reuse the frequency band and to make transmissions therein

(typically subject to a number of e.g. predetermined constraints) . However, if the secondary communication unit cannot decode the signature, e.g. due to the interference caused by the primary communication system and/or any other secondary communication unit already reusing the frequency, the secondary communication unit is not allowed to use the frequency band. Thus, by controlling the transmit power for the frequency reuse signature, the primary communication system can control the reuse and thereby the additional interference caused by the reuse.

Furthermore, the primary communication system can efficiently control the coverage region in which other communication units can reuse the system simply by controlling the transmit power for the signature signal.

The system may allow secondary communication units or systems to autonomously and independently determine whether to reuse a frequency band thereby allowing efficient yet simple reuse. Also, the system may automatically ensure that the frequency band is only being reused to an acceptable extent as the interference caused by the reuse contributes to the interference experienced by other secondary communication units thereby making it less likely that the signature signal can be not decoded for increasing reuse. Hence, an efficient, simple self regulating system for frequency reuse may be achieved.

FIG. 2 illustrates the primary base station 101 in more detail .

The primary base station 101 comprises a signature generator 201 which generates the signature. The signature may for example be a predetermined and known sequence of (e.g. binary) data symbols. The signature generator 201 is coupled to a spreading unit 203 which performs the spreading of the signature using well-known spread spectrum techniques. Specifically, the spreading unit 203 may implement a direct sequence spread spectrum generator which multiplies the generated signature by a spread spectrum chip code. In the specific example of a cellular communication system, the spread spectrum code may be a cell separation scrambling code and/or a channelisation code. The chip rate is selected such that the resulting spread signal has a bandwidth corresponding to the reuse frequency band.

It will be appreciated that in other embodiments, the spreading may be performed by e.g. a frequency hopping spread spectrum technique.

In the described example, the signature is continually repeated such that a continuous spread spectrum signature signal is generated. This signal is fed from the spreading unit 203 to a transceiver 205 which transmits the signature signal in the frequency band.

The primary base station 101 furthermore comprises a transmit power controller 207 coupled to the transceiver 205. The transmit power controller 207 controls the transmit power for the signature signal. The transmit power for the signature signal directly affects the probability that a given secondary communication unit will be able to decode the signature signal. Accordingly, the transmit power controller 207 can use the transmit power to control the likelihood of secondary communication units reusing the frequency. This allows the transmit power controller 207 to control the interference that is likely to be generated by reuse/sharing of the frequency band.

Furthermore, for a given interference scenario, the transmit power affects the coverage area for the signature signal and can thus be used to control the area in which the frequency may be reused. In the cellular example, the transmit power may for example be set to ensure that the reuse coverage area is smaller than the cell supported by the primary base station 101.

In some simple embodiments, a constant transmit power may simply be used for the signature signal. However, in more advanced embodiments the transmit power may be dynamically controlled to reflect the current conditions and preferences.

FIG. 3 illustrates the secondary communication unit 107 in more detail.

The secondary communication unit 107 is part of the secondary communication system which is independent of the primary communication system. For example, the secondary communication unit may be a mobile terminal providing Internet access via a Wireless Local Area Network (WLAN) .

The secondary communication system can comprise functionality for communicating in the frequency band of the primary communication system if this is acceptable to the primary communication system. Furthermore, the secondary communication system may use completely different transmission techniques than the primary communication system. For example, whereas the cellular communication system UMTS uses spread spectrum techniques, a typical WLAN system uses OFDM techniques (e.g. IEEE 802. Hx systems). Thus, the secondary communication system may comprise one or more access points which monitor for WLAN OFDM transmissions in the first frequency band. For these access points, the cellular spread spectrum (CDMA) signals will constitute interference but (partly due to the spreading) this interference may be sufficiently low to allow communications to the access point.

Thus, the secondary communication system can effectively reuse the first frequency band in scenarios where the interference from the primary communication system to the secondary communication system and from the secondary communication system to the primary communication system is acceptable. In the example, the secondary communication unit 107 automatically determines whether this is likely to be the case based on whether the signature signal can be correctly received or not.

The secondary communication unit 107 comprises a transceiver 301 which receives the signature signal from the primary base station 101. The received signature signal is fed to a signature receiver 303 which proceeds to de-spread the received signal using the same spread spectrum code that was used by the primary base station 101.

The resulting received signature is then fed to a comparison processor 305 which is furthermore coupled to a signature store 307. The comparison processor 305 retrieves the predetermined signature for the primary base station 101 from the signature store 307 and compares the received signature to the expected signature to determine if these correspond to each other.

In some embodiments, the comparison processor 305 simply determines that the signatures are corresponding if they are identical. In other embodiments, other criteria may be used such as e.g. that the difference between the received and stored signature is less than a certain number of data symbols (thereby allowing for one or more bit errors in the received signature) .

The comparison processor 305 is coupled to a transmit controller 309 which is further coupled to the transceiver 301. The comparison processor 305 feeds an indication of whether the received signature corresponds to the expected signature to the transmit controller 309. In response, the transmit controller 309 determines whether the frequency band is available for reuse by the secondary communication unit 107.

Specifically, if the correct signature is received, the frequency band is considered to be available for reuse and the transmit controller 309 controls the transceiver 301 to allow transmissions from the secondary communication unit 107. Thus, in this case the secondary communication unit 107 may proceed to transmit WLAN signals to an access point arranged to receive WLAN communications in the frequency band assigned to the primary cellular communication system. If the received signature is found to not correspond to the expected signature, the transmit controller 309 prohibits any transmissions within the first frequency band thereby preventing any interference to be caused.

Thus, the secondary communication unit 107 can autonomously, independently and separately of both the primary communication system and other secondary communication units decide whether to reuse the frequency band assigned to the first communication system. Thus, the system allows for a distributed frequency reuse thereby resulting in a low complexity and efficient reuse system which only requires a minimal interaction between the primary and secondary communication systems. Furthermore, the approach includes a self-regulation as reuse by one secondary communication unit introduces interference in the first frequency band thereby reducing the probability that other secondary communication systems can decode the signature and also reuse the frequency band.

The approach may be used in both licensed and unlicensed scenarios and with or without coordination between the involved communication units and systems. For example, in a licensed context, the primary communication system may be controlled by an operator/owner who is in charge of the transmission of the signature signal. In an unlicensed context, a regulator or other entity owning access point equipment may be responsible for transmitting the signature signal. The incentive for this equipment owner may be to enable spectrum reuse in a controlled manner . The use of the frequency band by the secondary communication unit 107 may be subject to various constraints or requirements. Specifically, a set of rules or policies may be employed which assures that the transmissions by the secondary communication unit 107 do not cause unacceptable problems to the service of the primary communication system.

The rules can specify various requirements for the transmission scheme used by the second communication unit 107. For example, the rules may specify a transmission format, maximum transmit power, spectral emission mask etc which must be met by any communication unit re-using the frequency spectrum. This approach may ensure an acceptable coexistence between the primary and secondary communication systems during frequency reuse.

The rules and requirements may be predetermined and known in advance by all the secondary communication units.

However, in other embodiments the primary communication system may transmit data which defines the transmission scheme requirements. For example, the primary base station 101 may transmit a data message which defines the individual transmission parameters and prescribes the requirement for each of these. Thus, a transmit scheme indication of an allowable transmission scheme can be transmitted. The secondary communication units 107 can be arranged to receive this data message and to select the transmission scheme depending on the requirements.

Furthermore, if a secondary communication units 107 is not able to use a transmission scheme that meets these requirements, it will not reuse the frequency band. In some embodiments, the signature itself is used to communicate the transmission scheme requirements. For example, a small number of transmission policies defining one or more requirements of the allowable transmission schemes may be defined and a separate signature may be assigned to each transmission scheme. Thus, depending on which requirements must be met by a communication unit reusing the first frequency band, the signature generator 201 may select the corresponding signature. When the secondary communication unit 107 decodes the signature, the comparison processor 305 compares this to a plurality of stored signatures. If the signature matches one of the stored signatures, the second communication unit 107 determines the transmission scheme requirements associated with this signature and applies these to the reuse process. Specifically, it proceeds to select the transmission scheme that meets the requirements or to refrain from the frequency reuse if no transmission scheme is available that can meet the requirements.

In some embodiments, the secondary communication unit 107 proceeds to transmit in the frequency band if the signature is correctly decoded (and the transmission scheme requirements are met) for a predetermined time interval. In other embodiments, the secondary communication unit 107 may continuously/repeatedly monitor the transmitted signature signal to determine whether the signature can be correctly received.

In this case, the secondary communication unit 107 may only proceed to reuse the frequency band for as long as the signature can be correctly received. Thus, in this example, the secondary communication unit 107 may start to reuse the first frequency band for transmissions when the signature is correctly decoded but will cease those transmissions if the interference level increases to such an extent that the signature no longer can be decoded. This approach may allow an automatic regulation wherein the interference level for the primary communication system is maintained at acceptable levels.

Similarly, the secondary communication unit 107 may only proceed to reuse the frequency band for as long as various constraints or requirements imposed for the reuse are met. Thus, if a set of rules or policies are employed to ensure that the transmissions by the secondary communication unit 107 do not cause unacceptable problems to the service of the primary communication system, adherence to these rules or policies may continuously be monitored and the reuse may terminate if they are no longer met.

As mentioned previously, the transmit power of the signature signal can be used by the primary communication system to control the frequency reuse. Thus, the transmit power of the signal is a critical parameter. A lower transmit power results in increased difficulties in decoding the signature and thus reduced frequency reuse and interference. A higher transmit power results in increased probability of decoding the signature and thus an increased frequency reuse and interference.

In the example of FIG. 2, the transmit power controller 207 dynamically modifies the first transmit power such that the frequency reuse is adjusted to the current conditions and requirements.

A minimum value for the transmit power may be given by the noise level and the coverage area in which the frequency reuse is allowable. For example, the minimum transmit power may be determined from propagation loss to and a noise level at a secondary communication unit at the edge of the coverage area. The minimum transmit power is e.g. determined such that for the given path loss the received signal level exceeds the noise level by a given margin reflected the required signal to noise ratio for successful decoding.

Furthermore, a maximum value may be given by the interference caused by the transmission of the signature signal. Specifically, the primary base station 101 may continue to support the cellular communication services during a frequency reuse. Accordingly, it is important that the signature signal does not introduce substantial interference to the cellular communications and the transmit power control of 207 can be arranged to determine an acceptable interference from the signature signal for the service currently being provided by this primary communication system.

This interference may depend on the current interference level in the system. For example, if the cellular communication system is relatively lightly loaded (which is typically the case when reuse is considered) the interference level is typically low and a relatively large interference contribution by the signature signal can be accepted. Once the acceptable interference contribution is determined, the transmit power controller 207 can proceed to set the transmit power such that the interference is kept below the required level.

In some embodiments, the transmit power controller 207 sets the signature transmit power in response to an interference level in the first frequency band.

Thus, the transmit power may be controlled such that the interference level in the combined communication system comprising both the primary and secondary communication systems is maintained at reasonable levels. For example, if the interference level is very low, the signature transmit power is increased thereby increasing the interference caused by the transmission of the signature and also increasing the probability of frequency reuse and thus the additional interference introduced by the reuse. In contrast, if the interference level is relatively high, the transmit power for the signature signal is reduced thereby reducing not only the interference caused by the transmission of the signature but also the interference caused by the reduced probability of frequency reuse.

It will be appreciated, that due to the spreading of the signature, the interference caused by the transmission of the signature is substantially reduced and is in many embodiments likely to be insignificant in comparison to the interference caused by frequency reuse and/or the other transmissions of the primary communication system.

The interference level used to control the transmit power may be a combined interference level reflecting the interference at different locations, the different communication units and/or the different services or may be a single interference level.

For example, in some embodiments the interference level at the primary base station 101 may be measured and used directly by the transmit power controller 207 to control the transmit power. This may provide an efficient yet simple implementation where an easy to generate indication of the loading of the cellular communication system is used to control the probability of frequency reuse. However, the approach may be less accurate in determining the actual interference conditions experienced by individual communication units of both the primary and secondary communication systems.

In some embodiments, the communication units of the primary communication systems can be arranged to measure interference levels and report these back to the primary base station 101. In this case, the transmit power controller 207 may generate an interference indication based on the reported interference levels and may set the transmit power in response to this interference level. This may allow an efficient control of interference in the primary communication system and may ensure that acceptable quality of service levels are provided despite the frequency reuse.

In some embodiments, communication units of the secondary communication system are additionally or alternatively arranged to measure interference levels and report these back to the primary base station 101. In this case (collaborative based between primary and secondary systems) , the transmit power controller 207 may generate an interference indication based on the reported interference levels and may set the transmit power in response to this interference level. This may allow improved consideration of the quality of service levels that can be achieved by the secondary communication system when reusing the first frequency band.

It will be appreciated that in many embodiments, complex rules and algorithms taking into account some or all of these interference measures can be implemented by the transmit power controller 207 when setting the transmit power level for the signature signal.

In some embodiments, the transmit controller 309 of the secondary communication unit 107 may also dynamically set the transmit power of the transmissions in the first frequency band during frequency reuse depending on the reception of the signature signal. Thus, the transmit power of the secondary communication unit 107 may not only be subject to a maximum power given by the transmission scheme rule and a minimum power given by the requirement to communicate successfully for the given interference level but may also take into consideration the effect on the decoding of the signature signal.

For example, if the secondary communication unit 107 successfully decoded the signature signal, it may proceed to set the transmit power at the maximum value given by the transmission scheme rule. It may then reduce the transmit power by a minimum path loss corresponding to a path loss to a nearby communication unit (say it may be considered that any other communication unit is at least, say, three meters away corresponding to an estimated path loss of x dB) . The resulting power level may then be added to the experienced interference level and it may be estimated if this increased interference level would allow the signature to be decoded. If so, the transmit power will be used and otherwise the transmit power will be reduced.

In some embodiments, one or more of the secondary communication units may furthermore be arranged to transmit information back to the primary base station 101 indicating whether it can successfully decode the signature or not. Specifically, the secondary communication unit 107 may transmit a message to the primary base station 101 if it cannot decode the signature. The primary base station 101 may accordingly comprise a feedback message receiver 209 coupled to the transceiver 205 and the transmit power controller 207. The feedback message receiver 209 receives the messages from the secondary communication unit 107 and forwards them to the transmit power controller 207 which may set the transmit power of the signature signal in response to the messages.

In such embodiments, the primary base station 101 may be arranged to modify the transmit power in response to indications of decode failures. For example, if the primary base station 101 receives a large number of indications that the signature cannot be decoded despite the primary base station 101 detecting that the interference (caused by both frequency reuse and communications within the primary communication system) is very low, it will proceed to increase the transmit power thereby increasing the probability of frequency reuse. Conversely, if the interference level is determined to be very high while the number of decode failure indications is very low, the primary base station 101 can reduce the signature signal transmit power to reduce the frequency reuse.

It will be appreciated that the approach allows a dynamic and self-regulating frequency reuse wherein secondary communication units can autonomously decide whether to use the frequency band. Furthermore, as any reuse increases interference thereby reducing the probability of other secondary communication units decoding the signature, the reuse is automatically limited without requiring any coordination between the secondary communication units or between the primary and secondary communication system.

FIG. 4 illustrates an example of a method of frequency reuse in accordance with some embodiments of the invention .

The method initiates in step 401, wherein a primary communication unit generates a signal by spreading a first signature using a first spread spectrum code.

Step 401 is followed by step 403 wherein the signal is transmitted in a first frequency interval at a first transmit power level. The first frequency interval is assigned to the first communication unit for transmissions Step 403 is followed by step 405 wherein the secondary communication unit receives the signal in the first frequency interval .

Step 405 is followed by step 407 and wherein a received signature is generated by de-spreading the signal using the first spread spectrum code.

Step 407 is followed by step 409 wherein it is determined if the received signature corresponds to the first signature .

If so, the method proceeds in step 411 wherein the secondary communication unit transmits in the first frequency interval and otherwise the method proceeds in step 413 wherein the secondary communication unit does not transmit in the first frequency interval.

It will be appreciated that the above description for clarity has described embodiments of the invention with reference to different functional units and processors. However, it will be apparent that any suitable distribution of functionality between different functional units or processors may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controllers. Hence, references to specific functional units are only to be seen as references to suitable means for providing the described functionality rather than indicative of a strict logical or physical structure or organization. The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors .

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate. Furthermore, the order of features in the claims does not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order.

Claims

1. A communication system comprising: a first communication unit comprising: means for generating a signal by spreading a first signature using a first spread spectrum code, a first transmitter for transmitting the signal in a first frequency interval at a first transmit power level, the first frequency interval being assigned to the first communication unit for transmissions; and a second communication unit comprising: a second transmitter; a receiver for receiving the signal in the first frequency interval; means for generating a received signature by de- spreading the signal using the first spread spectrum code; and means for determining if the received signature corresponds to the first signature; and wherein the transmitter is arranged to transmit in the first frequency interval only if the received signature corresponds to the first signature.

2. The communication system of claim 1 wherein the first communication unit is arranged to transmit to the second communication unit a transmit scheme indication of an allowable transmission scheme for transmission in the first frequency interval.

3. The communication system of claim 1 wherein the first communication unit comprises means for selecting the first signature in response to an allowable transmission scheme.

4. The communication system of claim 1 wherein the first communication system comprises a transmit power controller arranged to dynamically modify the first transmit power.

5. The communication system of claim 4 wherein the transmit power controller is arranged to determine the first transmit power in response to an interference level in the first frequency interval.

6. The communication system of claim 4 wherein the second communication unit comprises means for determining an interference level in the first frequency interval and to transmit an interference indication of the interference level to the first communication unit; and the transmit power controller is arranged to determine the first transmit power in response to the interference indication .

7. A communication unit comprising: a transmitter; a receiver for receiving a signal in a first frequency interval, the signal comprising a first signature spread using a first spread spectrum code and the first frequency interval being assigned to another communication unit for transmissions, means for generating a received signature by de- spreading the signal using the first spread spectrum code; and means for determining if the received signature corresponds to the first signature; and wherein the transmitter is arranged to transmit in the first frequency interval only if the received signature corresponds to the first signature.

8. A method of frequency reuse in a communication system, the method comprising: a first communication unit performing the steps of : generating a signal by spreading a first signature using a first spread spectrum code, transmitting the signal in a first frequency interval at a first transmit power level, the first frequency interval being assigned to the first communication unit for transmissions; and a second communication unit performing the steps of: receiving the signal in the first frequency interval; generating a received signature by de-spreading the signal using the first spread spectrum code; determining if the received signature corresponds to the first signature; and transmitting in the first frequency interval only if the received signature corresponds to the first signature .