GB2482197A

GB2482197A - Combined bulk and per-tone antenna selection in MIMO-OFDM systems with switching of selection method

Info

Publication number: GB2482197A
Application number: GB1012414.7A
Authority: GB
Inventors: Justin Coon
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2010-07-23
Filing date: 2010-07-23
Publication date: 2012-01-25
Anticipated expiration: 2030-07-23
Also published as: GB2482197B; GB201012414D0

Abstract

A MIMO-OFDM transmitter allocates antennae, from a plurality of available antennas, M, for transmission of signals on one or more sub-carriers using an antenna selection mechanism combining bulk and per-tone antenna selection. In one embodiment (Fig. 13) the device is arranged to switch the operation of the allocator between the combined antenna selection mechanism and a per-tone antenna selection mechanism or a bulk antenna selection mechanism based on one or more performance metrics such as SNR, BER, battery power or received signal strength (RSSI). In an alternative embodiment (Fig. 12, claim 4) the device varies the number, L, of antennas selected for use in the combined bulk/per-tone antenna selection mechanism based on one or more performance metrics.

Description

Method and Apparatus for Antenna Selection in Wireless Communication Systems

FIELD

Embodiments described herein relate generally to antenna selection in wireless communications systems transmitting frequency multiplexed signals.

BACKGROUND

Antenna selection in orthogonal frequency division multiplexing (OFDM) systems is a powerful technique for exploiting spatial diversity when limited channel state information (CSI) is available at the transmitter. Antenna selection methods in OFDM can be considered to fall into two categories: 1) bulk selection, whereby one or more antennas out of the total available set are used for transmission and/or reception for all frequencies, and 2) per-tone selection, which provides an additional degree of freedom that allows the antenna selection to differ across the utilised bandwidth. Both techniques have advantages and disadvantages. For example, bulk selection requires very little CSI feedback and exploits fewer radio frequency (RF) chains than per-tone selection. However, per-tone selection is capable of achieving much higher coding gains than bulk selection, i.e., the bit-error rate (BER) for per-tone selection is significantly lower than for bulk selection.

These two techniques are illustrated for transmit antenna selection diagrammatically in Figure 1 and Figure 2. In Figures 1 and 2 each illustrated segment represents a sub carrier, with a row of contiguous segments/sub carriers indicating the sub carriers associated with the particular antenna shown next to it. The shading denotes spatial/spectral regions that are used for transmission on the associated antenna. Blank areas indicate sub carriers that are not used for transmission on the associated antenna.

In Figure 1 transmitters TX 3 and TX 4 both transmit on all frequencies.

Thus, only two RE chains are required for this scenario. In contrast, Figure 2 shows all transmitters conveying information; however, only one transmitter is active on any given sub carrier. This is the so-called per-tone antenna selection approach, which requires four RF chains. Note that both these selection schemes can equally be performed at the receiver instead of the transmitter, or indeed at both the transmitter and the receiver.

Per-tone selection has been shown to perform exceptionally well.

However, its implementation requires a large amount of RF circuitry, which can draw a considerable amount of system resources (e.g., power) and reduce battery life in portable devices.

In the most general case, antenna selection can be performed such that a single antenna is chosen for transmission out of M available antennae, or a subset of L antennae are chosen out of the M available antennae. The former approach is herein labelled single-antenna selection', while the latter is termed subset selection'. Both approaches can be employed in bulk selection or per-tone selection scenarios.

The above discussed bulk antenna selection and per-tone antenna selection methods have recently been combined by the present inventors to yield a trade-off between complexity (which can be represented by the number of radio frequency (RF) chains required for implementation) and performance (which can be represented by bit error rate (BER)). In essence, this trade-off is achieved by constraining the overall transmission to be conveyed from a maximum of L < M of the M available antennae (using L RF chains), and performing a further per-tone selection using these L antennae. This bulk/per-tone antenna selection method is described in UK patent application no. 0919962.1, the entire disclosure of which is incorporated herein by this reference. It will be appreciated that the number of RF chains used by this antenna selection process is L, therefore saving transmit energy.

The above principle is illustrated by means of Figure 3. Figure 3 is a graph showing activity of antennas (labelled TXI to TX4) against frequency.

The available frequency band is subdivided into sub carriers, in accordance with usual practice. The sub carriers active on any particular antenna are shown as shaded boxes in the graph. In the example illustrated in Figure 3 the number of available antennae M =4. It can be seen in Figure 3 that transmitters TX2 and TX3 convey information, while TXI and TX4 remain inactive. The bulk selection part of the bulk/per-tone antenna selection method employed to arrive at the activation state of Figure 3 requires fewer RE chains to convey the data across the wireless channels and has used a subset of L =2 of the available antennae/RE chains. However, only one of TX2 and TX3 transmits on any given sub carrier. The per-tone selection element of the antenna selection algorithm employed in Figure 3 thus relies on a single antenna. Should the bulk selection part of the process have used a different number of antennae (in the example of Figure 3 the only different number useable would be L = 3), then the per-tone selection process may have used more than one antenna for transmitting signals on the sub carriers. For example, two antennae may be used for transmitting signals on each sub carrier. In such a case, spatial encoding methods, such as spatial multiplexing and space-time coding, can be used to encode the multiple-input signal.

It will moreover be appreciated that signals on some sub carriers can be transmitted using all of the antennae selected in the bulk processing part of the bulk/per-tone selection antenna algorithm, while others are transmitted using a limited number of the antennae selected in the bulk processing part of the bulk/per-tone antenna selection algorithm.

It will be appreciated that, although Figure 3 shows the selection/operation of transmit antennae, the embodiments also apply to the selectionloperation of receive antennae and/or a combination of selecting transmit and receive antennae.

It is known that per-tone selection provides the best performance in terms of bit-error rate (BER), capacity, outage probability, etc. However, per-tone selection selects antennae for the transmission of signals on the sub carriers from all M available RF chains/antennae. The requirement for the use of all available RE chains can be avoided in cases where the SNR of the received signals is high. In this case combined bulk/per-tone selection can be employed to yield the same performance as conventional per-tone selection.

For all antenna selection systems employing frequency multiplexed signals, some degree of channel state information (CSI) must be available at the transmitter (if transmit antenna selection is performed) and/or at the receiver (if receive antenna selection is performed). In particular, this information can be estimated using known channel estimation methods and either conveyed explicitly to the antenna selection modules in the transceiver or, in the case of transmit antenna selection, used to calculate the sub carrier/antenna allocation, which is then explicitly conveyed to the transmitter for implementation.

For systems employing transmit antenna selection and time-division duplex (TDD) communication, the acquirement of CSI at the transmitter for antenna selection can be simplified by exploiting the reciprocity in the channel to estimate the necessary CSI from the incoming signal. In frequency division duplex (FDD) systems, an explicit feedback channel is generally required to supply the transmitter with the required information. Note that only partial knowledge of the CSI is required to perform antenna selection; in particular, the amplitude of the channel frequency response must be estimated or known, whereas the phase information is irrelevant in most scenarios.

BRIEF DESCRIPTION OF IF-fE DRAWINGS

An embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 illustrates the concept of bulk antenna selection; Figure 2 illustrates the concept of per-tone antenna selection; Figure 3 illustrates the combined bulk/per-tone antenna selection method using a single-antenna per-tone selection approach; Figure 4 is a schematic illustration of a simple communication system comprising two multi antenna transmit/receive devices; Figure 5 is a schematic diagram of a transmit/receive device of Figure 4; Figure 6 is a schematic diagram of a transmitter driver of the transmit/receive device illustrated in Figure 5; Figure 7 shows one example of a flow diagram of a joint bulk and per-tone selection procedure; Figure 8 shows a second example of a flow diagram of a joint bulk and per-tone selection procedure; Figure 9 shows a third example of a flow diagram of a joint bulk and per-tone selection procedure; Figure 10 shows a fourth example of a flow diagram of a joint bulk and per-tone selection procedure; Figure 11 illustrates switching between per-tone selection and bulk/per-tone selection based on a received SNR threshold; Figure 12 Illustrates switching based on multiple received SNR thresholds; Figure 13 illustrates switching between per-tone, bulk/per-tone and bulk selection based on received SNR thresholds; and Figure 14 illustrates the results of simulations of the uncoded performance of an OFDM system employing per-tone, bulk/per-tone (labelled Joint') and bulk selection with a single-antenna selection strategy.

DETAILED DESCRIPTION

According to one embodiment there is provided a wireless communication device comprising a plurality of antennae and operable to emit a frequency multiplexed signal from one or more of said antennae over a communications channel. The device comprises an antenna allocator operable to allocate antennae from the plurality of antennae for transmission of signals on one or more sub carriers of the frequency multiplexed signal using an antenna selection mechanism that combines bulk and per-tone antenna selection. The device is arranged to switch the operation of the allocator between the antenna selection mechanism and either or both of a per-tone antenna selection mechanism and a bulk antenna selection mechanisms based on one or more performance metrics.

The one or more performance metrics can include one or more of a signal to noise ratio of signal received at one or more receivers of the system, a bit error rate of the received frequency multiplexed signal, a remaining battery power in a battery operated device and a strength of a signal received at one or more receiver of the system. In general terms, a system suitable for implementation of an embodiment is illustrated in Figures 4. Figure 4 illustrates a very schematic wireless communications system 10 comprising first and second transmit/receive devices 20, 30. Each of the transmit/receive devices 20, 30 is a multi antenna device, and antenna selection is well known to be a suitable way of making best use of the channel which can be formed between two such devices. The device may further comprise a means for determining the performance metric. It is envisaged that a decision to switch from one antenna selection technique to another can be based on a single performance metric. In some embodiments a combination of two or more performance metrics may be used as basis for switching between antenna selection techniques.

The device may further be operable to switch between the antenna selection mechanism combining bulk and per-tone antenna selection and the per-tone antenna selection mechanism based on a first level of a performance metric (for example one of the above discussed performance metrics) and between the antenna selection mechanism combining bulk and bulk antenna selection based on a second level of the same or a different performance metric.

The allocator can be arranged to sequentially operate two or more antenna selection mechanisms, each combining bulk and per-tone antenna selection. Each of the two or more antenna selection mechanisms may select a different number of antennae out of the plurality of antennae. The device may further be operable to switch between two of the two or more antenna selection mechanisms based on a performance metric, such as one of the above discussed performance metrics. The device may be operable to use different levels of one or more performance metrics to switch between a series of three or more bulk/per-tone antenna selection mechanisms.

The device may be arranged to switch between the two or more antenna selection mechanisms by selecting, as a next antenna selection mechanism that is to be used, an antenna selection mechanism that is operable to allocate the signals of the sub carriers to a number of the antennae that differs from a currently used number of antennae by one.

The above discussed antenna selection mechanisms may be employed by the device for the selection of transmit antennae or for the selection of receive antennae. The device may be an OFDM transmitter or receiver and can be a base station or a portable wireless device.

According to another embodiment there is provided a method of operating a wireless communication apparatus comprising a plurality of antennae and arranged to emit and/or receive a frequency multiplexed signal comprising a plurality of sub carriers. The method comprises allocating antennae from the plurality of antennae for transmission of the sub carriers using an antenna selection mechanism combining bulk and per-tone antenna selection and switching between the said antenna selection mechanism and a per-tone antenna selection mechanism and/or a bulk antenna selection mechanism based on one or more performance metrics. The device can be arranged to power down any radio frequency chains allocated to unused antennae, be that in a bulk selection mode or in a bulk/per-tone selection mode.

The device may then be operated using the selected antennae to transmit a signal on or receive a signal via the sub carriers.

According to another aspect of the invention there is provided a computer program product comprising computer executable instructions operable to configure a general purpose programmable communications device to perform any of the methods of the herein described embodiments. The computer program product may comprise a computer readable storage medium and/or a computer receivable signal.

The operation of the allocator may, for example, be switched from the pre-tone antenna selection method to the bulk/per-tone antenna selection method if the received signal to noise ratio or the received signal strength is determined to exceed a predetermined threshold, if the bit error rate is below a predetermined threshold or if the remaining battery power decreases to below a predetermined amount.

Figure 5 is a schematic diagram of the transmit/receive device 20 illustrated in Figure 4. The device 20 comprises a transmitter driver 22 operable to receive data from a data source/sink 26 for transmission on the antennas TXI to TX4 of the device 20. A receiver driver 24 receives and processes signals received on the same antennas and assist data to the data source sink 26 and channel state information to the transmitter driver 22. An array of suitable switches 28 is provided to enable the antennas to be used for both transmitting and receiving.

As shown in Figure 6, the transmitter driver 22 comprises channel coding 40, antenna selection 44 functional blocks operable to translate data to be transmitted to the signal applied to selected antennas at selected sub carrier frequencies. This is done on the basis of the channel state information fed to the antenna selection block 44.

For completeness four examples of bulk/per-tone antenna selection processes are provided in the following.

A first example of a process for bulk and per-tone selection will now be described, with reference to Figure 7 of the drawings. This can be described as an optimal bulk and per-tone antenna selection. In this example of implementation of the specific embodiment of the invention, the transmitter performs bulk and per-tone selection according to the following procedure: S1-0: Initialisation: Given L and M (the number of available RF chains), (M enumerate the U.) possible bulk selection allocations, and let the set of these allocations be denoted by S, writing the th element of the set as 5. Initialise = Si-I: Assign S1-2: Perform per-tone selection for the set of antennas contained in 5.

S1.-3: Evaluate the cost function related to the selection performed in step S1-2 and log this value.

S 1-4: If = 151, stop and choose the allocation that minimises the cost function. Otherwise, go to step Si-i.

A second example is illustrated in Figure 8. This is a joint bulk and per-tone selection to meet an objective. In this example, the transmitter performs bulk and per-tone selection according to the following procedure: (M S2-0: Initialisation: Given L and M enumerate the a.,) possible bulk selection allocations, and let the set of these allocations be denoted by S, writing the th element of the set as S1 Initialise t = 0 and choose a cost threshold V. S2-1: Assign t+1.

S2-2: Perform per-tone selection for the set of antennas contained in S2-3: Evaluate the cost function related to the selection performed in step S2-2 and log this value.

S2-4: If the cost is less than or equal toY, then choose this allocation and stop. Otherwise, if = ISI, stop and choose the allocation that minimises the cost function. Otherwise, go to step S2-1.

A third example is illustrated in Figure 9. In this example, an optimal sequential bulk and per-tone selection procedure is employed, according to the following procedure: (M S3-0: Initialisation: Given L and M, enumerate the.L) possible bulk selection allocations, and let the set of these allocations be denoted by S, writing the th element of the set as S3-1: Assign I +1 S3-2: Evaluate the bulk selection cost function for the assignment defined by S and log this value.

S3-3: If = 151, choose the bulk selection that minimises the associated cost function and go to step S3-4. Otherwise, go to step S3-1.

S3-4: Perform per-tone selection to minimise the chosen cost function for per-tone selection given the chosen bulk selection.

A fourth example is illustrated in Figure 10. In this example, a sequential bulk and per-tone selection process is used to meet a bulk selection objective.

To do this, the transmitter performs bulk and per-tone selection according to the following procedure: (M S4-0: Initialisation: Given L and M, enumerate the LJ possible bulk selection allocations, and let the set of these allocations be denoted by S writing the th element Of the set as S. Initialise 0 and choose a cost threshold r.

S4-1: Assign L-I-1 S4-2: Evaluate the bulk selection cost function for the assignment defined by S and log this value, S4-3: If the bulk cost is less than or equal to V, then choose this allocation and, go to step S4-4. Otherwise, if 151, choose the allocation that minim ises the bulk cost function and go to step S4- 4. Otherwise, go to step S4-1.

S4-4: Perform per-tone selection to minimise the per-tone cost function given the chosen bulk selection.

It will be appreciated that many combinations of bulk and per-tone selection exist, depending on the total number of antennas M and the number of available/active RF chains L. It has been realised that the bulk antenna selection method, the per-tone antenna selection method and the bulk/per-tone antenna selection method have advantages that can be used so that one antenna selection method complements one or more of the other antenna selection methods. An embodiment thus uses different antenna selection mechanisms for different performance criteria of ongoing data transmission or of channel properties.

Many different criteria can be employed to determine when to switch between different antenna selection mechanisms, such as per-tone, bulk/per-tone and bulk selection. Example criteria include received SNR, received signal strength and error performance. A threshold of a performance criterion may, for example, be set such that when the received SNR exceeds it, the system switches from per-tone selection to bulk/per-tone selection or from bulk/per-tone selection to bulk selection. If at a later time, the received SNR drops below this threshold, the system can switch back from bulk/per-tone selection to conventional per-tone selection or from bulk selection to bulk/per-tone selection.

In embodiments the performance criterion used for providing the switching threshold between the antenna selection methods is monitored. The embodiment may, for example monitor the received SNR, received signal strength or remaining battery power in the device employing the antenna selection method.

An illustration of the switching between bulk/per-tone selection and per-tone selection is provided in Figure 11. Thresholds can be set to exploit received signal strength and error performance as switching criteria in a similar manner.

In another embodiment, several thresholds may be set for switching between selection techniques. For example, when the received SNR exceeds lower threshold, the system may, starting from a per-tone selection process in which L = M, deactivate one RF chain, thus leaving L M -1 activated RF chains and M antennas for bulk/per-tone selection. When the received SNR surpasses the next threshold, another RF chain may be deactivated, and so on until only two RF chains are active. Two active RF chains is the minimum requirement to achieve performance identical to conventional per-tone antenna selection (with M RF chains) at high SNR when a single-antenna selection strategy is adopted. Thus, provided the largest threshold is high enough, optimal performance will be maintained as the received SNR increases and RF chains are sequentially deactivated. Again, a similar scheme can be devised for the case where received signal strength or error performance is used as switching criteria. An illustration of the concept of having multiple switching thresholds is shown in Figure 12.

The above discussed examples focus on the case where a system switches between per-tone and bulk/per-tone selection. In some space-time coded systems, such as in systems employing orthogonal space-time blocks, however, it is also beneficial to be able to switch to a pure bulk selection strategy. Thus, multiple switching thresholds can be used to specify for what ranges of, for example, received SNR a system should operate with per-tone, bulk/per-tone and bulk selection strategies, as indicated in Figure 13. It will be appreciated that the switching to/from bulk selection can also be used in the embodiments of Figures 11 and 12.

In battery-operated devices, remaining battery power levels can be used to instigate a switch between antenna selection techniques. For example, once the battery power drops below a given threshold, the system could switch from per-tone selection to bulk/per-tone selection to conserve power. Switches between different bulk/per-tone selection techniques.in which the number of activated RF chains is decreased also reduce energy consumption and may therefore also be instigated by a drop of remaining battery power below a predetermined threshold. At low battery power switch from bulk/per-tone antenna selection to bulk selection can be used to further conserve battery power.

When conventional per-tone selection is employed, information can be conveyed from all M antennas of a transmitter, thus using all M RF chains.

The power consumption of this approach can be high since each RE chain comprises its own circuitry (e.g., each chain may use a separate power amplifier). When a system switches from per-tone selection to bulk/per-tone selection where only L RE chains are required, the remaining M -L RF chains are powered down in the embodiment, thereby reducing the amount of power consumed. This approach allows maintaining the performance of the system, while improving the system's power efficiency, which, in turn, may prolong battery life in portable devices. Similarly, when a system switches from bulk/per-tone antenna selection method to a bulk selection method that uses fewer than the L RF chains used by the bulk/per-tone antenna selection method, all unused RF chains can be powered down.

The above discussed "powering down" approach is particularly beneficial in systems that utilise class A power amplifiers, which draw the same amount of power regardless of whether or not data is transmitted from them.

Moreover, power amplifier efficiency typically improves as output power increases. In per-tone antenna selection systems, power amplifiers must be rated such that each one can transmit data on all sub carriers since this is the largest transmit power likely to be encountered at one transmit branch in practice. However, due to the selection process, each antenna will likely only convey a subset of the total power. Consequently each power amplifier will output a fraction of the maximum rated power. Amplifier efficiency will thus be low. The above discussed embodiments can improve amplifier efficiency when the performance characteristics of the system employing the embodiment are good, for example when the received SNR is high, since fewer power amplifiers will be operational in this case, and each amplifier will output more power, corresponding to more data being transmitted from the corresponding transmit branch.

It will be appreciated that transmit antenna selection is considered in the illustrated examples above. Embodiments can, however, equally be embodied at the receiver or at both the transmitter and the receiver.

Figure 14 illustrates the BER of an OFDM system with M 4 employing per-tone, bulk/per-tone (L=2 with a single antenna selection strategy) and bulk selection, It can be seen in this figure that by employing a received SNR threshold of about 18 dB for switching between per-tone and bulk/per-tone selection, performance can be maintained and unused RF chains can be powered down when the received SNR exceeds this level.

While certain embodiments have been described, the embodiments have been presented by way of example only, an area not intended to limit the scope of the inventions. Indeed, the novel methods, apparatus and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.

Claims

CLAIMS: 1. A wireless communication device comprising a plurality of antennae and operable to emit a frequency multiplexed signal from one or more of said antennae over a communications channel, the device comprising an antenna allocator operable to allocate antennae from the plurality of antennae for transmission of signals on sub carriers of the frequency multiplexed signal using an antenna selection mechanism combining bulk and per-tone antenna selection, the device arranged to switch the operation of the allocator between the antenna selection mechanism and either or both of a per-tone antenna selection mechanism and a bulk antenna selection mechanism based on one or more performance metrics.
2. A device according to Claim 1, wherein said one or more performance metrics include one or more of: a signal to noise ratio of signal received at one or more receivers of the system; a bit error rate of the received frequency multiplexed signal; remaining battery power in a batter operated device; and a strength of a signal received at one or more receiver of the system; wherein the device further comprises a means for determining the performance metric.
3. A device according to Claim I or 2, the device further operable to switch between the antenna selection mechanism combining bulk and per-tone antenna selection and the per-tone antenna selection mechanism based on a first level of a said performance metric and between the antenna selection mechanism combining bulk and per-tone antenna selection and bulk antenna selection based on a second level of the said performance metric.
4. A device according to Claims 1, 2 or 3, wherein the allocator is arranged to operate two or more antenna selection mechanisms combining bulk and per-tone antenna selection, wherein each of the two or more antenna selection mechanisms selects a different number of antennae out of the plurality of antennae, the device further operable to switch between two of the two or more antenna selection mechanisms based on one or more performance metrics.
5. A device according to Claim 4, arranged to switch between the two or more antenna selection, mechanisms by selecting as a next antenna selectbn mechanism used an antenna selection mechanism that is operable to allocate the signals of the sub carriers to a number of the antennae that differs from a currently used number of antennae by one.
6. A device according to any preceding claim, wherein the device is a base station or a portable wireless device.
7. A device according to any preceding claim, wherein the device is an OFDM transmitter or receiver.
8. A system comprising a first device according to any of Claims 1 to 6 and operable to emit a frequency multiplexed signal from one or more of said antennae of the first device over the communications channel and a second device according to any of Claims 1 to 6, operable to receive the frequency multiplexed signal over the communications channel trough one or more of the antennae of the second device.
9. A method of operating a wireless communication apparatus comprising a plurality of antennae and arranged to emit and/or receive a frequency multiplexed signal comprising a plurality of sub carriers, the method comprising: allocating antennae from the plurality of antennae for transmission of the sub carriers using an antenna selection mechanism combining bulk and per-tone antenna selection and switching between the said antenna selection mechanism and a per-tone antenna selection mechanism and/or a bulk antenna selection mechanism based on one or more performance metrics.
10. A method according to Claim 9, wherein said one or more performance metrics include one or more of: a signal to noise ratio of signal received at one or more receivers of the system; a bit error rate of the received frequency multiplexed signal; remaining battery power in a batter operated device; and a strength of a signal received at one or more receiver of the system.
11. A method according to Claim 9 or 10, wherein said switching is between the antenna selection mechanism combining bulk and per-tone antenna selection and the per-tone antenna selection mechanism and is based on a first level of a said performance metric the method further comprising switching between the antenna selection mechanism combining bulk and per-tone antenna selection and the bulk antenna selection mechanism using a second level of the said performance metric.
12. A method according to Claim 9, 10 or 11, wherein the apparatus is arranged to operate two or more antenna selection mechanisms combining bulk arid per-tone antenna selection, wherein each of the two or more antenna selection mechanisms selects a different number of antennae out of the plurality of antennae, the method further comprising switching between two of the two or more antenna selection mechanisms based on a performance metric.
13. A computer program product comprising computer executable instructions operable to configure a general purpose programmable communications device to perform a method in accordance with any one of claims 9 to 12.
14. A computer program product in accordance with claim 13, comprising a computer readable storage medium.
15. A computer program product in accordance with claim 13, comprising a computer receivable signal.