Adaptive multi-channel transmitter linearization system using a shared feedback receiver

Pre-distortion techniques, devices and systems for a multi-channel transmitter are described. An adaptation time is provided for each of the transmit chains in the multi-channel transmitter. During each adaptation time an output of a transmit chain is coupled to a shared feedback receiver. The shared feedback receiver generates a feedback signal that is used to vary pre-distortion for that transmit chain. A commutation controller varies at least one of: (a) an amount of the adaptation time for a transmit chain and (b) an order in which each of the transmit chains receives its adaptation time.

TECHNICAL FIELD

The present invention relates generally to communications systems and in particular to methods, devices and systems associated with multi-channel transmitters.

BACKGROUND

As technology advances, the options for communications have become more varied. For example, in the last 30 years in the telecommunications industry, personal communications have evolved from a home having a single rotary dial telephone, to a home having multiple telephone, cable and/or fiber optic lines that accommodate both voice and data. Additionally cellular phones and Wi-Fi have added a mobile element to communications.

Until relatively recently, the primary use of cellular communications has been for voice communications between people. Even for this usage case, considerable optimization has been performed to carry the comparatively small amount of traffic in an optimal way, for instance the introduction of lower bit-rate codecs. In the last few years, the amount of information that needs to be transmitted has begun to increase dramatically with the increasing ubiquity of smart phones, wireless data services and users communicating with machines, particularly servers on the Internet. The next trend will likely be for mobile devices to be used to support machines communicating with other machines (e.g. “M2M”), for instance for remote monitoring and control and many other applications.

As is well known in cellular telecommunications, radiocommunication services are typically provided via an access point, e.g., a base station, which transmits signals to, and receives signals from, mobile devices (also sometimes called user equipment (UEs)). In order to support radiocommunications with a large number of devices which are within its transmission range, a base station will typically transmit and receive signals on a number of different channels and, therefore, will typically include a multi-channel transmitter.

Digital pre-distortion (DPD) is a widely applied technology which is used in the linearization of base stations' multi-channel transmitters. Power efficiency and system performance improvements are achieved by continuously monitoring and improving the output signal in an adaptive, closed loop. The output signal of each transmitter is fed back through a dedicated receiver and down-converted to its digital baseband frequency where a direct comparison with the reference, modem-generated downlink signal can be performed.

For example, as shown inFIG. 1, a traditional DPD system can include multiple transmitter channels or paths1, . . . , N. Each of the transmitter paths1, . . . , N includes a transmitter TX1, . . . , TX N and a power amplifier PA1, . . . , PA N. For each of the multiple transmitter paths signals are coupled from the output of a respective PA via couplers100,102,104, etc., and are fed back to a respective feedback receiver1, . . . , N for radio frequency processing and then on to a digital base band DPD unit106. However using a separate feedback receiver for each path occupies a lot of space in the base station and consumes significant power.

Another approach is, therefore, to provide a single feedback receiver200and switch its processing inputs between the various transmitter channels as shown inFIG. 2such that the feedback receiver200is shared among the various channels and provides feedback for each channel based on switched inputs. More information regarding an example of such a shared feedback receiver can, for example, be found in PCT Application WO 2010/054499.

However, there remains a need for improvement with respect to shared feedback receivers used in digital pre-distortion circuitry associated with multi-channel transmitters.

DL DownLink

LMS Least Mean Squares

LTE Long Term Evolution

MAC Medium Access Control

NCO Numerically Controlled Oscillator

PA Power Amplifier

PDCP Packet Data Convergence Protocol

PDU Packet Data Unit

RLC Radio Link Control

RMS Root Mean Square

TX Transmitter

UE User Equipment

UL UpLink

SUMMARY

Systems and methods according to the exemplary embodiments enable pre-distortion techniques, devices and systems for a multi-channel transmitter. An adaptation time is provided for each of the transmit chains in the multi-channel transmitter. During each adaptation time an output of a transmit chain is coupled to a shared feedback receiver. The shared feedback receiver generates a feedback signal that is used to vary pre-distortion for that transmit chain. A commutation controller varies at least one of: (a) an amount of the adaptation time for a transmit chain and (b) an order in which each of the transmit chains receives its adaptation time.

Among other advantages and benefits, embodiments provide for the sharing of feedback receiver resources, which in turn enables design of smaller size and lower cost radios. Over-design and over-specification of transmitter hardware can be minimized, which results in more economical and efficient systems. Performance of the single shared feedback receiver can be optimized according to embodiments by intelligent scheduling of the sharing and dynamic allocation of time for output observation.

According to one exemplary embodiment a method for adaptively providing pre-distortion in a multi-channel transmitter, wherein each channel is associated with a different transmit chain of elements, includes the steps of providing an adaptation time for each of the transmit chains, during which adaptation time an output of a respective transmit chain is coupled to a shared feedback receiver which generates a feedback signal that is used to vary pre-distortion for the respective transmit chain, and varying at least one of: (a) an amount of the adaptation time for a transmit chain and (b) an order in which each of the transmit chains receives the adaptation time.

According to another embodiment a multi-channel transmitter includes a plurality of different transmit chains, each associated with a different channel, a shared feedback receiver, and a commutation controller configured to provide an adaptation time for each of the transmit chains, during which adaptation time an output of a respective transmit chain is coupled to the shared feedback receiver which generates a feedback signal that is used to vary pre-distortion for the respective transmit chain, wherein at least one of: (a) an amount of the adaptation time for a transmit chain and (b) an order in which each of the transmit chains receives the adaptation time is varied over time.

DETAILED DESCRIPTION

In the context of digital pre-distortion, dedicating a feedback receiver to each transmitter output, e.g., as shown inFIG. 1, enables achieving the best dynamic performance of a linearizer system since any relevant changes in, e.g., the output transmit power, of a particular transmitter or channel can be fed back into the DPD unit virtually instantaneously for adaptation. On the other hand using as many feedback receivers as there are observation points at the transmitter outputs is inefficient in terms of power consumption, size and cost. The latter is a particular problem for units (e.g., base stations) of middle range power capability and more than one antenna port such as, but not limited to, remotely located radios supporting 2×2, 4×4 or larger MIMO ratios.

Alternatively, sharing a smaller number of feedback receivers than the number of observation points (transmitter channels), e.g., as shown inFIG. 2, improves the physical size and cost of the overall system. However, the performance of shared feedback receiver linearizers is limited by the speed of change of the power amplifier characteristics as a function of the input signal levels and ambient conditions. That is, during the period of time while the feedback receiver is processing signals from other transmit channels, a given transmit channel's performance will be negatively impacted if certain operating factors vary during that period of time. Thus, the longer that a transmitter is left without adaptation to its dynamically changing properties, the worse the output signal quality is over time. Accordingly, embodiments described herein provide for, among other things, dynamic adaptation of the usage of a shared feedback receiver which addresses this issue.

For example, according to an embodiment a system for transmitter linearization utilizing shared feedback receivers with dynamically controlled commutation can overcome the disadvantages of current linearizers. An embodiment employs a closed loop supervision system which uses information about the signal levels and statistics associated with the input transmit signals, as well as the spectrum of the feedback receiver signal currently connected to one of several transmitter outputs, to adapt usage of the shared feedback receiver.

A scheduling algorithm makes decisions about which transmitter is to be connected to the feedback receiver and for how long. These decisions can, for example, be based on analysis of the rate of change of the average power levels of each of the input signals, as well as the spectral performance of the transmitter outputs. The outputs of the various transmitters are observed and adapted, and their spectral performance is measured, e.g., on a regular basis. In case of a degradation of output performance of a transmitter or a significant change of input signal levels, priority usage of the shared feedback receiver can be given to a particular transmitter. Alternatively, in case of over-performance, a transmitter can be given reduced dwell time with the shared feedback receiver or can be skipped from the sharing sequence.

To provide some context for a more detailed discussion of such exemplary embodiments related to dynamic adaptation of usage of a shared feedback receiver, consider first an exemplary radiocommunication system as shown from two different perspectives inFIGS. 3 and 4, respectively. To increase the transmission rate of the systems, and to provide additional diversity against fading on the radio channels, modern wireless communication systems include transceivers that use multi-antennas—often referred to as MIMO systems—. The multi-antennas may be distributed to the receiver side, to the transmitter side and/or provided at both sides as shown inFIG. 3. More specifically,FIG. 3shows a base station320having four antennas340and a user terminal—also referred to herein as “user equipment” or “UE”—360having two antennas340. The number of antennas shown inFIG. 3is exemplary and is not intended to limit the actual number of antennas used at the base station320or at the user terminal360in the exemplary embodiments to be discussed below.

Additionally, the term “base station” is used herein as a generic term. As will be appreciated by those skilled in the art, in for example the LTE architecture an evolved NodeB (eNodeB) may correspond to the base station, i.e., a base station is a possible implementation of the eNodeB. However, the term “eNodeB” is also broader in some senses than the conventional base station since the eNodeB refers, in general, to a logical node. The term “base station” is used herein as inclusive of a base station, a NodeB, an eNodeB or other nodes specific for other architectures. An eNodeB in an LTE system handles transmission and reception in one or several cells, as shown for example inFIG. 4.

FIG. 4shows, among other things, two eNodeBs320and one user terminal or UE360. The user terminal360uses dedicated channels400to communicate with the eNodeB(s)320, e.g., by transmitting or receiving RLC PDU segments as described below. The two eNodeBs320are connected to a Core Network440.

One exemplary LTE architecture for processing data for transmission by an eNodeB320to a UE360, i.e., in the downlink (DL) is shown inFIG. 5. Therein, data to be transmitted by the eNodeB320, e.g., IP packets, to a particular user is first processed by a packet data convergence protocol (PDCP) entity500in which the IP headers can be compressed and ciphering of the data is performed. The radio link control (RLC) entity520handles, among other things, segmentation of—and/or concatenation of—the data received from the PDCP entity500into protocol data units (PDUs). Additionally, the RLC entity520provides a retransmission protocol, e.g., automatic repeat request (ARQ), which monitors sequence number status reports from its counterpart RLC entity in the UE36to selectively retransmit PDUs as requested. The medium access control (MAC) entity54is responsible for uplink and downlink scheduling via scheduler560, as well as hybrid-ARQ processes. A physical (PHY) layer entity580takes care of coding, modulation, and multi-antenna mapping, among other things. Each entity500-580shown inFIG. 5provides outputs to, and receives inputs from, their adjacent entities by way of bearers or channels as shown. The reverse of these processes are provided for the UE360as shown inFIG. 5for the received data, and it will be appreciated by those skilled in the art that, although not shown inFIG. 5, the UE360also has similar transmit chain elements as the eNB320for transmitting on the uplink (UL) toward the eNB320and the eNB320also has similar receive chain elements as the UE360for receiving data from the UE360on the UL. It will be appreciated that although an exemplary LTE system is described here for context, that the present invention is not limited to implementation in an LTE system and may be implemented in any radiocommunication system.

Having described some exemplary devices in which aspects of dynamic adaptation of the usage of a shared feedback receiver according to embodiments can be implemented, the discussion now returns to such embodiments with a discussion ofFIG. 6. Therein, a functional block diagram of an adaptive multi-channel transmitter linearization system using a shared feedback receiver is illustrated, which system is generally referenced by numeral600.FIG. 6shows N transmitters or channels associated with the transceiver or system600, however same reference numerals are used in each transmit chain, it being understood that each of the other N−1 transmitters include the same or similar components.

Taking TX1as exemplary, this transmit chain includes, for example an input TxGain block602which receives a modulated, digital data signal being prepared for transmission and which controls the gain of the transmitter TX1based on various inputs described below. A digital pre-distortion (DPD) module604operates, based on various inputs described below, to provide pre-distortion to the signal to be transmitted. The digital-to-analog converter (DAC) and upconverter606transforms the digital signal to be transmitted into an analog signal and upconverts the signal to a desired RF frequency for transmission, and the power amplifier (PA)608amplifies the analog signal prior to transmission via an antenna (not shown). Note that the term “transmitter”, “transmit chain” and “channel” are used interchangeably herein due to the close association between a transmitter, e.g., TX1, its transmit chain of elements and the corresponding channel on which data processed via that transmitter is transmitted.

Each transmitter TX1, . . . , TXN output has a coupler610to couple the output RF signal to the shared feedback receiver612. In this embodiment a single shared feedback receiver612serves all of the transmitters in the system600, e.g., a base station transmitter, however it will be appreciated that other embodiments could provide for more than one shared feedback receiver which operate in accordance with the following discussion. The shared feedback receiver612down-converts one of the RF signals which are provided to it by the couplers610and provides a down-converted feedback signal614to the DPD module604in the transmit chain associated with that RF signal. A commutation controller615selects which transmitter's TX1, . . . , TXN coupled signal is to be connected to the shared feedback receiver612at a given time or time period via the FBRX_CTRL signal616, e.g., as an input to a switch or multiplexer (not shown) disposed upstream of the shared feedback receiver612. The commutation controller615also generates other control signals, which other control signals are illustrated inFIG. 6and described in more detail below, for use in adapting the behavior of the transmission system600to, among other things, dynamically adapt the shared usage of the shared feedback receiver612to changing conditions.

The down-converted signal614from the shared feedback receiver612is, according to this embodiment, also passed to a digital spectrum analyzer (SA)618where the RF output spurious emission level associated with the down-converted feedback signal614is measured. This measured level (SA_LEVEL) is passed to the commutation controller615for processing via signal620. The commutation controller615controls the operation of the spectrum analyzer618via the SA_CTRL signal622.

FIG. 7shows a functional block diagram of an exemplary spectrum analyzer618for one frequency channel only. The system600can however be designed to include, for example, multiple spectrum analyzer channels tuned to different frequencies to provide for time-frequency measurements across the observed bandwidth. The down-converted signal614from the shared feedback receiver612is fed into a mixer700in the spectrum analyzer618. The numerically controlled oscillator (NCO) block702provides the tuning required to select the appropriate frequency offset for the center of a spectral power measurement associated with the signal coupled from the currently selected one of the transmitters TX1, . . . , TXN. Two or more low-pass filters704,705, each having different pass-bands, provide for a configurable resolution bandwidth of the spectral power measurement (e.g., 100 kHz and 1 MHz), one of which can be selected by the multiplexer706based on an input control signal, e.g., signal622. The range select function708provides a variable gain stage in the spectrum analyzer618which is used to adjust the feedback signal614to an optimal level for digital power measurement. The digital power detector710performs instantaneous power measurement and the low-pass filter712provides an averaging function to generate a spectral RMS power reading.

Returning toFIG. 6, according to an embodiment, each TxGain block602receives an input from a respective level limiter624. The level limiter624measures the signal level at both the input and output of its respective TxGain block602to detect input signal level changes for which DPD re-adaptation is needed. The level limiter624in each transmitter compensates for the detected change by either increasing or decreasing the gain of the TxGain block602to temporarily maintain the same signal level into DPD604. Since the shared feedback receiver612is shared with other transmitters, this level compensation temporarily avoids the need for DPD adaptation until the commutation controller615can schedule the shared feedback receiver612for the transmitter where a change is detected and thus, allowing the DPD604to adapt.

Thus, once a level limiter624detects a change in the input signal level having a magnitude which exceeds a threshold, information about a signal level change is sent from the level limiter624to the commutation controller615via the LEVEL_CHANGEx signal. This action results in an identifier associated with this transmitter TX1, TX2or TXN being, for example, placed into a priority queue to be serviced by the commutation controller615as described in more detail below. When the commutation controller615allocates the shared feedback receiver resource612to this transmitter, the commutation controller615sends a LEVEL_CTRLx signal to the corresponding level limiter624to tell the level limiter to remove its temporary level compensation so that the DPD604adapts to the changed or changing signal level for that transmitter.

In addition, there is illustrated inFIG. 6an optional MAC_LAYER_LEVELx input signal to the level limiter block624from the medium access control (MAC) layer, or any other function(s) from the baseband modem which have prior knowledge of a change in the signal level, which MAC_LAYER_LEVELx signal provides early information about a change in signal level. For a system which has access to the MAC layer to obtain early information via the signal MAC_LAYER_LEVELx, about the signal level change, the level limiter624can make use of this information to initialize the start of a compensation process without transient latency. This option requires knowledge of the boundary of the symbol/frame of the data synchronized to the MAC_LAYER_LEVELx control bus. Having such MAC layer information helps avoid power level glitches from passing through in cases of rapid power variations which otherwise are compensated by a more complicated design of the AGC algorithm in the level limiter624. For example, if the system600is employed in a so-called 4G or LTE system, the MAC_LAYER_LEVELx input signal could be provided on a per frame basis, a per symbol basis or over any pre-defined period of time.

According to one embodiment, when the commutation controller615is ready to service a particular transmitter by providing its coupled feedback signal to the shared feedback receiver612, the commutation controller615can instruct the level limiter624to completely remove its previously imposed gain compensation. According to another embodiment, instead of removing the level compensation all at once, the level limiter624can do so gradually in one adaptation period or incrementally during several adaptation periods. In other words, if the signal level change is significantly large and/or requiring a much longer DPD adaptation period than allocated by the commutation controller615, then the level limiter624can perform a partial removal of the amount of level compensation per unit time. Complete or partial level compensation removal can be performed, for example, gradually toward the end of the adaptation period (e.g., following a linear ramp profile) and, if necessary, further level compensation removal can continue in the next allocated DPD adaptation period for that particular transmitter.

Also shown inFIG. 6is an adaptive PA model626which is used, in this embodiment, in each transmit path. The PA model626is an adaptive model of the PA608's nonlinear characteristic as a function of input RMS level and ambient conditions. The characteristic variation due to ambient condition is expected to be slow and gradual in system600. However, the characteristic variation due to input RMS level changing is expected to be significant relative to the magnitude of the change in input RMS level. Therefore, in a shared feedback receiver system, the DPD604should be adapted immediately whenever there is a significant change in input RMS level. Thus, according to this embodiment, the DPD604in each transmit path can intermittently use the output from its PA model626for adaptation instead of continually relying on the output from the actual PA608. Since the feedback receiver612is a shared resource, having an accurate PA model626reduces the need for the DPD604to rely solely on the feedback receiver612with the actual PA608output for adaptation, thus, allowing the feedback receiver612to be shared among more transmitters TX1, . . . , TXN.

The PA model626in a transmit path can be updated whenever the shared feedback receiver612is switched to this particular transmit path. The commutation controller615controls when the PA model626is to be updated via the PAModel_CTRLx signal. The PA model626is optional and can, for example, be omitted in some systems.

Having described an exemplary multi-channel transmitter600including dynamic adaptation of the usage of a shared feedback receiver612according to an exemplary embodiment, a more detailed description of the operation of a commutation controller615is now provided with reference to the flow diagram ofFIG. 8. As discussed above, the commutation controller615receives input from the level limiters624and the spectrum analyzer618. A commutation algorithm processes these inputs and generates appropriate control signals to the PA model626, level limiters624, the DPD modules604, and the shared feedback receiver612based, e.g., on the steps illustrated inFIG. 8.

Therein, upon enabling the commutation controller615, the algorithm starts by checking for channels in the priority queue at step800. According to an embodiment, the priority queue contains a FIFO list of the channels reported by the various level limiters624as having an input signal level change which exceeds the threshold. If the priority queue is empty, the process follows the “No” path from block800and the next channel is selected based on a round-robin scheme at step802. The commutation controller615then switches the shared feedback receiver612to the chosen channel at step804. For example, if TX1had previously received an adaptation period from the commutation controller615, then the coupled signal from coupler610associated with TX2could be passed to the shared feedback receiver612at step804. Then, the commutation controller615enables DPD adaptation for this particular channel at step806. The spectrum analyzer618is then activated, at step808, by the commutation controller615to perform a spectral RMS power reading for the currently selected channel. The commutation controller615then updates its control parameters at step810(which are discussed in more detail below) and returns back to step800to check for channels in the priority queue.

If, at step800, the priority queue is not empty then the flow proceeds along the “Yes” branch from block800to step812, wherein the next channel chosen for adaptation using the shared feedback receiver612is the first channel in the queue. The commutation controller615then switches the shared feedback receiver612to the chosen channel at step814. The commutation controller615enables the DPD604associated with the selected channel at step816and then signals the level limiter624associated with the selected channel/transmitter to remove the level compensation at step818, e.g., either completely or gradually as described above. If a significantly large signal level change is associated with placement of this transmitter in the priority queue, the commutation controller615could, for example, signal the corresponding level limiter624to only partially remove the level compensation during this adaptation period and to perform further removal of level compensation during the next adaptation period which is assigned to this channel/transmitter. This effectively controls the ramp of the input signal level to ensure all transmitters in the system600are adequately served by the shared feedback receiver612. If the full level compensation is removed for a particular transmitter, then that transmitter/channel is removed from the priority queue. If only partial level compensation is removed, the channel is moved to the bottom of the priority queue according to this embodiment.

Continuing on inFIG. 8, after step818, the commutation controller615then activates the spectrum analyzer618at step808to measure the spurious emission level and updates its control parameters at step810. In both cases described above, i.e., switching the feedback receiver612to evaluate a coupled signal from a channel/transmitter in the priority queue or the next channel/transmitter based on some arbitration scheme, e.g., round robin, the results of the spectrum analyzer measurement are used to adaptively update the system parameters. Since embodiments are adaptive in order to potentially avoid the need for factory calibration of system parameters, the results from the spectrum analyzer measurement are used to adaptively update the following parameters (which are not intended to be exhaustive):1. Time period for DPD adaptation of a transmitter TX1, . . . , TXN in the case when the signal level change does not exceed the threshold as indicated by the level limiter.2. Time period for DPD adaptation of a transmitter TX1, . . . , TXN in the case when the signal level change exceeds the threshold as indicated by the level limiter. The adaptation time can be proportional to the magnitude of the signal level change (increase or decrease).3. The amount of level compensation to be removed by the level limiter624during each DPD adaptation period.4. The threshold used by the level limiter624to detect whether the magnitude of the signal level change requires immediate DPD adaptation.

Updates of the system parameters at step810can be performed adaptively using, for example, a common least mean squares (LMS) algorithm (in small iterations proportional to an error function value leading to the overall minimization of such error function), however such updates are not limited to this particular type of algorithm. These adaptive updates of the system parameters are useful to optimize the system performance with respect to the sharing of the feedback receiver612. The first exemplary parameter which can be updated at step810is the DPD adaptation time of a transmitter for an input signal level change which does not exceed the level limiter threshold. During each of the adaptation periods, the spectrum analyzer618measures the spurious emission level. If the margin is adequate, the update algorithm reduces the DPD adaptation time for subsequent operations on transmitters where the input signal level change does not exceed the threshold. If the margin is not adequate, the update algorithm increases the adaptation time for subsequent operations.

The second exemplary parameter which can be updated at step810is the DPD adaptation time of a transmitter with an input signal level change which does exceed the level limiter threshold. Corresponding to the magnitude of the input signal level change, the adaptive algorithm allocates a specific DPD adaptation time. Based on the spectrum analyzer's detected spurious emission margin for this channel, the update algorithm increases or decreases the adaptation time corresponding to the magnitude of signal level change accordingly.

The third parameter exemplary parameter which can be updated at step810is the amount of level compensation to be removed by the level limiter624during each DPD adaptation period. A larger input signal level change (i.e., particularly an increase) will result in more DPD adaptation time being provided to that transmitter. If an input level change for a given transmitter is too large, i.e., requiring significant DPD adaptation time which is more than what can be fairly allocated without negatively impacting the adaptation of the other transmitters, the allowable input signal level change can be regulated to allow only partial increase or decrease associated with removing the level limiter624's influence on the TXGain block602during a particular DPD adaptation period. According to an embodiment, it is however advantageous for the level limiter624to remove as much level compensation as possible during each DPD adaptation period to avoid needing multiple high priority DPD adaptation periods for a particular transmitter. The adaptive update algorithm monitors the spectrum analyzer spurious emission results and either increases or decreases the allowable amount of input signal level change accordingly.

The fourth exemplary parameter which can be updated at step810is the threshold used by the level limiter624to detect whether the magnitude of the input signal level change requires substantially immediate DPD adaptation as handled by the priority queue. The adaptive update of this parameter can be performed by monitoring the spectrum analyzer measurement. For example, if the threshold is set too high and the signal level change is allowed through by the level limiter624without registering the channel into the priority queue of the commutation controller615, then the spectrum analyzer608will measure a spurious emission margin which would have decreased. This is an indication that the threshold should have been set lower to register this channel into the priority queue for high priority DPD adaptation. As a result, the threshold will be lowered accordingly during the update process810. Increasing of the threshold is done similarly by monitoring the spectrum analyzer measurement. If the channel is registered in the priority queue when it is not necessary (i.e. no significant change in spurious emission margin), the threshold will be increased accordingly.

The preceding examples describe an approach for adaptively updating each of the four exemplary system parameters. However it will be appreciated by those skilled in the art that this set of parameters is not intended to be exhaustive—i.e. there can be other system parameters which can be updated similarly using the available information regarding the input signal level provided by the level limiter and the output spurious emission level provided by the spectrum analyzer. Alternatively, any subset of the four parameters described above could be adaptively updated, and others could remain with fixed values.

The preceding embodiments provide for a number of advantages and benefits. For example, sharing feedback receiver resources enables design of smaller size and lower cost radios. Over-design and over-specification of transmitter hardware can be minimized, which results in more economical and efficient systems. The performance challenges which arise naturally from the usage of a single shared feedback receiver, as opposed to using e.g., one shared feedback receiver per transmit chain, are overcome according to embodiments by smart scheduling of the sharing and dynamic allocation of time for output observation. The input to the control algorithm is composed of direct measurement of the input signals, as well as the actual spectral performance of the transmitter outputs. Embodiments include methodologies which are entirely adaptive, rely on real-time system and signal measurement and have minimal calibration requirements, making systems which employ such methodologies easier to manufacture and sustain. The added benefit of having continuous spectral measurements of the transmit outputs increases the overall base station robustness and reliability to violation of emissions requirements.

According to one embodiment, a method for adaptively providing pre-distortion in a multi-channel transmitter, wherein each channel is associated with a different transmit chain of elements, can include the steps illustrated in the flowchart ofFIG. 9. Therein, at step900, an adaptation time can be provided for each of the transmit chains, during which adaptation time an output of a respective transmit chain is coupled to a shared feedback receiver which generates a feedback signal that is used to vary pre-distortion for the respective transmit chain. As indicated by step902, at least one of: (a) an amount of the adaptation time for a transmit chain and (b) an order in which each of the transmit chains receives the adaptation time, can be varied over time. For example, relative to (a), the method ofFIG. 9could enable TX1to receive twice as much adaptation time (usage of the shared feedback receiver) as TX2inFIG. 6if, for example, signal level changes in the signal being processed by TX1were substantially more significant than signal level changes in the signal being processed by TX2for a given time period. As another example, relative to (b), the method ofFIG. 9could change the order or sequence in which adaptation was provided to the transmitters by, for example, initially providing adaptation time to the transmitters in the order of TX1, TX2, . . . , TXN, and then later providing adaptation time in the order of TX2, TX1, TX2, TX3, TX2, . . . , TXN, e.g., if signal level changes in the signal being processed by transmitter TX2warranted an increased periodicity of adaptation. This provides, among other things, a dynamic nature to the method and system which balances the pre-distortion needs of the various transmitters.

Systems and methods for processing data according to exemplary embodiments of the present invention can be performed by one or more processors executing sequences of instructions contained in a memory device. Such instructions may be read into the memory device from other computer-readable mediums such as secondary data storage device(s). Execution of the sequences of instructions contained in the memory device causes the processor to operate, for example, as described above. In alternative embodiments, hard-wire circuitry may be used in place of or in combination with software instructions to implement the present invention.

The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items.