Abstract:
A forward link transmitter in a sectored cell includes a baseband processor having traditional baseband signal digital processing circuitry in addition to including a digital hybrid matrix (vector and delay compensated transformation module) whose phase and amplitude (vector) and delay may be adjusted to compensate for downstream errors that are introduced and detected by a feedback circuit. Accordingly, the baseband processor, by monitoring an output of an analog hybrid matrix producing modulated and amplified radio frequency (RF) signals just prior to propagation from an antenna, can determine errors produced by the analog circuitry including the analog hybrid matrix and may compensate for the same by introducing an amplitude, phase and delay adjustment (in the digital domain) into output digital waveform signals to compensate for the error introduced downstream to the baseband processor.

Description:
TECHNICAL FIELD OF THE INVENTION 
   This invention relates generally to wireless communication systems and, more particularly, to radio frequency (RF) transmitters used within radio transceivers of such wireless communication systems. 
   DESCRIPTION OF RELATED ART 
   Communication systems are known to support wireless and wire line communications between wireless and/or wire line communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet, to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), universal mobile telephone systems (UMTSs), local multi-point distribution systems (LMDSs), multi-channel-multi-point distribution systems (MMDSs), and/or variations thereof, including wireless LAN networks such as IEEE 802.11, Bluetooth, etc. 
   For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via a public switched telephone network (PSTN), via the Internet, and/or via some other wide area network. 
   As is known by those of average skill in the art, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna. 
   Typically, in a sectored cellular network wherein each cell is divided into three or more cell sectors, each having its own amplification and transmission circuitry, beam forming antennas typically are used to create a forward link transmission pattern that fills the cell sector without overlapping in adjacent cell sectors. While one or two amplifiers could be used in a cell having more than two sectors, it is common to use one amplifier per cell sector. One problem that has been addressed by the prior art is that of amplifier failure in one of the sectors. A pair of N×N hybrid matrices are used in prior art. The first matrix will divide a signal at an input port of the first N×N hybrid matrix into N equal components, with a taper applied to each of the components. The N signals are then applied to N high power amplifiers, whereafter the amplified signals are fed to a second N×N hybrid matrix such that the original signal will only appear at one of the second N×N hybrid matrix output ports. One benefit of using the N×N hybrid matrix for this is that each signal is amplified partially by each of the amplifiers that are operational. Thus, if one amplifier were to fail, all output signals could be amplified sufficiently for transmission through all of the cell sectors (though in a degraded mode of operation). In the hybrid matrix amplifier (prior art), the hybrid matrix is fixed so that the degraded mode of operation impacts the signal-to-noise ratio. Such power sharing further has an advantage in that each forward link amplifier need not be designed to accommodate maximum power loads because additional power may be obtained from one or more other power amplifiers for maximum power requirements (across all the sectors). Thus, lower cost power amplifiers may be utilized. 
     FIG. 1  is a functional block diagram of a prior art cellular network cell having three cell sectors. More specifically, a cell  02  includes three cell sectors  04 . Approximately in the center of cell  02  exists a base station transceiver set (BTS)  06  that includes an amplifier  08  and an antenna  10  for each cell sector  04 .  FIG. 1  shows the amplifiers  08  and antennas  10  well within its corresponding cell sector  04  to show the relationships therefor. It is understood, however, that the amplifiers  08  and antennas  10  for the cell sectors  04  are located approximately in the center of cell  02 . The antennas  10  are so called sector antennas that radiate a pattern to fill cell sectors  04  without overlapping into an adjacent cell sector. For a system as shown in  FIG. 1  in which distinct amplifiers are used but in which a hybrid matrix is not included for power sharing, each of the amplifiers  08  must be designed to satisfy maximum power level demands for the sector. 
     FIG. 2  is a prior art transmitter that includes a pair of analog hybrid matrices. A baseband radio  14  produces a plurality of digital waveform signals to a digital-to-analog conversion module  16  to generate a corresponding plurality of analog signals. The plurality of analog signals are then up-converted by a plurality of mixers  18  that up-convert the plurality of analog signals by multiplying the baseband signals with a local oscillation signal to create output RF signals. The output RF signals are then produced to a first hybrid matrix  20  that produces a corresponding number of transformed signals. More specifically, if the first hybrid matrix  20  receives signals sig_ 1 , sig_ 2  and sig_ 3 , it produces three transformed analog signals having components of all three signals sig_ 1 , sig_ 2  and sig_ 3 . 
   A power amplifier module  22  includes a plurality of power amplifiers that are coupled to receive the 1 st , 2 nd , and 3 rd  transformed analog output signals from the first hybrid matrix  20  and amplifies them. A second hybrid matrix  24  then receives the 1 st , 2 nd , and 3 rd  transformed and amplified signals and recombines them to create amplified versions of sig_ 1 , sig_ 2 , and sig_ 3  at the second hybrid matrix  24  outputs. In operation, the second hybrid matrix  24  adds the signals at the sum port and cancels out signal portions at the output ports of the second hybrid matrix  24 . To effectively cancel unwanted signal components at the output ports, however, the relative component vector (phase and amplitude) and delay must be as expected. If a vector and/or delay error is introduced in or between the first hybrid matrix  20  or the second hybrid matrix  24 , then perfect cancellation does not occur at the undesired ports and a resulting waveform continues to include components of other waveforms. Accordingly, it is desirable to eliminate the effects of introduced relative vector and delay errors. 
   While utilizing hybrid matrices are advantageous for the described reasons, including power sharing, hybrid matrices are analog devices that introduce vector and delay errors in the output RF signal. Accordingly, what is needed is a system that allows for power sharing to achieve the benefits of an analog hybrid matrix amplifier pair but that produces output signals with the ability to compensate for vector and delay errors. 
   BRIEF SUMMARY OF THE INVENTION 
   A base station transmitter in a sectored cell includes a baseband processor having traditional baseband digital signal processing circuitry for transmitting forward link communication signals. In addition, the base station transmitter includes a digital signal processor that includes modules that form a digital hybrid matrix having logic for vector and delay adjustments to compensate for downstream vector and delay errors that are introduced. Accordingly, the baseband processor, by monitoring an output of an analog hybrid matrix producing modulated and amplified radio frequency (RF) signals just prior to propagation from an antenna, can indirectly determine relative vector and delay errors produced by the analog hybrid matrix, amplifiers, mixers, up-converters and connection circuitry coupled downstream from the digital signal processing circuitry and may compensate for the same by introducing a vector and delay adjustment (in the digital domain) into output digital waveform signals to compensate for the errors introduced downstream to the baseband processor. Thus, an output signal of the analog hybrid matrix after compensation has far less, or perhaps even no, vector (phase and amplitude) or delay errors despite the addition of these errors from the downstream circuitry mentioned above. 
   More specifically, the baseband processor includes a first processing module for generating a plurality of digital waveform signals, wherein the plurality of digital waveform signals represents a corresponding plurality of RF analog signals that are to be transmitted within corresponding cell sectors of a cellular network cell. A second processing module receives the plurality of digital waveform signals to produce a plurality of transformed digital waveform signals each containing a portion of each of the plurality of digital waveform signals. The second processing module includes a vector and delay detection module and a vector and delay compensated transformation module. The second processing module includes a vector and delay compensated transformation module that transforms and modifies the received digital waveform signals in phase, amplitude and delay and produces its output to a third processing module. The output of the second processing module is a plurality of transformed digital waveform signals that compensate for downstream vector and delay errors. The third processing module is coupled to receive the outputs of the second processing module and includes a baseband pre-distortion (BBPD) module, that adjusts for amplifier distortion and a peak power reduction (PPR) module that reduces peak power for a given digital waveform signal thereby reducing the peak power demand of the power amplifier without significant signal degradation. The third processing module produces a plurality of transformed and adjusted digital waveform signals. 
   The plurality of transformed and adjusted digital waveform signals output from the third processing module is then produced to a digital-to-analog conversion module for converting to an analog (analog signal) domain. A plurality of transformed analog signals produced by the digital-to-analog conversion module is then produced to an up-conversion module for mixing a local oscillation signal and are up-converted from a baseband frequency, or intermediate frequency (IF) if an IF stage is used, to a radio frequency to produce a plurality of transformed RF analog signals. At least one power amplifier module is coupled to receive the plurality of transformed and amplified RF analog signals to produce a plurality of amplified RF analog signals wherein each of the plurality of amplified RF analog signals corresponds to each of the plurality of digital waveform signals. 
   A hybrid matrix module, which, in the described embodiment of the invention is an analog hybrid matrix, is coupled to receive the plurality of transformed and amplified RF analog signals to create a plurality of amplified RF analog signals that are to be transmitted within corresponding cell sectors of a cellular network cell. Finally, the inventive transmitter includes feedback circuitry coupled to receive the plurality of RF analog signals and produces a digital representation of the plurality of amplified RF analog signals to the second processing module of the baseband processor module (by way of a digital-to-analog converter). Accordingly, the second processing module is able to indirectly determine relative vector and delay errors produced by the analog hybrid matrix, amplifiers, mixers, up-converters and connection circuitry coupled downstream from the digital signal processing circuitry and may compensate for the same by introducing a vector and delay adjustment (in the digital domain) into the plurality of transformed digital waveform signals to compensate for the errors introduced downstream to the baseband processor. The second processing module also includes a digital power amplifier failure compensation module for adjusting the signals in case of an amplifier failure such that power is steered to the required sectors with the best possible signal-to-noise ratio (best performance). 
   These and other features, aspects and advantages of the present invention will be more fully understood when considered with respect to the following detailed description, appended claims and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a functional block diagram of a prior art cellular network cell having three cell sectors; 
       FIG. 2  is a prior art transmitter that includes a pair of analog hybrid matrices; 
       FIG. 3  is a functional block diagram of a radio transmitter formed according to one embodiment of the present invention; 
       FIG. 4  is a functional block diagram of a radio transmitter illustrating one aspect of the present invention; and 
       FIG. 5  is a flow chart illustrating a method for generating forward link communication signals according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  is a functional block diagram of a radio transmitter formed according to one embodiment of the present invention. A baseband processor  30  includes a plurality of modules that produce a plurality of transformed and adjusted digital waveform signals having compensation components that compensate for errors that are introduced downstream. More specifically, a first processing module  32  generates a plurality of digital waveform signals, each of which is a digital bit stream that represents an analog radio frequency (RF) signal (i.e., a digital representation of an “analog” RF signal) that is to be transmitted to a cell sector. A second processing module  34  receives the plurality of digital waveform signals and produces a plurality of transformed digital waveform signals wherein each of the plurality of transformed digital waveform signals include digital representations of portions of each of a plurality of RF analog signals represented by the plurality of digital waveform signals produced by the first processing module  32 . 
   The second processing module  34  includes an indirect vector and delay detection module  36  and a vector and delay compensated transformation module  38 . The indirect vector and delay detection module  36  uses the amplitude of the signals at the output ports to determine the degree of summation and cancellation. Based on the degree of summation and cancellation, the vector and delay compensated transformation module  38  is formed to introduce either one or both of a vector and delay component to the plurality of digital waveform signals by adjusting the vector and delay compensated transformation module  38 . A plurality of transformed digital waveform signals produced by the second processing module  34  of the baseband processor  30  is thus modified in amplitude, phase and delay according to detected vector and delay errors introduced downstream. A third baseband processor  40  then applies a number of further processing functions to each of the plurality of transformed digital waveform signals at the output of the second processing module  34 . The functions include baseband pre-distortion, peak power reduction and a number of filter functions. The baseband processor  30  and, more specifically, the third processing module  40 , then produces the plurality of transformed and adjusted digital waveform signals to a digital-to-analog conversion module  46  wherein the plurality of transformed and adjusted digital waveform signals are converted from a digital domain to an analog domain to create a plurality of transformed analog signals. The converted plurality of transformed analog signals are then produced by the digital-to-analog conversion module  46  to an up-conversion module  48  where they are up-converted from a baseband frequency to a radio frequency (RF) to create a plurality of transformed RF analog signals. 
   The plurality of transformed RF analog signals is then produced from the up-conversion module  48  to a power amplification module  50  wherein the plurality of transformed RF analog signals is amplified to create a plurality of transformed and amplified RF analog signals. The plurality of transformed and amplified RF analog signals is then produced by the power amplification module  50  to a hybrid matrix module  52 . 
   The hybrid matrix module  52  receives the plurality of transformed and amplified RF analog signals and produces a plurality of amplified RF analog signals to the appropriate sum and cancellation ports for transmission into an appropriate cell sector. Additionally, the plurality of amplified RF analog signals is also coupled to a feedback loop  54 . The feedback loop  54  includes a switching module  56  coupled to receive and select between each of the plurality of transformed and amplified RF analog signals before the hybrid matrix module  52  and the plurality of amplified RF analog signals after the hybrid matrix module  52 . The selected output of the switching module  56  is then produced to a down-conversion module  58  where it converts the selected amplified RF analog signal to a baseband or intermediate frequency. The down-converted signal is then produced to an analog-to-digital conversion module  60  that converts the signal to the digital domain. The digitally converted signals are produced by the analog-to-digital conversion module  60  to the third processing module  40 , and more specifically, to a peak power reduction module  44  and to a pre-distortion module  42 . Pre-distortion module  42  and peak power detection module  44  are operable to compensate for distortion and reduce peak power for a given digital waveform signal, respectively. The digitally converted signals are also produced to the indirect vector and delay detection module  36  of the second processing module  34 . 
   The indirect vector and delay detection module  36  of the second processing module  34  then determines the degree of error of the sum and cancellation ports relative to desired values. The vector and delay compensated transformation module  38  of the second processing module  34  compensates and adjusts the amplitude, phase and delay of the corresponding components of the plurality of digital waveform signals produced by the first processing module  32 , based on the errors determined by the indirect vector and delay detection module  36 , by adjusting the vector and delay compensated transformation module  38  to compensate for the errors introduced downstream from the baseband processor  30 . 
   For example, if the digital signal represents a first amplified RF analog signal, and the vector and delay compensated transformation module  38  determines that the first amplified RF analog signal from the hybrid matrix module  46  has a component that is lagging by 10 degrees due to introduced phase errors, then the vector and delay compensated transformation module  38  advances the corresponding component in the corresponding transformed and adjusted digital waveform signal by 10 degrees. 
   In this example, the phase shift of the component of the first amplified RF analog signal has been compensated by adding 10 degrees to the corresponding transformed and adjusted digital waveform signal. Similar compensation may also be made for the other signal components as necessary. For example, the indirect vector and delay detection module  36  is operable to detect vector (phase and amplitude) and delay errors and to compensate therefor. 
     FIG. 4  is a functional block diagram of a radio transmitter illustrating one aspect of the present invention. A baseband processor  62  includes a first processing module  32 , a second processing module  64  and a third processing module  40 . First and third processing modules  32  and  40  are as described in  FIG. 3 . Second processing module  64 , however, further includes a digital power amplifier failure compensation module  66 . 
   The digital power amplifier failure compensation module  66  is, among other functions, for defining how the configuration of the vector and delay compensated transformation module  38  will change to compensate for a condition where one of the paths between the baseband processor  62  and a hybrid matrix  74  has failed, giving the best possible system performance under the given failure condition. 
   Statistically, all three sectors will not be fully loaded and since power is shared between all the amplifiers, the amplifier size can be reduced while still achieving the required total power across all sectors. Without power sharing, the amplifier power has to be high enough to handle the fully loaded sector. But, if the sector is under-loaded, the power of the amplifier power is under-utilized. Thus, power sharing allows the individual amplifier sizes to be reduced. The power sharing capability is a result of the transformation process. 
   Many of the components of  FIG. 3  are shown in  FIG. 4 . Accordingly, those components will not be described further here in the description of  FIG. 4 .  FIG. 4  further illustrates a feedback loop  70  that includes a plurality of directional couplers  72  that are connected between the power amplifiers for each branch and hybrid matrix  74 , and a plurality of directional couplers  76  that are connected between hybrid matrix  74  and antennas through which RF is propagated. The feedback loop  70  further includes a six-way switch  78 . In the example of  FIG. 4 , the six directional couplers  72  and  76  are coupled to the six-way switch  78  (or, alternatively, a multiplexer) that selects one of the six inputs provided by the six directional couplers  72  and  76  and produces the selected input to a down-conversion module  84 . 
   The down-conversion module  84  then produces a baseband or intermediate frequency signal to an analog-to-digital converter  82  for converting the signal to the digital domain for processing and analysis by the baseband processor  62 . The six directional couplers  72  and  76 , the six-way switch  78 , the down-conversion module  84  and the analog-to-digital converter  82  all are shown here in  FIG. 4  as being part of the feedback loop  70 . The feedback loop  70  produces the selected signal to the baseband processor  62  and, more particularly, to the second processing module  64  and third processing module  40  (and the modules included therein) for analysis as described herein and for phase, amplitude and delay of the corresponding signals responsive thereto. 
     FIG. 5  is a flowchart illustrating a method by a base station for generating forward link communication signals according to an embodiment of the invention. Initially, a baseband processor produces a plurality of transformed and adjusted digital waveform signals where the digital waveform signals represent a corresponding plurality of amplified RF analog signals (step  90 ). In general, the radio transmitter transmits an amplified RF analog signal to mobile terminals within a cell or cell sector. Because the baseband processor operates in the digital domain, however, it generates a plurality of transformed and adjusted digital waveform signals where the digital waveform signals represent a corresponding plurality of amplified RF analog signals that are to be transmitted from antennas within the corresponding cell sectors. 
   Thereafter, a digital-to-analog conversion module in the radio transmitter converts each of the plurality of transformed and adjusted digital waveform signals from a digital domain to an analog domain to produce a plurality of transformed analog signals (step  92 ). The transformed analog signals are then up-converted from a baseband frequency to radio frequency (RF) to produce a plurality of transformed RF analog signals (step  94 ). The radio transmitter then amplifies the plurality of transformed RF analog signals produced by the up-conversion module to produce a plurality of transformed amplified RF analog signals (step  96 ). 
   The hybrid matrix module is coupled to receive the plurality of transformed amplified RF analog signals and produces amplified RF analog signals to an antenna for propagation (step  98 ). Each of the amplified RF analog signals only includes components for the amplified RF analog signal for transmission into a specific cell sector. The transmitter produces the amplified RF analog signals to an antenna for propagation through a cell sector as well as to a feedback loop (step  100 ). In addition to propagating the amplified RF analog signals, the feedback loop(s) need to be utilized to provide the baseband processor the ability to determine what downstream error has been introduced to facilitate compensation therefore. Accordingly, the invention includes selecting, in a six-way switching module in one embodiment of the invention, among the plurality of transformed and amplified RF analog signals prior to the hybrid matrix module and the plurality of amplified RF analog signals being produced after the hybrid matrix module and produce the selected signal to a mixer for down-conversion from RF to baseband or an intermediate frequency (step  102 ). 
   Thereafter, the amplified RF analog signals are converted to a baseband or intermediate frequency in the described embodiment of the invention (step  104 ). The method then includes conversion of the baseband or intermediate frequency analog signals to the digital domain (step  106 ). The digital domain signals are then produced to the baseband processor and, more particularly, to the second and third processing modules of the baseband processor (step  108 ). The baseband processor or, more particularly, the second processing module of the baseband processor, then determines an amount and type of error introduced downstream of the baseband processor (step  110 ). Finally, the invention includes introducing a corresponding compensation into the digital waveform signals to compensate for the determined error introduced downstream from the baseband processor (step  112 ). 
   The invention disclosed herein is susceptible to various modifications and alternative forms. Specific embodiments therefore have been shown by way of example in the drawings and detailed description. It should be understood that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the claims. For example, while the described embodiment of the invention has been discussed in terms of a 3 by 3 hybrid matrix, the invention specifically includes a matrix of any size (N×N).