Abstract:
A communication system interface between a baseband unit and a radio frequency (RF) unit is configured to advantageously use a common set of lines to carry both transmit and receive baseband analog signals between the baseband and RF unit, thereby enabling a relatively lower signal count and permitting loopback testing of elements within the baseband and the RF units.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 12/043,862 entitled, “Analog Baseband Interface For Communication Systems” filed Mar. 6, 2008 which claims priority of U.S. Provisional Patent Application 60/896,249, entitled “Analog Baseband Interface For Communication Systems” filed Mar. 21, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments described in this specification relate generally to communications systems and more particularly to an analog baseband interface between wireless communication units. 
         [0004]    2 Description of the Related Art 
         [0005]    Wireless communications systems generally use radio frequency (RF) signals to transmit data from a transmitter to one or more receivers. Wireless communication systems are frequently used to implement wireless local area networks (LANs) in which data is transmitted and received between computers, servers, Ethernet switches, hubs, and the like. A wireless LAN may, for example, allow web page data to be transferred between a server and a computer. 
         [0006]    Many wireless communication systems may be divided into two or more units. A typical division occurs between an RF unit and a baseband unit. The RF unit may convert transmit baseband analog signals into RF signals that may be transmitted through an antenna. The RF unit may also receive an RF signal from an antenna and convert the RF signal to a receive baseband analog signal. The baseband unit, working in conjunction with the RF unit, may create the transmit baseband analog signal the RF unit processes and transmits and may also receive a receive baseband analog signal from the RF unit that has been generated from a received RF signal. 
         [0007]    The baseband unit is typically coupled to other units within the wireless communication system. Other typical elements of wireless communication systems may include elements configured to process data to be transmitted. For example, data may need to be encoded by an encoding element before the data can be processed by the baseband unit and then coupled to the RF unit for transmission. Still other typical elements of a wireless communication system may also include one or more digital signal processing units that can further process the data generated by the baseband unit from the analog signal received from the RF unit. 
         [0008]      FIG. 1  illustrates an exemplary prior art wireless communication system  100  including a baseband unit  101  and an RF unit  111 . Baseband unit  101  includes a digital to analog converter (DAC)  102  and an analog to digital converter (ADC)  103 , whereas RF unit  111  includes a transmitter (TX)  112  and a receiver (RX)  113 . One or more antennas  110  may be coupled to RF unit  111 . In many cases, RX  113  and TX  112  may share antenna  110  (shown). 
         [0009]    TX  112  of RF unit  111  is coupled via lines to DAC  102  in baseband unit  101 . Many wireless communication systems are configured to transmit more than one RF signal contemporaneously. For example, two quadrature RF signals are usually transmitted to support orthogonal frequency-division multiplexing (OFDM) defined by wireless communication standards IEEE 802.11a or 802.11g. Therefore, TX  112  is usually configured to accept two transmit baseband analog signals. The two transmit baseband analog signals are also often relatively high bandwidth signals in order to support relatively high data transfer rates. 
         [0010]    Differential line pairs are often used for high bandwidth signals in order to increase, among other things, noise immunity and performance. One embodiment of a differential line pair encodes a signal with a positive component and a negative component. These two components are typically implemented with two lines, each line carrying one component. 
         [0011]    Oftentimes, the coupling between DAC  102  and TX  112  is through two differential line pairs.  FIG. 1  shows two differential line pairs  107  (i.e. I+, I−, Q+, and Q−) from DAC  102  in baseband unit  101  to TX  112  in RF unit  111 . RX  113  receives an RF signal through antenna  110  and recovers one or more receive baseband analog signals. As shown in  FIG. 1 , two differential line pairs also couple RX  113  to ADC  103  in baseband unit  101 , thereby facilitating the contemporaneous receipt of two receive baseband analog signals (for the same reason as described above in the transmit case). Thus, wireless communication system  100  includes four differential line pairs coupling baseband unit  101  and RF unit  111  (i.e. eight discrete lines in total). 
         [0012]    Multiple-input multiple-output (MIMO) wireless LAN architectures may provide improved performance when compared to single-input single-output architectures. The improved performance may be provided by, in part, using a plurality of transmitters and receivers (transceivers) to process RF signals.  FIG. 2  illustrates a portion of an exemplary multiple transceiver wireless communication system  200 , which can be characterized as an extension of the system configuration of  FIG. 1  (but is not known to be implemented or discussed in the prior art). System  200 , like system  100  ( FIG. 1 ), includes a baseband unit  201  and an RF unit  211 . However, in system  200 , baseband unit  201  and RF unit  211  are divided into three sub-units, i.e. A, B, and C (indicated by the suffix of each reference number). Note that in other embodiments, baseband unit  201  and RF unit  211  may be divided into two sub-units or more than three sub-units. 
         [0013]    A first baseband sub-unit  201 A includes a first DAC  202 A and a first ADC  203 A, a second baseband sub-unit  201 B includes a second DAC  202 B and a second ADC  203 B, and a third baseband sub-unit  201 C includes a third DAC  202 C and a third ADC  203 C. A first RF sub-unit  211 A includes a first transmitter (TX)  212 A and a first receiver (RX)  213 A, a second RF sub-unit  211 B includes a second TX  212 B and a second RX  213 B, and a third RF sub-unit  211 C includes a third TX  212 C and a third RX  213 C. 
         [0014]    System  200 , like system  100 , uses differential line pairs to couple the elements in baseband unit  201  to the elements in RF unit  211 . In system  200 , two differential line pairs couple the DACs to the TXs and two differential line pairs couple the RXs to the ADCs. Therefore, to couple baseband unit  201  to RF unit  211 , twenty-four discrete, inter-unit lines (i.e. lines between baseband unit  201  and RF unit  211 ) are required. 
         [0015]    Note that wireless communication system  200  may be configured to enable loopback testing. Loopback testing is a testing method that allows a user to test or calibrate portions of a wireless communication system without the need to transmit or receive data to or from a second wireless communication system. Loopback testing, therefore, advantageously makes possible some amount of testing or calibration of the wireless communication system without relying on a separate wireless communication system. 
         [0016]    Typically, during loopback testing, data passes through a loopback processing chain of elements that includes a DAC, a TX, a RX, and an ADC. Oftentimes, the loopback processing chain is configured such that the DAC is coupled to the TX that is coupled to the RX that is further coupled to the ADC. All the elements within the loopback processing chain may function contemporaneously to process test data. Specifically, the test data is often introduced into the loopback processing chain at the DAC, proceeds from the DAC to the TX, continues from the TX to the RX and finally travels to the ADC. The testing and calibration may come about by understanding the test data that is introduced to the loopback processing chain and examining the data that is returned from the loopback processing chain. 
         [0017]    For example, using system  200  to test sub-unit A in loopback fashion, test data would be introduced to DAC  202 A; DAC  202 A would send data to TX  212 A. The output of TX  212 A would be sent to RX  213 A. The output of RX  213 A would then be sent to ADC  203 A. The data from ADC  203 A would then be examined. Thus, when wireless communication system  200  is configured in this fashion, the elements within the loopback processing chain may function contemporaneously, and one or more of the elements within the first baseband sub-unit  201 A (i.e. DAC  202 A and  203 A) and the first RF sub-unit  211 A (i.e. TX  212 A and RX  213 A) may be tested or calibrated. 
         [0018]    One drawback to the architecture of system  200  is the relatively high inter-unit line count between baseband unit  201  and RF unit  211 . Specifically, consider a typical implementation of wireless communication system  200  in which baseband unit  201  and RF unit  211  are on separate integrated circuits (ICs). In this implementation, each line coupling baseband unit  201  to RF unit  211  may require two pins, i.e. each IC may require one pin to connect to each line. Thus, each differential line pair may require four pins, i.e. each IC may require two pins to connect to each differential line pair. As a result, the twelve differential line pairs coupling baseband unit  201  and RF unit  211  may require twenty-four pins on each IC or forty-eight pins in total. As is well-known, relatively greater amounts of pins can significantly and undesirably increase the cost of an IC package. 
         [0019]    Another drawback is that relatively large numbers of high-speed, differential line pairs, particularly differential traces used to couple baseband unit  201  and RF unit  211 , may be relatively difficult to design. Specifically, differential traces may have relatively more stringent design rules than other, low speed traces. As is well-known, more stringent design rules generally require more design effort than less stringent design rules, such as those that may be required for low speed traces. Therefore, more differential lines pairs generally increase the design effort required to a design wireless communication system. 
         [0020]    Therefore, a need arises to reduce the number of lines between baseband and RF units in a wireless communication system while still retaining the advantages of loopback testing. 
       SUMMARY OF THE INVENTION 
       [0021]    A wireless communication system that advantageously reduces the number of lines between baseband and RF units is provided. This wireless communication system can include a baseband unit, an RF unit, and a plurality of sets of inter-unit lines. The baseband unit can include a plurality of baseband sub-units, wherein each baseband sub-unit can include a digital to analog converter (DAC) and an analog to digital converter (ADC). The RF unit can include a plurality of RF sub-units, wherein each RF unit can include a transmitter (TX) and a receiver (RX). Each set of inter-unit lines can connect a baseband sub-unit and a corresponding RF sub-unit. Moreover, each baseband sub-unit and its corresponding RF sub-unit can form a processing section. Notably, a DAC and a TX of one processing section and an RX and an ADC of another processing section can use the same set of inter-unit lines to communicate. This inter-unit line configuration minimizes the number of lines between the baseband and RF units. 
         [0022]    This wireless communication system can further include a plurality of intra-unit lines for connecting DACs and ADCs of different baseband sub-units and for connecting TXs and RXs of different RF sub-units. These intra-unit lines, along with the above-described sets of inter-unit lines, can advantageously facilitate loopback testing. In one embodiment to further facilitate loopback testing, the DACs and the RXs can have enabled/disabled output terminals. Note that each set of inter-unit lines can include I/Q differential lines or other types of lines. Notably, in one embodiment, the baseband and RF units can be implemented on different integrated circuits (ICs) and the plurality of sets of inter-unit lines can be connected to pads of those ICs. 
         [0023]    Another, more generalized, wireless communication system that reduces the number of lines between baseband and RF units is provided. This wireless communication system can include a baseband unit, an RF unit, and a plurality of sets of inter-unit lines. The baseband unit can include a plurality of baseband sub-units and the RF unit can include a plurality of RF sub-units. Each set of inter-unit lines can connect a baseband sub-unit and a corresponding RF sub-unit. Moreover, each baseband sub-unit and its corresponding RF sub-unit can form a processing section. Advantageously, communication from a first baseband sub-unit to a first RF unit of a first processing section and from a second RF sub-unit to a second baseband sub-unit of a second processing section can share a set of inter-unit lines. 
         [0024]    This generalized wireless communication system can further include a plurality of intra-unit lines for connecting components of different baseband sub-units and for connecting components of different RF sub-units. In one embodiment, certain components of each baseband sub-unit and RF sub-unit can have enabled/disabled output terminals to coordinate sharing of the inter-unit lines. Note that each set of inter-unit lines can include I/Q differential lines or other types of lines. In one embodiment, the baseband and RF units can be implemented on different ICs and the plurality of sets of inter-unit lines can be connected to pads of the first and second ICs. 
         [0025]    A method of communicating in a wireless system having the above-described configurations can include communicating from a first baseband sub-unit to a first RF unit of a first processing section and from a second RF sub-unit to a second baseband sub-unit of a second processing section by sharing a common set of inter-unit lines. In other words, communicating from the first baseband sub-unit to the first RF unit of the first processing section and from the first RF sub-unit to the first baseband sub-unit can be advantageously accomplished using inter-unit lines of different processing sections. To provide the desired loopback configuration, two sets of inter-unit lines, intra-unit lines within the baseband and RF units, and antenna lines of the targeted processing section can be used. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a block diagram illustrating a portion of a prior art wireless communication system. 
           [0027]      FIG. 2  is a block diagram illustrating portion of an exemplary multiple transceiver wireless communication system. 
           [0028]      FIG. 3  is a block diagram illustrating a portion of an exemplary multiple transceiver wireless communication system configured to reduce the number of lines between baseband and RF units. 
           [0029]      FIG. 4  is a block diagram illustrating an exemplary embodiment of a portion of a multiple transceiver wireless communication system configured according to the specification. 
           [0030]      FIG. 5  is a block diagram illustrating another exemplary embodiment of a portion of a multiple transceiver wireless communication system configured according to the specification. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    Many wireless communication systems include baseband and RF units where each baseband and RF unit includes a plurality of baseband and RF sub-units, respectively. It is advantageous to reduce the relative number of inter-unit lines used to couple a baseband sub-unit to an RF sub-unit because as the number of sub-units increase, so may the number of coupling lines. As described above, reducing the relative amount of the inter-unit lines may help ease system design and reduce costs. 
         [0032]    One method to reduce the number of inter-unit lines between a baseband unit and an RF unit configures those units to use the same lines to carry both receive baseband analog signals and transmit baseband analog signals. This is possible because, oftentimes in normal operation, the baseband unit is either processing data to transmit or to receive, but generally not processing transmit and receive data simultaneously. In one embodiment, a DAC and an RX may have outputs that may be controllably enabled. The outputs of the DAC may be enabled when transmitting RF signals and the outputs of the RX may be enabled when receiving RF signals. In some embodiments, configuration may be accomplished by controlling tri-state drives. In other embodiments, switches may selectively couple the active output to the inter-unit line. 
         [0033]      FIG. 3  illustrates a portion of an exemplary multiple transceiver wireless communication system  300  configured to reduce the number of lines between a baseband unit  301  and an RF unit  311 . In system  300 , baseband unit  301  and RF unit  311  are each divided into three sub-units (i.e. A, B, and C). A first baseband sub-unit  301 A includes a first DAC  302 A and a first ADC  303 A, a second baseband sub-unit  301 B includes a second DAC  302 B and a second ADC  303 B, and a third baseband sub-unit  301 C includes a third DAC  302 C and a third ADC  303 C. Similarly, a first RF sub-unit  311 A includes a first TX  312 A and a first RX  313 A, a second RF sub-unit  311 B includes a second TX  312 B and a second RX  313 B, and a third RF sub-unit  311 C includes a third TX  312 C and a third RX  313 C. 
         [0034]    First DAC  301 A is coupled to first TX  212 A through a set of I/Q differential inter-unit lines  315 . Notably, first RX  313 A is coupled to first ADC  302 A through the same set of I/Q differential inter-unit lines  315 . Second and third baseband sub-units are coupled to the second and third RF sub-units, respectively, using similar sets of I/Q differential inter-unit lines  315 . Note that, for simplicity in  FIG. 3  (and subsequent figures), sets of I/Q differential lines have been reduced to a single line). Thus, in this embodiment, each line in the figure (other than the I/Q differential lines in and between baseband sub-unit  301 A and RF sub-unit  311 A) represents I/Q differential lines. Therefore, in this example, four I/Q differential lines couple each baseband sub-unit with its corresponding RF sub-unit. Note that the black circles represent full connections between the lines. 
         [0035]    Advantageously, wireless communication system  300  of  FIG. 3  has relatively few I/Q differential inter-unit lines between baseband unit  301  and RF unit  311  because receive baseband analog signals and transmit baseband analog signals may be carried on the same I/Q differential inter-unit lines. In one embodiment, a DAC and a receiver may have outputs that may be controllably enabled/disabled during actual operation. For example, to transmit an RF signal through first TX  312 A, first DAC  302 A may have its output enabled while the output of first RX  313 A is not enabled. Data from first DAC  302 A may then be provided to first TX  312 A. On the other hand, to receive an RF signal through first RX  313 A, the output of first RX  313 A is enabled while the output of first DAC  302 A is not enabled. Data from first RX  313 A may then be provided to first ADC  303 A. 
         [0036]    Note that while wireless communication systems  200  and  300  ( FIGS. 2 and 3 , respectively) are similarly configured with each system including three baseband sub-units and three RF sub-units, system  200  uses twenty-four I/Q differential inter-unit lines whereas system  300  advantageously uses only twelve I/Q differential lines to couple the baseband unit to the RF unit. One disadvantage, however, of system  300  is that loopback testing of a selected RF unit cannot easily be configured. 
         [0037]    Loopback testing, as described above, configures a loopback processing chain of elements within a baseband sub-unit and an RF sub-unit such that the elements may contemporaneously process test data, thereby enabling testing or calibration of one or more of the elements within the baseband sub-unit and the RF sub-unit. However, because wireless communication system  300  cannot be so configured, loopback testing cannot be implemented. For example, referring to first baseband sub-unit  301 A and first RF sub-unit  311 A, note that the output of first DAC  302 A is coupled to the input of first ADC  303 A. Thus, first TX  312 A and first RX  313 A may not be tested because the test data may pass substantially between first DAC  302 A and first ADC  303 A, thereby bypassing first TX  312 A and first RX  313 A. Because second and third baseband sub-units  301 B/ 301 C and RF sub-units  311 B/ 311 C are similarly configured, those sub-units may not be configured for loopback testing either. 
         [0038]      FIG. 4  illustrates a portion of an exemplary wireless communication system  400  configured to provide loopback testing. Wireless communication system  400  includes a baseband unit  401  and an RF unit  402 , each of which is divided into three sub-units. A first baseband sub-unit  401 A includes a first DAC  402 A and a first ADC  403 A, a second baseband sub-unit  401 B includes a second DAC  402 B and a second ADC  403 B, and a third baseband sub-unit  401 C includes a third DAC  402 C and a third ADC  403 C. A first RF sub-unit  411 A includes a first TX  412 A and a first RX  413 A, a second RF sub-unit  411 B includes a second TX  412 B and a second RX  413 B, and a third RF sub-unit  411 C includes a third TX  412 C and a third RX  413 C. 
         [0039]    As described in further detail below, baseband sub-units  401 A,  401 B, and  401 C include I/Q differential intra-unit lines  416 . RF sub-units  411 A,  411 B, and  411 C similarly include I/Q differential intra-unit lines  416  and antenna lines  430  (note that antenna lines  430  are shown as being on-chip, but could also be implemented off-chip). Using intra-unit lines  416 , first DAC  402 A is coupled to second ADC  403 B, second DAC  402 B is coupled to third ADC  403 C, third DAC  402 C is coupled to first ADC  403 A, first TX  412 A is coupled to second RX  413 B, second TX  412 B is coupled to third RX  413 C, and third TX  412 C is coupled to first RX  413 A. Antenna lines  430  connect antenna  410 A to TX  412 A and RX  413 A, antenna  410 B to TX  412 B and RX  413 B, and antenna  410 C to TX  412 C and RX  413 C. 
         [0040]    In system  400 , each baseband sub-unit can be characterized as having a corresponding RF sub-unit. For example, baseband sub-unit  401 A has a corresponding RF sub-unit  411 A, baseband sub-unit  401 B has a corresponding RF sub-unit  411 B, and baseband sub-unit  401 C has a corresponding RF sub-unit  411 C. Note that in other embodiments, baseband unit  401  and RF unit  402  may include two or more than three sub-units. As used herein, the term “processing section” refers to a baseband sub-unit and its corresponding RF unit. Thus, system  400  includes three processing sections  420 A,  420 B, and  420 C. 
         [0041]    As described in further detail below, each processing section includes a set of I/Q differential inter-unit lines that connect a baseband sub-unit and its corresponding RF sub-unit. For example, processing section  420 A includes a set of I/Q differential inter-unit lines  415 A. Similarly, processing section  420 B includes a set of I/Q differential inter-unit lines  415 B, and processing section  420 C includes a set of I/Q differential inter-unit lines  415 C. 
         [0042]    Notably, in contrast to wireless communication system  300  ( FIG. 3 ), the transmit baseband analog and the receive baseband analog signals associated with a specific processing section in wireless communication system  400  are not carried on the same set of inter-unit lines. For example, first DAC  402 A is coupled to first TX  412 A using inter-unit lines  415 A, whereas first RX  413 A is coupled to first ADC  403 A using inter-unit lines  415 C; second DAC  402 B is coupled to second TX  412 B using inter-unit lines  415 B, whereas second RX  413 B is coupled to second ADC  403 B using inter-unit lines  415 A; and third DAC  402 C is coupled to third TX  413 C using inter-unit lines  415 C, whereas third RX  413 C is coupled to third ADC  403 C using inter-unit lines  415 B. This exemplary arrangement of lines advantageously enables the configuration of one or more loopback processing chains, as is described below in greater detail. 
         [0043]    In one embodiment, the output terminals of the DACs and the RXs may be controllably enabled to allow the transmit baseband analog signal and the receive baseband analog signal to be carried on the same lines. During normal operation, wireless communication system  400  may be configured to transmit and receive RF signals in a manner similar to wireless communication system  300  ( FIG. 3 ). For example, to transmit an RF signal through first TX  402 A, first DAC  402 A may have its output terminal enabled while the output terminal of second RX  413 B is not enabled (noting that first DAC  402 A and second RX  413 B share the same set of I/Q differential inter-unit lines). Data from first DAC  402 A may then be provided to first TX  412 A. On the other hand, to receive an RF signal through second RX  413 B, the output terminal of second RX  413 B is enabled while the output terminal of first DAC  402 A is not enabled. Data from second RX  413 B may then be provided to second ADC  403 B. 
         [0044]    Advantageously, wireless communication system  400  of  FIG. 4  may be configured to enable loopback testing. In one embodiment, a baseband sub-unit may be coupled to an RF sub-unit and the elements within those sub-units may form a loopback processing chain comprised of a DAC, a transmitter, a receiver and an ADC. All elements within the loopback processing chain may function contemporaneously and process test data using two sets of I/Q differential inter-unit lines, I/Q different intra-unit lines extending across two or more sub-units, and the antenna lines of the targeted processing section. 
         [0045]    For example, first baseband sub-unit  401 A and first RF sub-unit  411 A (i.e. processing section  420 A) may be configured for loopback testing. In this case, a loopback processing chain may be configured that includes first DAC  402 A, first TX  412 A, first RX  413 A, and first ADC  403 A. First DAC  402 A is coupled to first TX  412 A (using a first set of I/Q differential inter-unit lines). TX  412 A is coupled to first RX  413 A using antenna lines  430  of RF sub-unit  411 A. Notably, RX  413 A is coupled to first ADC  403 A using I/Q differential intra-unit lines in RF sub-units  411 A,  411 B, and  411 C, a second set of I/Q differential inter-unit lines, and I/Q differential intra-unit lines in baseband sub-units  401 A,  401 B, and  401 C (see dotted line  417  showing total connected path). 
         [0046]    Loopback testing is possible within system  400  because all elements within a loopback processing chain may function contemporaneously and may be coupled together in a manner that permits loopback testing. That is, in our example, the set of I/Q differential inter-unit lines coupling first DAC  402 A to first TX  412 A are separate from the set of I/Q differential inter-unit lines coupling first RX  413 A to first ADC  403 A. Therefore, the output of first DAC  402 A and first RX  413 A may both be enabled, thereby allowing the test data to be processed by the loopback processing chain. System  400  is configured such that the other baseband and RF sub-units shown in  FIG. 4  may advantageously be tested in a similar manner. 
         [0047]    Thus, wireless communication system  400  of  FIG. 4  advantageously minimizes the number of inter-unit lines required to couple baseband unit  401  to RF unit  411 . Relatively fewer inter-unit lines may reduce the cost of the IC package including system  400  because there are relatively fewer pins required to connect to the inter-unit lines. Design costs may also be reduced because fewer inter-unit lines, such as high bandwidth I/Q differential lines, may have to be designed to couple the baseband sub-units to the RF sub-units. The described line configuration of system  400  also advantageously enables loopback testing. Thus, portions of system  400  may be tested or calibrated without the need to transmit or receive data to or from a second wireless system. 
         [0048]    In the exemplary wireless communication system of  FIG. 4 , the baseband sub-units include one DAC and one ADC. In other embodiments, a baseband sub-unit may include two or more DACs and two or more ADCs. Similarly, in other embodiments an RF sub-unit may include two or more transmitters and two or more receivers (i.e. two or more transceivers). 
         [0049]    Exemplary wireless communication system  400  of  FIG. 4  illustrates one embodiment of a wireless communication system that may be configured to reduce the number of inter-unit lines while enabling loopback testing. In other embodiments, the elements in the RF unit and the baseband unit may be coupled together in a different manner. For example, first DAC  402 A may be coupled to third ADC  403 C and first TX  412 A may be coupled to third RX  413 C. In this case, a loopback processing chain may still be configured with DAC  402 A, TX  412 A, RX  413 A and ADC  403 A. 
         [0050]    Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying figures, it is to be understood that the invention is not limited to those precise embodiments. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. As such, many modifications and variations will be apparent. For example, although  FIG. 4  illustrates DACs  402  being coupled to TXs  412  using fewer I/Q differential intra-unit lines compared to those used to couple RXs  413  and ADCs  403 ,  FIG. 5  illustrates a wireless communication system  500  that reverses this configuration, i.e. DACs  402  being coupled to TXs  412  using more I/Q differential intra-unit lines compared to those used to couple RXs  413  and ADCs  403 . Moreover, note that in other embodiments, the components of the baseband unit may be connected to the components of the RF unit with non-differential lines, such as single-ended lines or the like. In still other embodiments, the number of baseband sub-units may differ from the number of RF sub-units. Accordingly, it is intended that the scope of the invention be defined by the following Claims and their equivalents.