Patent Publication Number: US-7590390-B2

Title: Method and apparatus for signal power loss reduction in RF communication systems

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/217,699, entitled “Method And Apparatus For Signal Power Loss Reduction In RF Communication Systems” filed Aug. 13, 2002. 
    
    
     COPYRIGHT NOTICE 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to reduction of signal power loss in communications systems. The invention is more particularly related to the reduction of signal power loss in wireless RF systems. In addition, the invention relates to implementing antenna diversity in wireless systems. 
     2. Discussion of Background 
     The use of multiple antennas has become a popular method of improving performance of wireless devices. A simple technique is called “switch diversity”, where a radio can alternatively transmit and receive through separate antennas, using only one antenna at a time. The radio can then decide which antenna to use based upon the success of previous communications.  FIG. 1  is a diagram of a traditional “switch diversity” architecture. A pair of antennas, Antenna A and Antenna B (diversity antennas  100 ) are located at diverse physical locations and/or are antennas having diverse physical properties (gain, directivity, etc.). The diversity antennas  100  are connected to a diversity switch  110  that connects the antennas to a transmit receive switch  120 . When transmitting, RF Mixers/Baseband device  140  feeds a signal to a power amplifier  132  in RF front end  130 . The transmit receive switch  120  directs the amplified signal to the diversity switch  110  which directs the amplified transmit signal to a selected one of the diversity antennas. When receiving a reverse signal flow occurs, except that the received signal is boosted by an LNA  134  prior to being received by the Mixers/Baseband device  140 . 
     The diversity switch  110  is controlled by software or other electronics that selects one of the diversity antennas for reception/transmission. Selection criteria is typically based on quality of signal, S/N ratio, and/or other identifiers, such as packet reception errors, etc. For example, a typical arrangement would call for the baseband and mixers  140  to include some processing or algorithm which activate a control signal to perform the switching. The processing would include receiving packets on each of the antennas for a length of time (or number of packets), and then compare the number of packet errors received by each of the antennas. The antenna with the least number of errors or the highest S/N ratio is then selected for broadcast/reception. Once an antenna is selected, transmission/reception continues on the selected antenna. Periodically, the other antenna(s) are re-tested. In the event a re-test indicates an environment change or other factor is degrading performance of the selected antenna compared to the other antenna(s), the selected antenna is changed to the then best performing antenna. 
     SUMMARY OF THE INVENTION 
     The present inventors have realized various inefficiencies, particularly signal losses, that occur through standard diversity and transmit/receive architectures. The present inventors have developed certain improvements in wireless signal reception, particularly when applied to antenna diversity architectures, and transmit/receive architectures. One problem recognized is the loss incurred through the diversity and transmit/receive switches. The switches attenuate the RF signal and introduce noise, which degrades the performance of the PA and LNA when measured from the antenna port. 
     The present invention provides an architecture, method, and device wherein redundancies at the front end of an RF device can reduce losses that occur at the diversity switch. In highly integrated radio systems, silicon area is inexpensive and the cost of redundancy is low. The losses incurred by the diversity switches can be reduced by duplicating the number of PAs/LNAs as well as moving the antenna diversity switches from between the PA/LNA and antenna to after the PAs/LNAs. 
     In one embodiment the present invention is an RF front end, comprising, at least one power amplifier configured to transmit signals to at least two antenna ports, and at least two LNA devices, each LNA device configured to receive signals from one of the antenna ports. The invention may also be embodied in an RF device, comprising, at least two antenna ports, an RF front end having at least two PA amplifiers, each PA amplifier having an output coupled to one of the antenna ports, an RF signal generating device, and an antenna diversity switch coupled to the RF signal generating device and the RF front end, wherein said antenna diversity switch is configured to direct RF signals generated by the RF signal generating device to one of the PA amplifiers. 
     The present invention may also be embodied as a method of transmitting RF signals, comprising the steps of, preparing an RF signal for transmission, selecting an antenna best suited for transmitting the prepared RF signals from a set of at least two antennas, and feeding the prepared RF signals to a power amplifier coupled to the selected antenna; and/or a method of receiving RF signals, comprising the steps of, determining at least one antenna best suited for receiving RF signals, and switching a signal receiving line to an output of an LNA coupled to one of the best suited antennas. 
     Portions of both the device and method may be conveniently implemented in programming, data sequences, and/or control signals executed or generated on a general purpose computer. Any components of the present invention represented in a computer program, data sequences, and/or control signals may be embodied as an electronic signal broadcast (or transmitted) at any frequency in any medium including, but not limited to, wireless broadcasts, and transmissions over copper wire(s), fiber optic cable(s), and co-ax cable(s), etc. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a block diagram of a traditional switch diversity antenna architecture; 
         FIG. 2  is a block diagram of a redundant front end architecture in a diversity antenna device according to an embodiment of the present invention; 
         FIG. 3  is a block diagram of a half redundant front end in a diversity antenna device according to an embodiment of the present invention; 
         FIG. 4  is a block diagram of a beam forming architecture using multiple integrated PA and LNA blocks according to an embodiment of the present invention; 
         FIG. 5  is a circuit diagram of a conventional implementation of an on-chip LNA; 
         FIG. 6  is a circuit diagram of an example implementation of an on-chip LNA having switching pairs for steering current according to an embodiment of the present invention; 
         FIG. 7  is a circuit diagram of an example implementation of an on-chip amplifier that enables beam-forming according to an embodiment of the present invention; 
         FIG. 8  is a circuit diagram of an example implementation of a beam-forming PA according to an embodiment of the present invention; 
         FIG. 9  is a graph of example antenna radiation patterns resulting from the circuit of  FIG. 8 ; 
         FIG. 10  is a prior art implementation of a switchable power combiner using parallel shunt PA devices; 
         FIG. 11  is a block diagram of a transmitter PA and a receiving LNA operating with shunt switches in conjunction with a single antenna according to an embodiment of the present invention; 
         FIG. 12  is an example implementation of a PA with a built-in shunt switch according to an embodiment of the present invention; 
         FIG. 13  provides an example implementation of beam forming by signal phase shifting according to an embodiment of the present invention; 
         FIG. 14A  is a radiation diagram for a pair of antennas configured without the additional mixers shown in  FIG. 13 ; 
         FIG. 14B  is a radiation diagram illustrating the jB combinations according to the example implementation of  FIG. 13 ; 
         FIG. 14C  is an effective radiation diagram combining A+-jb radiation patterns of  FIG. 14B  with the radiation patterns of  FIG. 14A ; 
         FIG. 15  is an example circuit implementation of the LNAs and 1st combiner from  FIG. 13  according to an embodiment of the present invention; and 
         FIG. 16  is a circuit diagram of an example implementation of the mixers M 1 , M 2 , and adder of  FIG. 13  according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring again to the drawings, wherein like reference numerals designate identical or corresponding parts, and more particularly to  FIG. 2  thereof, there is illustrated a block diagram of diversity antenna according to an embodiment of the present invention. The illustrated embodiment provides a framework from which signal losses incurred due to a diversity switch can be reduced. 
     The signal affected most by inefficiencies, and therefore presenting the greatest design challenge is the signal as it is transmitted to and received from the antenna. The signal is received, for example, at antenna A, and is then directed, by transmit/receive switch  205 , to the LNA  220 . In the previous designs, a diversity switch was also included in the signal path from the antennas to the LNA. However, in the invention, RF front end  200  has an architecture that uses redundant front end components that eliminates the need for a diversity switch between the front end and the antennas. 
     In  FIG. 2 , the baseband and mixers provide a signal to be transmitted. The signal to be transmitted is amplified as required to counteract any losses that occur at a diversity switch  240 . The diversity switch  240  sends the signals to be transmitted to one of a first PA  215  and a second PA  225 . The diversity switch  240  is set to send the signal to be transmitted to PA  215  when Antenna A is the selected antenna for transmissions. Reversing that process, when receiving signals, and antenna A is the selected receiving antenna, signals received at antenna A are routed through LNA  220 , where they are amplified, and then sent through diversity switch  240  to the baseband and mixers  250 . When the selected antenna is antenna B. similar processes occur except that the transmit signals are routed through PA  225  and received signals are routed through LNA  230 . 
     Therefore, redundant architecture of RF front-end  200  implements a switch diversity scheme without the need for a diversity switch between the RF front-end and the antennas, improving the system performance. The diversity switching function is accomplished in a less performance critical section of the radio. Additional power usage is averted because the unused half of the RF front-end can be powered down (i.e., when transmitting/receiving on antenna A, PA  225  and LNA  230  are powered down). 
     If the PA and LNA are integrated parts of a larger radio-on-a-chip, the incremental cost of the silicon area to duplicate the PA and LNA layouts is relatively small. The primary cost of this approach is the need to multiply the number of package pins required for the PA and LNA ports by the number of diversity antennas being implemented. The transmit/receive switches  205  and  210  can either be integrated or off-chip. 
     This architecture could also be used with discrete components for the PAs and LNAs, but the cost of discrete components could be prohibitive because of the need to buy multiple PA and LNA components. If discrete external components are necessary due to performance constraints, a modified version of the redundant diversity architecture allows for the use of a single external component. 
       FIG. 3  is a block diagram of a half redundant front end  300  in a diversity antenna device according to an embodiment of the present invention. The half redundant front end  300  is attached to diversity switches  340 . The diversity switches  340  direct incoming signals from either LNA  305  or LNA  310 , each on-chip devices, to the baseband and mixers  250 . Signal to be transmitted, from the baseband and mixers  250  are routed to an external Power Amplifier (PA)  315 . The external PA then powers one of the antennas according to setting of switches  320  and  325 . 
     The half-redundant architecture of RF front-end  300  duplicates the layout of the on-chip components, mainly the LNA, where the incremental cost is small, while avoiding the cost of duplicating external components. In the example above, an external PA and a redundant LNA is used, but a similar architecture with a redundant PA and an external LNA could be used. 
     Using the half-redundant architecture, the performance of the external component may be compromised because of having to drive two switches (e.g., switches  320  and  325 ). The finite isolation of the switch that is off will allow some fraction of power to be diverted from the intended path. However, the use of an external component may provide enough power margin to overcome this loss. In the example above, the performance of the external PA may be compromised, but the loss from the antenna to the LNAs is still lower than a traditional architecture with a diversity switch between the RF front-end and the antennas. 
     In another embodiment, multiple LNA and/or PA devices are implemented on-chip, one dedicated to each antenna. More than one of these devices can operate at a given time, enabling the simultaneous use of multiple antennas, or equivalently the use of one multi-element antenna. 
       FIG. 4  is a block diagram of a beam forming architecture using multiple integrated PA and LNA blocks according to an embodiment of the present invention. RF front end  400  includes a power amplifier (PA) (e.g., PAs  405  and  410 ) and an LNA (e.g., LNAs  415  and  420 ) for each antenna. Although the figures presented herein mainly show two antennas, any number of antennas may be utilized. In the beam forming embodiment, for example, three, four, or more antennas may be utilized, each antenna having an additional set of LNA and PA that is preferably on-chip. 
     Each PA is shown feeding a transmit receive switch and a corresponding antenna. For example, PA  410  feeds transmit/receive switch  430  and antenna A, and PA  405  feeds transmit/receive switch  435  and antenna B. 
     An input signal to be transmitted (from baseband and mixers  460 ) is split to feed each of the PAs. In a component based implementation a splitter  441  is utilized to split the signal. In an integrated implementation a shared line may be utilized. 
     One of the PAs (e.g.,  405 / 410 ) is operational when the device is being operated in the single antenna mode, and both PAs are operational when transmitting in a beam forming or multiple antenna mode. Again, other PAs and corresponding antennas may be utilized and included in the beam forming or multi antenna element modes, or any one or more of the PAs and corresponding antennas may be shut down when the corresponding antenna is either being used for receiving or not utilized. Any number of combinations of transmitting, receiving, and/or unutilized PA and antenna combinations is possible to match any system requirements. 
     Each LNA is fed a received signal from one of the antennas, through a corresponding transmit/receive switch. Outputs of the LNAs are combined (at adder  450 ) when the device is in a multi antenna element configuration. Any one or more LNA outputs may be excluded from the adder operation if that LNA/antenna are idle, or, when that LNA/antenna are being used in a transmit capacity. 
     Thus, in the embodiment of  FIG. 4 , the on-chip diversity switches discussed previously are replaced by appropriate signal combiners (e.g., adder  450 ) at the outputs of the LNAs and signal splitters which provide the inputs to the PAs. The properties and advantages of multi-element antennas are well documented in literature. By adjusting the phase with which the signals are combined from or split between the multiple antennas, the radiation diagram of the antennas (beam forming) can be adjusted and significantly improve the antenna gain towards the desired direction, with respect with what is achieved with a single antenna. The adjustment may be made by trial and error or based on an algorithm that tries different phase adjustments and extrapolates the results of the different phase adjustments to a best phase from which to combine the signals with.  FIG. 4  provides a preferred configuration. Blocks A /1 , A /2 , A p1 , A p2 ( 442 - 448 ) are programmable phase shifters. Using appropriate circuit techniques, these blocks can be implemented in a way that does not incur an appreciable cost in terms of performance degradation, power consumption, or silicon area. The implementation is facilitated if the phase shifters are capable to implement only a finite set of signal phase rotation (for example, 0°, 180°, and 90°) as opposed to implementing an arbitrary signal phase rotation which is also realizable. The finite number of phase rotations reduces performance with respect to an arbitrary phase rotation, but by only a small amount and reduces complexity and cost. The use of multiple LNAs and PAs does come at the price of increased power consumption, but it is possible that lower performance and lower power circuit blocks can be used and still benefit from the new architecture. Because the performance improvement provided by this technique might be enough to eliminate an external LNA or PA, this technique can be used to provide significant system power and form factor reduction. 
     To illustrate the benefit of this configuration in an integrated receiver consider, for example, that the signal is being received with the same power from the two antennas and its phase is adjusted in the blocks All and A /2  such that the two signals add in phase. The signal level is then 6 dB higher than in the case that a single antenna is used. The noise received by the two antennas and introduced by the transmit/receive switches and all the circuitry in front of the combiners is uncorrelated in the two paths and the output of the combiner has 3 dB higher noise level than the LNA output in the single antenna case. Therefore, the SNR improvement is 3 dB. In addition, since the signal level is higher at the combiner output than in the single active LNA case, the noise contribution of the rest of the receiver chain (mixers and baseband) is lower. 
     On the transmitter side, consider an example where the two PAs are fed with equal amplitude signals, shifted in phase appropriately by blocks A p1 , and A p2  such that the outputs of the two antennas add constructively in phase at the desired direction. Then the transmitted signal is 6 dB higher than in the single antenna case assuming that the input signal to the single PA is equal to the signal to each one of the multiple PAs. Since the amount of transmitted power at the desired direction is usually defined by the system requirements, we can reduce the signal power fed to each PA, with a significant linearity benefit with respect to the single PA case. Since each PA needs to transmit only the fourth of the power of the single PA case, the power consumption in the two PAs can be significantly lower than twice the power consumption in the single PA case. 
     As stated before, the on-chip diversity switches, as well as the signal combiners and splitters can be implemented on-chip in various efficient ways. 
       FIG. 5  is a circuit diagram of a conventional implementation of an on-chip LNA. LNA  500  includes Cascode devices (e.g.,  506 / 508 ) provide for reverse isolation and stability. The dual outputs  510 A/ 510 B, and dual inputs  505 A and  505 B indicate a differential design. Bias and other details are omitted to enhance clarity. As with all the circuit diagrams presented herein, values of specific components shown, if any, are merely exemplary. 
     Diversity, as discussed with reference to  FIGS. 2 and 3 , can be achieved by using two of the LNAs described in  FIG. 5  (or another LNA design) with a common pair of load inductors (e.g.,  622 / 624 —which are used for tuning the frequency to which the device is sensitive) as shown in  FIG. 6 . The gate voltages of the cascode devices can be controlled by logic signals control A and control B to turn off the LNA that needs to be disconnected (e.g., LNA  600  or LNA  602 ). For example, if LNA  600  was to be used, then Control B would be grounded, and the four transistors on the right, that constitute LNA  602 , would effectively be eliminated. Preferably, LNA  600  is connected to a first antenna, and LNA  602  is connected to a second antenna. And, with the redundant amplifiers, the antennas are more directly connected to the amplifiers. 
     An implementation consistent with diversity as implemented in  FIG. 6  has an additional advantage in that much less physical space is taken up than a design that simply made redundant LNAs. In the redundant LNA design of  FIG. 5 , the inductors (e.g.,  522 / 524 ) take up more physical space than other parts of the circuit, therefore the redundant LNA uses more die space and has greater manufacturing cost. Since the inductors take up a large percentage of space even for a single LNA, a design consistent with  FIG. 6  can be implemented on a die space nearly equivalent to the die space of a single LNA, but it is a dual LNA implementation. 
       FIG. 7  is a circuit diagram of an example implementation of an on-chip amplifier that enables beam-forming. The amplifier may be, for example, an LNA or a PA pre-driver. As illustrated in  FIG. 7 , diversity as well as beam forming, as discussed with reference to  FIG. 4 , can be implemented by replacing the cascode devices with switching pairs (e.g. switching pairs  710 ,  720 ,  730 , and  740 ) which can steer the current to the one or the other load inductor. By controlling the gates of the transistors of the switching pairs with logic signals (A 1 p, A 1 n, A 2 n, A 2 p, B 1 n, B 1 p, B 1 p, B 1 n) we can obtain the signal from either antenna A or antenna B alone, the sum of antenna A and antenna B, or the difference between antenna A and antenna B. By adding additional circuits, these properties may be propagated to triple, quad, or more antenna arrays. 
     For example, the current-steering amplifier topology of  FIG. 7  can be used either as an LNA or as a PA driver, depending on how the inputs  711 ,  721 ,  731 , and  741  and outputs  750  and  752  are connected. Control Logic signals are attached to gates on switching pairs  710 ,  720 ,  730  , and  740  (A 1 p, A 1 n, A 2 n, A 2 p, B 1 n, B 1 p, B 1 p, B 1 n). For LNA: antennas are attached to gates at inputs  711 ,  721 ,  731 , and  741 ; and receive mixers are connected to outputs  750  and  752 . For PA driver: transmit mixers are attached to gates at inputs  711 ,  721 ,  731 , and  741 ; and antennas are attached to outputs  750  and  752 . 
     A notable variation between  FIG. 6  and  FIG. 7  is that  FIG. 6  provides two control (logic) signals so that either amplifier A (e.g.,  600 ) is on or amplifier B (e.g.,  602 ) is on. In  FIG. 7 , the switching pairs allow a summing operation. By manipulating the logic signals, the amplifiers may be configured only antenna A, only antenna B, A+B, or A−B. Now, since the antennas are to be located in different places, radiation patterns having peaks and valleys are formed (e.g., see  FIG. 9 ). Depending on where a signal that a radio is trying to listen to, or, in the case of a transmitter, where a transmitter is trying to transmit to, there will be an optimum combination that provides the best communications link. 
     For example, in terms of receiver, depending on where the transmitting station is, the receiver may get the clearest reception by subtracting the two signals received from the different antennas, or by adding them. In one embodiment, the present invention includes a processing device that tries different combinations of A, B, A+B, A−B, etc., and then determines which combination/single antenna gives the best reception, and then that combination is used for the remaining transmission. In one embodiment, the selected antenna combination is periodically updated. 
     The penalty introduced by the implementation of  FIG. 7  is the small gain reduction of each LNA with respect to the single antenna implementation, because of the parasitic capacitance introduced by all the cascode devices and the lower load inductor value required to resonate it. However, the benefits are considerable and, in most applications, easily justify this small gain reduction 
     In one embodiment, not shown in  FIG. 7 , to mitigate the gain reduction penalty, the switching pairs of one of the two LNAs are replaced with single cascode devices, since it suffices to be able to change the sign of the signal of one of the two LNAs only. Some asymmetry between the two paths may be introduced, but that disadvantage may be outweighed by the mitigating effects. 
     An implementation of a redundant PA to implement switch diversity would be to duplicate the PA layout, drive the PAs in parallel, and power down the amplifier driving the unused antenna. Implementing beam steering requires more complexity compared to the LNA case, as current steering devices in the output stage would reduce the amount of voltage that can be delivered to the antenna which in turn would reduce the PA efficiency.  FIG. 8  proposes an alternate transmitter implementation where two sets of current steering drivers (similar to  FIG. 7 )  805  and  810  are used to drive two output stages  815  and  820 . The control bits ( 807  and  812 ) to the drivers direct the TX signal paths to be driven in phase or complementary, or alternatively, the control bits can power down one PA or the other. 
       FIG. 9  is a graph of example antenna radiation patterns resulting from the circuit of  FIG. 8 . In  FIG. 9 , the radiation pattern is shown for a single antenna, pattern  910  (e.g., antenna A or antenna B transmitting alone). The difference of antenna A and antenna B is shown as patterns  920 A and  920 B, and the sum of antenna A and antenna B is shown as patterns  930 A and  930 B. 
     The phase shifting techniques presented above in  FIG. 6  and  FIG. 7  is configured to shift the signal 0 or 180 degrees. However, referring back to  FIG. 4 , an implementation with programmable phase shifters can implement additional, or programmable phase shifts of the antenna radiation pattern. Other phase shifting techniques may also be utilized. 
     For example, the present invention provides a higher degree of phase shift programmability by replicating a larger part of the receive and transmit chains instead of the LNAs and/or PAs only. Phase shift by 90 degrees can be achieved by shifting a Local Oscillator (LO) signal that drives a mixer by 90 degrees. The I and Q components of the LO signal are often available on-chip and are utilized in this manner to implement a signal phase shift by 90 degrees.  FIG. 13  provides an example implementation of received signal phase shifting by 0°, 90°, and 180° according to an embodiment of the present invention. Antenna A and antenna B each feed received signals to corresponding LNAs  1310  and  1320 . The amplified signals are combined by a first combiner  1330  to produce combined signals x and y, which are described as:
         (x,y),{(A, 0 ),(B, 0 ),(A+B, 0 ),(−A+B, 0 ),(A,B),(−A,B)}
 
Therefore, x can be either A, −A, B, A+B, or −A+B, and y can be B or 0. Mixer  1  multiplies x with a signal LO I , (I component of a local oscillator), and mixer  2  multiplies y with a signal LO Q  (Q component of the local oscillator). The LO I , and LO Q  signals are 90° out of phase. Adder  1350  then combines each of the x and y phase shifted signals to a combined phase shifted signal z. The signal z is described as:
 
z∈{A,B,+/−A+B,+/−A+jB}, where j 2 =−1, and
 
it is also noted that the signal jB represents the B amplified signal rotated by 90°. Furthermore, in this implementation, it is also noted that Mixer M 2  only needs to be powered on when it is desired to obtain z=+/−A+jB because all the other combinations are obtainable via combiner  1330 .
       

     Although  FIG. 13  is directed to receiving signals, the same basic architecture described in  FIG. 13  may be applied to a transmit chain where signals are combined and phase shifted prior to broadcast on the antennas. 
       FIG. 14A  is a radiation diagram for a pair of antennas configured without the additional mixers shown in  FIG. 13 . The radiation diagram is based on assumptions of antenna omnidirectivity and that a distance between antennas A and B is ½ of a wavelength. A pair of lobes  1410  A and  1410  B illustrate radiation patterns of antennas A and B (and, conversely, reception sensitivity when receiving signals) for the case of A+B. Lobes  1420  A and  1420  B illustrate radiation patterns of antennas for the case of A−B. The bold line highlights the outer edges of each of the lobes. The present invention includes programming or other logic that recognizes directivity needed for a signal to be broadcast and directivity strength in received signals. The programming or other logic also selects the most advantageous combination of, +/−A, B, jB, etc., for broadcast or received signals based on that directivity. Therefore, the bold line illustrates the effective radiation pattern of the antenna because the programming and logic allows the best characteristics of each radiation pattern to be taken advantage of. 
     By adding more combinations of antenna patterns and including additional programming and or logic to select the additional antenna patterns when needed, the effective radiation pattern of the antennas is further increased.  FIG. 14B  is a radiation diagram illustrating the jB combinations (B rotated 90°) according to the example implementation of  FIG. 13 . Lobe  1440  illustrates the A−jB radiation patterns, and lobe  1450  illustrates the A+jB radiation pattern. These additional radiation patterns are added to those illustrated in  FIG. 14A , resulting in an effective radiation pattern as shown by the bold line in  FIG. 14C . 
       FIG. 15  provides an example circuit implementation of the LNAs and 1st combiner from  FIG. 13 . Differential amplifier LNA A amplifies signals A(+) and A(−), and LNA B amplifies signals B(+) and B(−). Logic signals S 1 , S 2 , S 3 , and S 4  control whether LNA amplified signals A(+), A(−), B(+), or B(−) are applied individually or in combination to differential output signals x(+), x(−), and y(+), y(−). For example, to get an (x, y)=((A−B),  0 ) from the combiner, logic signals are applied so that S 1 =1, S 2 =0, S 3 =1, and S 4 =0, therefore, the x differential outputs each have +/−A and−/+B applied to them, resulting in an A−B signal. Table 1 provides a logic chart identifying control signals and outputs for  FIG. 15 : 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 S 1   
                 S 2   
                 S 3   
                 S 4   
                 x(+) 
                 x(−) 
                 y(+) 
                 y(−) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 A 
                 0 
                 1 
                 0 
                 1 
                 A(+) 
                 A(−) 
                 B(+) 
                 B(−) 
               
               
                 A − B 
                 1 
                 0 
                 1 
                 0 
                 A(−) + B(+) 
                 A(+) + B(−) 
                 0 
                 0 
               
               
                 −A 
                 1 
                 0 
                 0 
                 1 
                 A(−) 
                 A(+) 
                 B(+) 
                 B(−) 
               
               
                 A + B 
                 0 
                 1 
                 1 
                 0 
                 A(+) + B(+) 
                 A(−) + B(−) 
                 0 
                 0 
               
               
                 B 
                 0 
                 0 
                 1 
                 0 
                 B(+) 
                 B(−) 
                 0 
                 0 
               
               
                   
               
            
           
         
       
     
       FIG. 16  is a circuit diagram of an example implementation of the mixer  1 , mixer  2 , and adder of  FIG. 13 . The topology follows a Gilbert Cell topology, but applied for use in the present invention. Switches S 5  and S 6  are closed to shut down mixer  2  under the conditions described above. 
     The present invention also provides an implementation that eliminates off-chip receive/transmit switches. Off-chip diversity and Receive-Transmit switches are usually implemented as series or shunt switches. Switches are often implemented with diodes or FET transistors. Open series switches and closed shunt switches are ideally lossless terminations and can be used to reflect the signal power and direct it to a desired device. The present invention includes the implementation of on-chip series or shunt switches as a way to eliminate the off-chip devices. Such an implementation saves the cost of the external devices, and lowers the system power consumption in the case that the on-chip devices replace external diode switches, since diodes consume DC power when they are on. More significantly, they can potentially reduce the front-end signal power loss. 
     The on-chip switches, implemented with FET transistors are not ideal. Their on resistance is finite, while the lossy capacitance of the source and the drain represent some finite resistance to the substrate at high frequency. Parasitic series resistance of on switches is less harmful in terms of power loss when the impedance of the block to which the switch is connected in series is high. Parasitic shunt resistance is less harmful when the impedance of the block to which the switch is connected in series or in shunt is low. 
     The input of the LNA is usually very sensitive to any lossy components connected to the gate of the input device. If a switch isn&#39;t used at the LNA input, when powered down, the LNA possibly represents only a small load to the PA, relative to the load represented by the lossy drain region of the large PA output transistors. However, a good on-chip shunt switch at the PA output can be implemented by using the PA output devices, without the need for new switch devices, which will introduce more losses. Therefore, while in concept this invention would require two on-chip shunt switches, in practice only the PA shunt switch may be needed. However, an advantage may still exist when including both the PA and LNA shunt switches in a design. For example, large amounts of power transmitted by the PA may have enough power leakage to the LNA that the LNA may be in danger of being damaged. In this case, the LNA shunt switch would help prevent damage. 
       FIG. 10  is a prior art implementation of switchable power combiner using parallel devices. Each of a dual PA arrangement includes a shunt switch that grounds the unused PA output. This basic idea can be extended to transmit/receive switches. 
     The present invention leverages from the switchable power combiner in  FIG. 10  to provide an improved transmit/receive switch arrangement for associating a PA or a LNA to an antenna.  FIG. 11  is a block diagram of a transmitter PA  1100  and a receiving LNA  1105  operating without external switches in conjunction with a single antenna  1110 . Each of the components are coupled to a ¼ wavelength wires or equivalent networks  1115  and  1120 . And, no grounding switch for the LNA  1105  is needed. 
       FIG. 11  exploits a property of ¼ wavelength wires that a short circuit on one end of the wire appears as an open circuit at the other end. In the circuit shown, during receive, the shunt switch  1102  on the transmit (PA) side is shorted to ground. This causes the other end of wire  1120  to appear as an open circuit, and thus the received signal is coupled from the antenna  1110  to the LNA  1105  with a minimum of loss onto the transmit circuitry. Thus, the shunt switch  1102  is opened during transmit and closed during receiving. The shunt switch  1102  is operated via circuitry and/or programming. 
     The same principle applies during transmit mode, when signals coupled from the PA  1100  to the antenna  1110  with a minimum of loss into the receive circuitry. In concept, this is accomplished by placing a shorted switch  1107  on the LNA side of wire  1115 . In practice, the present inventors have found that the loss of signal power during transmit is minimal, and may not require switch  1107  at the LNA input. This is due to the relatively small input capacitance of the LNA (in this implementation). Also, since the presence of a switch at the LNA input would degrade the receiver performance (even when the switch is open), the preferred implementation of  FIG. 11  uses only the single shunt switch  1102  at the output of the PA  1110 . 
     Individuals skilled in the art of RF design can replace the ¼ wavelength lines with equivalent lump element circuits or matching networks. 
       FIG. 12  is an example implementation of a PA with a built-in shunt switch according to an embodiment of the present invention. With reference to  FIG. 12 , the following operations are performed: 
     During Transmit, the PA output device delivers power to the antenna Rfout  1210 . Switch (A) is closed so that the DC current is provided to the PA output device  1200 . Bias is set at a DC voltage appropriate for amplifying the input RF signal. 
     During Receive, the shunt switch is active, and no DC power should be consumed. Switch (A) is open to disconnect DC power from the PA output device. Bias is connected to VDD so that the PA output device  1200  forms a shunt-switch to ground. Note that switch (A) can either be placed between the inductor and PA device  1200  as shown in  FIG. 12  or between the inductor and supply VDD. 
     Although the present invention has been described herein with reference to diversity antennas A and B, any number of antennas may be accommodated by adding additional circuits and/or other hardware as described herein, which will be apparent to the ordinarily skilled artisan based upon review of the present disclosure. 
     The present invention is intended to be applicable to any range of frequencies and numerous antenna combinations. In one embodiment, an RF front end according to the present invention is configured for IEEE 802.11a wireless communications. In another embodiment, the RF front end is configured for 802.11b wireless communications. In yet another embodiment, diversity antennas are utilized in an 802.11a and 802.11b combined radio device. Either the antennas themselves are dual band antennas, or the one or more of the antennas attached to the RF front end are specifically for 802.11a communications and one or more other of the antennas attached to the RF front end device are specifically for 802.11b communications. Again, however, any combination protocols or broadcast frequencies may be supported by the devices and processes elaborated herein. 
     In describing preferred embodiments of the present invention illustrated in the drawings, specific terminology (e.g., component values, transistor types, differential design, etc.) is employed for the sake of clarity. However, the present invention is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical and design equivalents which operate in a similar manner. For example, when describing power amplifier and combination of electrical components, including, but not limited to, transistors, resistors, capacitors, etc. may be employed in making that part, and, any other device having an equivalent function or capability, whether or not listed herein, may be substituted therewith. Furthermore, the inventors recognize that newly developed technologies not now known may also be substituted for the described parts and still not depart from the spirit and scope of the present invention. All other described items, including, but not limited to LNAs, Splitters, combiners, switches, and antennas, etc should also be consider in light of any and all available equivalents. 
     Portions of the present invention may be conveniently implemented using a conventional general purpose or a specialized digital computer or microprocessor programmed according to the teachings of the present disclosure, as will be apparent to those skilled in the computer art. 
     Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will be apparent to those skilled in the software art. The invention may also be implemented by the preparation of application specific integrated circuits or by interconnecting an appropriate network of conventional component circuits, as will be readily apparent to those skilled in the art based on the present disclosure. 
     The present invention includes a computer program product which is a storage medium (media) having instructions stored thereon/in which can be used to control, or cause, a computer to perform any one or more processes of the present invention. The storage medium can include, but is not limited to, any type of disk including floppy disks, mini disks (MD&#39;s), optical discs, DVD, CD-ROMS, micro-drive, and magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, DRAMs, VRAMs, flash memory devices (including flash cards), magnetic or optical cards, nanosystems (including molecular memory ICs), RAID devices, remote data storage/archive/warehousing, or any type of media or device suitable for storing instructions and/or data. 
     Stored on any one of the computer readable medium (media), the present invention includes software for controlling both the hardware of the general purpose/specialized computer or microprocessor, and for enabling the computer or microprocessor to interact with a human user or other mechanism utilizing the results of the present invention. Such software may include, but is not limited to, device drivers, operating systems, and user applications. Ultimately, such computer readable media further includes software for performing the present invention, as described above. 
     Included in the programming (software) of the general/specialized computer or microprocessor are software modules for implementing the teachings of the present invention, including, but not limited to, setting of switches, (e.g., diversity switches, transmit/receive switches, etc.), setting PA amplification levels, packet testing, signal strength evaluation, software mixing, combining, or other functions to implement any aspect of the present invention. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.