Abstract:
Systems and techniques are disclosed relating to communications. The systems and techniques involve transmitting a signal through a first antenna, receiving feedback related to the signal transmission through the first antenna, selecting between the first antenna and a second antenna as a function of the feedback, and transmitting the signal through the selected antenna. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or the meaning of the claims.

Description:
BACKGROUND 
   1. Field 
   The present invention relates generally to communications, and more specifically, to communication devices with switched antenna transmit diversity. 
   2. Background 
   In wireless communications, transmitted signals are reflected and scattered by obstacles in their path, often resulting in multiple copies of the signal arriving at the receiver at different times. Depending on the location of the receiving antenna relative to the transmitting antenna, and the obstacles in the signal path, the multiple copies of the signal may combine constructively or destructively at the receiving antenna. In narrow band mobile applications, this phenomenon may cause fluctuations in the signal when the user travels even a small distance. This is often referred to as fast fading. The use of a wide band code division multiple access (CDMA) signal may significantly reduce the impact of fast fading. CDMA is a modulation and multiple access scheme based on spread-spectrum communications which is well known in the art. 
   Another technique to mitigate fast fading in mobile applications is to use multiple antennas to increase the gain of the signal due to spatial diversity of the antennas. Currently, there are a number of commercially available mobile devices with dual antenna arrangements. However, these mobile devices employ spatial diversity combining techniques for the received signal only, using a single antenna to transmit. In these mobile devices, it would be advantageous to employ a methodology that uses both antennas to achieve transmit antenna diversity. 
   SUMMARY 
   In one aspect of the present invention, a method of communications includes transmitting a signal through a first antenna, receiving feedback related to the signal transmission, selecting between the first antenna and a second antenna as a function of the feedback, and transmitting the signal through the selected antenna. 
   In another aspect of the present invention, a communications apparatus configured to transmit a signal to a remote source includes first and second antennas, an antenna selection module responsive to feedback from the remote source, the feedback being related to the signal transmission, and a transmitter selectively coupled between the first and second antennas under control of the antenna selection module. 
   In yet another aspect of the present invention, computer readable media embodying a program of instructions executable by a computer program performs a method of transmitting a signal to a remote source, the method including receiving feedback relating to the signal transmission, selecting between the first antenna and a second antenna as a function of the feedback, and generating a signal to couple a transmitter to the selected antenna. 
   In a further aspect of the present invention, a communications apparatus configured to transmit a signal to a remote source includes first and second antennas, means for selecting between the first and second antennas as a function of feedback received from the remote source, the feedback being related to the signal transmission, a transmitter, and means for coupling the transmitter to the selected antenna. 
   It is understood that other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only exemplary embodiments of the invention by way of illustration. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Aspects of the present invention are illustrated by way of example, and not by way of limitation, in the accompanying drawings wherein: 
       FIG. 1  is a conceptual block diagram of an exemplary CDMA communications system; 
       FIG. 2  is a simplified functional block diagram of an exemplary base station supporting a power control loop; 
       FIG. 3  is a simplified functional block diagram of an exemplary subscriber station with switched transmit antenna diversity; and 
       FIG. 4  is a flow chart illustrating an exemplary procedure implemented by an antenna selection module for switching a transmitter between two antennas. 
   

   DETAILED DESCRIPTION 
   The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practiced. The term “exemplary” used throughout this description means “serving as an example, instance, or illustration,” and should not necessarily be construed as preferred or advantageous over other embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form in order to avoid obscuring the concepts of the present invention. 
   In the following detailed description, various techniques will be described in the context of a CDMA communications system. While these techniques may be well suited for use in this environment, those skilled in the art will readily appreciate that these techniques are likewise applicable to other wireless networks. Accordingly, any reference to a CDMA communications system is intended only to illustrate various inventive aspects of the present invention, with the understanding that these inventive aspects have a wide range of applications. 
     FIG. 1  is a conceptual block diagram of an exemplary CDMA communications system. A base station controller  102  may be used to provide an interface between a network  106  and all base stations dispersed throughout a geographic region, such as Base Station (BS)  104 . The geographic region is generally divided into smaller regions known as cells. Each cell typically includes a base station capable of serving all subscriber stations in that cell. In more densely populated regions, the cell may be divided into sectors with a base station serving each sector. For ease of explanation, only one base station  104  is shown. A subscriber station  108  may access the network  106 , or communicate with other subscriber stations, through one or more base stations under control of the base station controller  102 . 
   The base station  104  may be equipped with any number of antennas depending on the particular application and overall design constraints. In the CDMA communications system shown in  FIG. 1 , the base station  104  includes a transmit antenna  110  and two receive antennas  112 A and  112 B. The two receive antennas  112 A and  112 B may be used by the base station  104  to receive a signal transmission from the subscriber station  108 . This approach increases the gain of the signal transmission due to the spatial diversity of the receive antennas  112 A and  112 B and the combining techniques employed by the base station  104 . The transmit and receive antennas  110 ,  112 A,  112 B may be spatially separated individual radiating elements such as dipoles, open-ended waveguides, slots cut in waveguides, or any other type of radiating elements. 
   The subscriber station  108  is shown with a dual antenna arrangement; however, as those skilled in the art will appreciate the subscriber station  108  may be configured with any number of antennas. The two antennas  114 A and  114 B may be embedded in the subscriber station  108 . This approach enhances the aesthetics of the subscriber station as well as provides increased user convenience by eliminating the need to deploy the antennas during use. Alternatively, the two antennas  114 A and  114 B may be whips, helices, or any other type of radiating elements. 
   In the exemplary embodiment shown in  FIG. 1 , the two antennas  114 A and  114 B may be used to provide spatial diversity for the received signal on the forward link transmission. The forward link refers to a signal transmission from the base station  104  to the subscriber station  108 . The two antennas  114 A and  114 B may also be used to support switched transmit diversity on the reverse link. The reverse link refers to a signal transmission from the subscriber station  108  to the base station  104 . Switched diversity may be affected by optimally switching the reverse link signal transmission between the two antennas  114 A and  114 B in accordance with a control procedure. 
   The control procedure may be implemented in various ways depending on the particular application, overall design constraints, and/or other relevant factors. In at least one embodiment of the subscriber station, feedback from the base station  104  may be used to optimally switch the signal transmission between the two antennas  114 A and  114 B. The feedback may take on various forms, but should generally provide some indication of the reverse link signal quality. The feedback may be any reverse link quality metric computed at the base station and fed back to the subscriber station over an air traffic or overhead channel. Examples of reverse link quality metrics include the bit energy-to-noise density (E b /I o ), the bit error rate (BER), the frame error rate (FER), the carrier-to-interference ratio (C/I), or any other known parameter. Alternatively, the subscriber station may utilize existing feedback loops in conventional CDMA communication systems. By way of example, a power control loop used by conventional subscriber stations to control the power of the reverse link transmission may be used to control the switching of the antennas. 
     FIG. 2  is a simplified functional block diagram of an exemplary base station  104  supporting a power control loop. The base station  104  includes two receive antennas  112 A and  112 B coupled to a receiver  202 . The receiver  202  includes various high frequency and signal processing components, however, only those components pertinent to the inventive concepts described throughout this disclosure will be discussed. An analog front end  204  may be used to amplify, filter and downconvert the signals received by the antennas  112 A and  112 B to baseband signals. The baseband signals from the antennas  112 A and  112 B may be separately demodulated and then combined with a rake receiver (not shown) in a demodulator  206 . A decoder  208  may be used to de-interleave and decode the combined signal from the demodulator  206 . 
   The demodulator  206  may also be used to generate a received signal strength indicator (RSSI) for the reverse link transmission from the combined signal. The RSSI may be provided to a power control module  210  where it may be compared to a power set point to produce a power control signal. The power control signal may be used as a feedback signal by the subscriber station to increase the reverse link transmission power if the RSSI is less than the power set point, and decrease the reverse link power if the RSSI is more than the power set point. Because the power set point is typically determined from the FER of the decoded signal, it has a direct correlation to the reverse link signal quality. Accordingly, the power control signal is a good choice for controlling the switching of the antennas at the subscriber station during reverse link signal transmissions. 
   The power set point is a threshold value against which the measured signal strength, specifically RSSI in the present embodiment, is compared. Alternate embodiments may use alternate measures of signal strength or signal quality and employ an alternate threshold metric. In one embodiment, multiple thresholds are used to determine increases and/or decreases in transmit power on the reverse link. For example, the use of one threshold to determine decreases in transmit power and the use of a different lower threshold to determine increases in transmit power. Another example may incorporate multiple ranges, wherein the ranges are associated with control information regarding the transmit power adjustment. In this way, if the measured RSSI is within a given range, such range indicates an increase in transmit power by a predetermined amount. Other ranges may indicate other adjustment amounts. Similarly, the control values of each range may be dynamically adjusted based on the current values of another range. Historical information may determine changes in the control values, such as changes in the threshold values and/or ranges, as well as changes in the associated control decisions. 
   The power control signal may be provided to a transmitter  212 . The transmitter  212  includes various high frequency and signal processing components, however, only those components pertinent to the inventive concepts described throughout this disclosure will be discussed. A puncture element  214  in the transmitter  212  may be used to puncture the power control signal from the power control module  210  into a traffic channel or overhead channel. The traffic or overhead channel from the puncture element  214  may then be provided to a modulator  216  before being upconverted to a carrier frequency, filtered and amplified by an analog front end  218  for transmission on the forward link via the transmit antenna  110 . 
     FIG. 3  is a simplified functional block diagram of an exemplary subscriber station  108  with switched transmit antenna diversity. As explained earlier, the base station  104  uses two receive antennas  112 A and  112 B for reverse link reception, and therefore, achieves gain due to spatial diversity of the antennas  112 A,  112 B and the combining techniques utilized by the rake receiver (not shown). By employing switched transmit antenna diversity at the subscriber station  108  further improvements in reverse link signal quality may be achieved. 
   The subscriber station  108  includes a receiver  302  and transmitter  304  which share the same two transmit/receive antennas  114 A and  114 B. A separate duplexer  306 A and  306 B may be used to connect both the receiver  302  and the transmitter  304  to each transmit/receive antenna  114 A and  114 B. The duplexers  306 A and  306 B prevent transmitter leakage from desensitizing or damaging the receiver  302  while at the same time ensuring weak signals received by the transmit/receive antennas  114 A and  114 B are directed to the receiver  302 . The receiver  302  may be coupled to both of the transmit/receive antennas  114 A and  114 B, and the transmitter  304  may be switched between the transmit/receive antennas  114 A and  114 B using a microwave switch  308  or similar device. A high intercept point microwave switch with good linearity may be used to reduce out-of-band emissions during high power transmissions. Both the receiver  302  and the transmitter  304  include various high frequency and signal processing components, however, only those components pertinent to the inventive concepts described throughout this disclosure will be discussed. 
   An analog front end  310  in the receiver  302  may be used to amplify, filter and downconvert the signals received by the transmit/receive antennas  114 A and  114 B to baseband signals. The baseband signals from the analog front end  310  may be separately demodulated and then combined with a rake receiver (not shown) in a demodulator  312 . The power control signal may then be extracted from the combined signal and provided to an antenna selection module  314 . In a manner to be described in greater detail later, the antenna selection module  314  may use the power control signal to control the switching of the transmitter  304  between the transmit/receive antennas  114 A,  114 B via duplexers  306 A and  306 B. 
   The power control signal may also be provided to the transmitter  304  to control the reverse link transmission power. In the exemplary embodiment shown in  FIG. 3 , a modulated signal may be provided to an analog front end  316  for filtering and upconversion to a carrier frequency suitable for transmission over the reverse link. A power amplifier (not shown) in the analog front end  316  of transmitter  304  may be used to generate a high power transmission. The power control signal from the demodulator  312  of receiver  302  may be provided to the power amplifier of analog front end  316  to control the reverse link signal gain. The reverse link signal transmission from the power amplifier of analog front end  316  may be switched to the appropriate transmit/receive antenna through the microwave switch  308  under control of the antenna selection module  314 . 
   The antenna selection module  314  may be embodied in software capable of being executed on a general purpose processor, specific application processor, or in any other software execution environment. The software may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk, a CD-ROM, or any other storage medium known in the art. Alternatively, the antenna selection module  314  may be embodied in hardware or in any combination of hardware and software. By way of example, the antenna selection module  314  may be an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, any combination thereof, or any other equivalent or nonequivalent structure designed to perform one or more of the described functions. 
   The purpose of the antenna selection module  314  in at least one embodiment of the subscriber station is to select an antenna for transmission that will result in the best reverse link signal quality. Because of signal fluctuations experienced by the base station  104  as the subscriber station  108  travels through the cellular region (or sector), the antenna capable of the best reverse link signal quality will vary with time. The antenna selection module  314  may use feedback from the base station  104  to select the transmit/receive antenna with the best reverse link signal quality. The actual procedure used to select the transmit/receive antenna may vary depending on a variety of factors such as cost and performance tradeoffs as well as other design constraints. In at least one embodiment of the antenna selection module  314 , the power control signal may be used to gain insight into the reverse link signal quality. More specifically, if the power control signal indicates that the base station  104  is requesting more power, the antenna selection module  314  may switch the transmitter  304  to the other antenna  114 A,  114 B and monitor whether a decrease in power is requested by the base station  104  through the power control signal. A request for less power indicates that the reverse link signal quality from this selected antenna is better. This procedure may be continued for the duration of the call. 
     FIG. 4  is a flow chart illustrating in more detail this exemplary procedure implemented by the antenna selection module for switching the transmitter  304  between the two antennas  114 A,  114 B. Initially, a first transmit antenna is selected in step  402 . The initial selection of the first transmit antenna may be random, or may based on some other criteria. Next, a signal is generated in step  404  that may be used to connect the transmitter to the first transmit antenna. At this point, the subscriber station  108  is ready to transmit. In step  406 , the antenna selection module  314  extracts feedback from the base station  104  relating to the reverse link transmission. The feedback in this case is the power control signal. If the antenna selection module  314  determines in step  408  that the power control signal is requesting a decrease in the reverse link power, then the signal used to connect the transmitter  304  to the first transmit antenna is maintained in step  410 , and the antenna selection module  314  extracts the next power control signal in step  406 . 
   Returning to step  408 , if the antenna selection module determines that the power control signal is requesting an increase in the reverse link power, then the antenna selection module  314  generates a signal in step  412  to connect the transmitter to a second transmit/receive antenna. Once the transmitter is connected to the second transmit/receive antenna, the subscriber station is ready to transmit. In step  414 , the antenna selection module  314  extracts the next power control signal relating to the reverse link transmission. If the antenna selection module  314  determines in step  416  that the power control signal is requesting a decrease in the reverse link power, then the signal used to connect the transmitter to the second transmit antenna is maintained in step  418 , and the antenna selection module  314  extracts the next power control signal in step  414 . Conversely, if the antenna selection module  314  determines in step  416  that the power control signal is requesting an increase in the reverse link transmission power, then the antenna selection module  314  generates a signal in step  404  to connect the transmitter  304  to the first transmit antenna. 
   The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
   The methods or algorithms described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor may read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal. 
   The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.