Abstract:
A mobile communications device includes at least two receivers and a memory module. The power consumption of the mobile communications device is controlled by selectively awakening and sleeping the receivers as needed to ensure that the desired quality of reception to check the control channel for messages from an associated base station and to handle incoming communications is maintained. The mobile communications device may operate in single receiver mode or dual receiver diversity mode. In the dual receiver diversity mode, the mobile communications device may switch between one of a number of diversity combining techniques based on the quality of reception.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to mobile communications devices for use in analog and digital-based cellular communication systems, and, in particular, to a method for conserving power in such mobile communications devices using two receivers for improved diversity reception. 
     Mobile communications devices or terminals are becoming ubiquitous in modem society. Typical mobile communications devices include pagers, personal digital assistants, mobile phones, and the like, however, all such devices shall be referred to herein as mobile terminals. One of the primary concerns of consumers is directed to the size and weight of the mobile terminals. Many people carry two or more mobile communications devices, such as a pager and mobile phone. If they have to carry a mobile terminal, they want it to be as small and unobtrusive as possible. As a result, there is increasing pressure to shrink the mobile terminal and its internal components. 
     A second concern exists about the battery capacity of the mobile terminals, and especially the operating time that the battery provides before needing to be recharged. However, this concern is directly at odds with the desire to miniaturize the mobile terminals. Batteries power the myriad of functions available on the mobile terminal and are critical to their operation. While advances have been made in the size and weight of the battery, pressures to make the mobile terminals smaller and lighter have frequently exceeded the ability of the battery designers to produce a battery which supplies the needed power for the desired long interval between recharging while fitting within the condensed mobile terminal. 
     Therefore, it has become increasingly imperative that the manufacturers of the mobile terminals improve on ways to conserve energy within the mobile terminals. One method to conserve battery power is to put the mobile terminal into a “sleep” mode. During sleep mode, the mobile terminal is effectively on standby mode waiting for incoming communication. During a sleep period, the receiver in the mobile terminal is not powered so that there is less drain on the battery. A low-power timer in the mobile terminal periodically “wakes up” the receiver to process the control channel from the associated base station to determine if there are any incoming messages for the mobile terminal. If not, then the mobile terminal returns to sleep mode. If so, then the mobile terminal remains awake and processes the incoming message according to the instructions of the control channel from the associated base station. 
     Sleep mode is different from merely powering a mobile terminal off in that sleep mode presupposes waking up periodically to check the control channel. In contrast, turning off may allow for repowering, but it does not presuppose returning to an active state periodically to check the control channel. Two examples of sleeping techniques are seen in U.S. Pat. Nos. 5,224,152 and 5,568,513, which are incorporated by reference. 
     A third concern is quality of reception. A common problem that occurs in the radio transmission of signals is that the signals are sometimes lost or distorted as a result of multipath fading and interference. One known method of reducing interference and multipath fading is to use a plurality of antennas, and more preferably a plurality of receivers. That is, the mobile terminal includes two receiver circuits, which are then utilized together using interference cancellation or other known performance-enhancing techniques to provide a clear audio signal for the user in the case of a voice call and improved data throughput in the case of a data call. This two receiver methodology is also in direct conflict with both size reduction and battery conservation. The additional circuitry adds a drain on the battery, thereby increasing the frequency of recharging. 
     With the above concerns in mind, there exists a need in the mobile communications industry to provide a mobile communications device which has the benefits of improved power conservation and the two receiver diversity. 
     SUMMARY OF THE INVENTION 
     The present invention meets this need by providing a mobile communications device with at least two receivers and a memory. The power consumption of the mobile communications device is controlled by selectively awakening and sleeping the receivers as needed to ensure the desired quality of reception to check the control channel for messages from an associated base station and to handle incoming communications. A plurality of diversity combining techniques are used to improve the performance of the receivers in the mobile communications device. The performance of the receivers is evaluated using the Carrier-to-Interference ratio (C/I), Bit-Error-Ratio (BER), Frame Erasure Rate (FER), Received Signal Strength Indicator (RSSI), soft information confidence, voice quality or a related performance measure or some combination of these performance measurements. 
     In the preferred embodiment, during periodic waking periods, the mobile communications device checks the individual receivers to see if they can provide adequate reception before relying on both receivers and one of the plurality of diversity combining techniques. The device increments through the diversity combining techniques in order of required power consumption until a desired quality of reception is achieved. Conversely, if reception is adequate, the device checks to see if a lower level of power consumption is possible during the periodic waking periods. This is accomplished by determining whether or not the diversity combining technique can be decremented or scaled back to a single receiver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a mobile terminal according to the present invention; 
     FIG. 2 is a state diagram illustrating the various operating modes of themobile terminal of FIG. 1; 
     FIG. 3 is a flow chart illustrating the operation of the mobile terminal of FIG. 1; 
     FIGS. 4 and 5 are a flow chart illustrating the procedure for selecting an operating mode for the mobile terminal of FIG. 1; 
     FIG. 6 is a flow chart of an alternate procedure for selecting an operating mode for the mobile terminal of FIG. 1; and 
     FIG. 7 is a flow chart illustrating a procedure for monitoring the performance of the mobile terminal of FIG. 1 during the course of a call and changing operating modes. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While adaptable to a myriad of mobile communications devices or terminals such as pagers, personal digital assistants and the like, the present invention is particularly well suited for use in a cellular telephone. 
     FIG. 1 is a block diagram of a mobile terminal  10  made according to the present invention. Mobile terminal  10  includes control logic  12 , memory  14 , a transceiver section  16 , a baseband processing section  18 , a pair of diversity antennas  20 ,  22 , and a switching unit  30 . Control logic  12  controls the operation of the mobile terminal  10  according to program instructions stored in memory  14 . Control logic  12  may comprise a single dedicated processor or, alternately, may comprise multiple processors, each performing different control functions. Memory  14  represents the hierarchy of memory normally present in a mobile terminal  10 . Memory  14  stores the operating system programs and data used by the mobile terminal  10  to control operation of the mobile terminal  10 . Memory  14  may also store application programs and user data. 
     Transceiver section  16  comprises a first receiver  24 , a second receiver  26 , and a transmitter  28 . Receivers  24 ,  26  are connected to respective antennas  20 ,  22  via switching unit  30 . The receivers  24 ,  26  can be operated independently of one another in a single receiver mode or, alternately, may be operated together in a dual receiver diversity mode. The receivers  24 ,  26  receive electromagnetic waves representative of a signal on a control channel via the antennas  20 ,  22 . The receivers  24 ,  26  process the received electromagnetic waves and extract the signal in a conventional manner. In dual receiver diversity mode, the receivers  24 ,  26  process the received electromagnetic waves concurrently and extract the signal in a redundant manner such that the same signal is extracted separately by each receiver  24 ,  26 . It will be appreciated by those skilled in the art that the signal may comprise any desired information, such as voice or data information. The transmitter  28  is operatively connected to antenna  20  and operates in a conventional manner. 
     The baseband section  18  is responsible for processing signals for transmission by the transmitter  28  and for processing signals received by receivers  24 ,  26 . The baseband section  18  is also responsible for any diversity combining performed by the mobile terminal  10 . In contrast to the prior art, the baseband section  18  is programmed to use a variety of diversity-combining techniques and may switch between different diversity techniques as needed to ensure an acceptable signal quality while consuming the least possible amount of power. 
     Switching unit  30  operatively connects receivers  24 ,  26  to antennas  20 ,  22 , respectively. Switching unit  30  comprises a pair of switches  32 ,  34 . Control logic  12  selectively actuates switches  32 ,  34 , and receivers  24 ,  26  to select the desired operating mode as will be described in more detail below. In general, control logic  12  selects the operating mode that is the least power consumptive and ensures a minimum received quality standard. 
     There are presently several techniques by which two receivers can be combined to provide better reception. These are all loosely called diversity combining techniques, and include techniques such as Equal Gain Combining (EGC), Maximal Ratio Combining (MRC), Interference Rejection Combining (IRC) and the like. MRC involves more computations than EGC and is thus more power consumptive. IRC is a refinement of the MRC technique. While allowing better quality of reception, IRC is more power consumptive than MRC. A mobile terminal that uses IRC is described in U.S. Pat. No. 5,680,419, which is hereby incorporated by reference. 
     Presently, mobile terminals are only programmed to use one diversity combining technique and cannot switch between them as needed or desired to conserve power. In the present invention, the mobile terminal  10  is able to switch between single receiver mode and dual receiver diversity mode as needed. Further, in dual receiver diversity mode the mobile terminal  10  can choose between a variety of different diversity combining techniques depending on the quality of the received signal. The receivers  24 ,  26  are configured to measure the signal quality of the electromagnetic wave and generate respective signal quality indicators representative of the signal quality of the electromagnetic wave, and hence, the signal, received by each receiver  24 ,  26 . 
     The particular operating mode selected by control logic  12  is based on signal quality measurements. Appropriate signal quality measurements include Carrier-to-Interference Ratio (C/I), Bit Error Ratio (BER), Frame Erasure Rate (FER), Received Signal Strength Indicator (RSSI), some other similar performance measure, or some combination of these factors. Based on some predetermined decision criteria, the control logic  12  selects either single receiver mode or dual receiver diversity mode. In essence, the mobile terminal  10  selects single receiver mode when the desired signal quality criteria can be met using either one of the receivers  24 ,  26 . If neither receiver  24 ,  26  provides the desired signal quality, then dual receiver diversity mode is selected. In dual receiver diversity mode, the control logic  12  can select between a variety of different diversity-combining techniques. Initially, the mobile terminal  10  selects the diversity technique that is least consumptive from a power standpoint. During a call, the mobile terminal  10  continues to monitor the signal quality and can change operating modes, or change the diversity technique being used in order to minimize power consumption. 
     The process of changing operating modes or diversity techniques is shown in the state diagram seen in FIG.  2 . In FIG. 2, the circles represent the different operating modes. The least power consumptive mode, represented by reference character S, is the single receiver mode. The most consumptive mode which yields the best quality, respresented by reference character  1 , is diversity combining mode using IRC. The intermediate state, represented by reference character D, is diversity combining mode using EGC or MRC. 
     In operation, the mobile terminal  10  goes into a sleep mode or standby mode when it is not actively involved in a call. In sleep mode, both receivers  24 ,  26  and transmitter  28  are powered off. The mobile terminal  10  periodically wakes up and monitors the control channel to determine if it is being paged or needs to perform some other task. When the mobile terminal  10  wakes, it turns on one or both receivers  24 ,  26  as described more fully below. Mobile terminal  10  initially selects the operating mode that was last used and begins to process the control channel. The mobile terminal  10  will then determine whether a different operating mode is required based on measurements of the received signal. In general, the mobile terminal  10  will move to a more power consumptive mode if the received signal quality is unsatisfactory and to a less consumptive mode if possible to conserve power provided that a predetermined minimum signal quality standards can be met. The majority of the time, the mobile terminal  10  will process the control channel and discover that there are no incoming calls or other tasks to perform and return to sleep mode. Occasionally, however, there will be an incoming call or some other task to perform. Mobile terminal  10  will receive the call using the operating mode that was required to process the control channel. At the end of the call, the mobile terminal  10  stores the last used settings and returns to sleep mode. In particular, a flag mode is set in memory  14  to indicate the mode last used by the mobile terminal (e.g., dual-receiver diversity vs. single receiver). Also, a primary receiver flag is set in memory  14  to indicate which of the receivers  24 ,  26  is designated as the primary receiver. 
     FIG. 3 is a flow diagram illustrating the operation of the mobile terminal  10 . It is assumed that the mobile terminal  10  is initially in sleep mode, i.e., meaning that both receivers  24 ,  26  are deactivated. While in sleep mode, mobile terminal  10  periodically checks to determine whether it is time to awaken (block  105 ). If not, the mobile terminal  10  remains in sleep mode. An internal timer may indicate that it is time to awaken. When it is time to awaken, mobile terminal  10  activates one or both receivers  24 ,  26  and receives the control channel. The initial operating mode will be the last used mode as indicated by the mode flag in memory  14 . While receiving the control channel, the mobile terminal  10  will select an operating mode based on the received signal on the control channel (block  110 ). The possible operating modes include single receiver mode and dual receiver diversity modes. 
     In single receiver mode, the mobile terminal  10  can receive signals on either of its two receivers  24 ,  26 . In dual receiver diversity mode, mobile terminal  10  preferably has a choice of two or more diversity-combining techniques, such as EGC, MRC, and IRC. The initial operating mode is either single receiver mode or dual receiver diversity mode, with the least power consumptive technique being preferred. 
     The majority of the time, the mobile terminal  10  will process the control channel, using the selected operating mode, discover that there are no incoming calls (block  115 ) or other tasks to perform, and return to sleep. In that case, the mobile terminal  10  stores the last used operating mode in memory and returns to sleep mode (block  125 ). The mobile terminal  10  also notes which of the receivers  24 ,  26  is designated as the primary receiver by setting the primary receiver flag. Occasionally, however, there will be an incoming call or some other task to perform, such as neighbor list scanning. If so, the mobile terminal  10  begins to receive the call (block  120 ) using the operating mode selected while the control channel was being received. At the end of the call, the mobile terminal  10  stores the last used settings and returns to sleep mode (block  125 ). 
     During a call, the mobile terminal  10  may continuously monitor received signal quality and determine whether a different operating mode is required based on measurements of the received signal. In general, if the mobile terminal  10  determines that the signal quality is unsatisfactory, the mobile terminal  10  may move to a more power consumptive operating mode. If the mobile terminal  10  determines that the received signal quality is satisfactory and a less consumptive mode is possible to conserve power, the mobile terminal  10  may switch to a less consumptive operating mode, provided that a predetermined minimum signal quality standards can be met in the new mode. 
     FIGS. 4 and 5 illustrate an exemplary procedure for selecting the operating mode. The procedure shown in FIGS. 4 and 5 correspond to block  110  in FIG.  3 . When the mobile terminal  10  awakens, the mobile terminal  10  checks the mode flag in memory  14  to see if both receivers  24 ,  26  were needed in the previous awake cycle (block  205 ). As a default, when the mobile terminal  10  is powered up the first time, the mode flag is set to indicate single receiver mode and the first receiver  24  is set as the primary receiver. Where only one receiver was used in the last awake cycle, the mobile terminal  10  awakens the primary receiver and receives the control channel (block  210 ). The primary receiver may be either the first receiver  24  or the second receiver  26  as indicated by the primary receiver flag in memory  14 . While receiving the control channel, the mobile terminal  10  checks to see if the performance of the primary receiver alone is adequate to ensure the desired quality of reception (block  215 ). If performance is acceptable, the mobile terminal  10  remains in single receiver mode with the primary receiver used to receive the control channel (block  235 ). The control information is processed conventionally in single receiver mode (block  240 ). The secondary receiver, which likewise may be either the first receiver  24  or the second receiver  26 , is left in sleep mode, i.e., turned off. 
     If the signal quality for the currently designated primary receiver is inadequate (block  215 ), the mobile terminal  10  awakens (i.e. turns on) the secondary receiver and receives the control channel with both receivers (block  220 ). While receiving the control channel, the mobile terminal  10  determines if the performance of the secondary receiver alone is adequate to provide the desired quality of reception (block  225 ). If the secondary receiver can provide the desired signal quality, the mobile terminal  10  deactivates the primary receiver and the former secondary receiver is designated as the new primary receiver (block  230 ). The original primary receiver (now asleep) is designated as the secondary receiver. The primary receiver flag is set in memory  14  to indicate this reversal of designation. After switching the designations, the control channel is received with the new primary receiver (block  235 ) and the control channel is processed conventionally for mobile terminals with one receiver (block  240 ). If the secondary receiver cannot by itself provide the desired signal quality, the mobile terminal  10  receives the control channel with both receivers and switches to dual receiver diversity mode (block  245 ). The received signals are then processed (block  240 ) using diversity combining techniques. 
     Turning now to FIG. 5, if the last used mode when the mobile terminal awakens is dual receiver diversity mode, the mobile terminal  10  initially awakens both receivers (block  250 ), and receives the control channel on both receivers (block  255 ). While receiving the control channel on both receivers, the mobile terminal  10  determines whether either of the receivers  24 ,  26  alone is adequate (blocks  260 ,  270 ). Preferably, the mobile terminal  10  checks the primary receiver first (block  260 ). If the primary receiver alone is sufficient, the mobile terminal  10  deactivates the secondary receiver and changes the mode flag to note that dual receiver diversity mode is not needed (block  265 ). The control channel is then received conventionally using just the primary receiver and processed (blocks  235 ,  240  in FIG.  4 ). 
     If the primary receiver does not provide adequate signal quality, the mobile terminal  10  determines whether the secondary receiver alone is adequate (block  270 ). If not, the mobile terminal  10  continues to receive the call in diversity mode using both receivers  24 ,  26  (block  245 , FIG.  4 ). In the preferred embodiment the control channel is processed using the MRC technique, however other diversity combining techniques such as IRC could be used. 
     If the secondary receiver is sufficient to provide the desired signal quality, the primary receiver is deactivated and the mobile terminal  10  changes the primary receiver flag in memory  14  (block  275 ). The original secondary receiver becomes the new primary receiver and the original primary receiver (now already asleep) becomes the new secondary receiver. The mobile terminal  10  sets the primary receiver flag so that the appropriate receiver  24  or  26  is designated the primary receiver. The mobile terminal  10  also changes the mode flag to note that both receivers are not needed. The mobile terminal  10  then receives the control channel with the new primary receiver conventionally (block  235 , FIG. 4) and processes the control channel in single receiver mode (block  240 , FIG.  4 ). 
     A second implementation of the present invention is seen in FIG.  6 . In contrast to the method of FIGS. 4 and 5, where the mobile terminal  10  wakens the receiver(s) last needed, the second implementation routinely wakes up both receivers and deactivates one if possible. This improves the likelihood that the control channel is successfully processed, but at the expense of greater initial power consumption. Specifically, the mobile terminal  10  determines whether it is time to awaken (block  305 ). If not, then the mobile terminal checks periodically until it is time to awaken. This may be accomplished through the use of an internal timer. When it is time to awaken, the mobile terminal  10  activates both receivers and begins receiving the control channel (block  310 ) using one of the diversity combining techniques discussed earlier. While the mobile terminal  10  is receiving with both receivers, the mobile terminal  10  is simultaneously testing to see if one receiver alone is adequate. Specifically, the mobile terminal  10  first checks whether the primary receiver alone is adequate (block  315 ). If so, the mobile terminal  10  deactivates the secondary receiver and changes the mode flag (block  320 ). The mobile terminal  10  then receives the control channel with just the primary receiver (block  325 ). The control information is processed conventionally (block  345 ) and the mobile terminal  10  returns to sleep (block  350 ) after completion of the processing. 
     If the primary receiver is inadequate by itself, the mobile terminal  10  determines whether the performance of the secondary receiver alone is adequate (block  330 ). If so, the mobile terminal  10  puts the primary receiver to sleep (block  335 ). The mobile terminal  10  then reverses the designations of the primary receiver and the secondary receiver and sets the primary receiver flag to indicate which receiver is the primary receiver. The new primary receiver continues to receive the control channel (block  325 ) and processes the information conventionally (block  345 ). After completion of the processing, the mobile terminal  10  returns to sleep (block  350 ) to restart the process. 
     If neither receiver  24 ,  26  itself is adequate, the mobile terminal  10  continues to receive the control channel with both receivers in dual receiver diversity mode (block  340 ). After the control channel is processed (block  345 ), the mobile terminal  10  returns to sleep mode (block  350 ). 
     As already noted, when a call is received the mobile terminal  10  may continue to monitor performance of both receivers  24 ,  26  while it is receiving the call. In particular, while operating in dual receiver diversity mode, the mobile terminal  10  continuously monitors the received signal to determine if single receiver mode is acceptable. If so, the mobile terminal  10  switches from dual receiver diversity mode to single receiver mode. This monitoring procedure is illustrated in FIG.  7 . The mobile terminal  10  is assumed to be receiving in dual receiver diversity mode initially (block  400 ). Performance of the primary and secondary receivers is periodically checked (blocks  410 ,  425 ). Initially, the performance of the primary receiver alone is checked to see if the primary receiver alone is adequate (block  410 ). If so, then the secondary receiver is put to sleep mode (block  415 ) and reception continues using just the primary receiver (block  420 ). If not, the performance of the secondary receiver is checked to see if it is adequate (block  425 ). If so, then the primary receiver is put to sleep (block  430 ) and reception continues on the secondary receiver (block  435 ). As described above, the designations are reversed and reception continues until sleep mode is entered again. If neither receiver  24 ,  26  is sufficient, reception continues using a simple diversity combining technique such as MRC or EGC (block  440 ). However, in contrast to the previously described systems, the performance of the two-receiver reception is evaluated (block  445 ). If the performance is adequate, then the process recycles and continues to try to sleep one receiver to conserve power as described above on the hope that the reception improves. However, if performance is not adequate, then the control function increments to a more power consumptive diversity technique such as interference rejection combining (IRC) (block  460 ) to process the incoming signal. The switch to IRC is a last resort activity to improve performance because IRC technique involves increased computational calculations resulting in faster battery drain. If performance is adequate, the mobile terminal  10  may also determine whether a less power consumptive diversity technique can be used (block  455 ). If so, the mobile terminal  10  decrements the diversity technique (block  455 ). 
     The process described above repeats continuously while looking for an opportunity to decrement to a lower power consuming technique, either by deactivating one of the receivers or reverting back to EGC or MRC. Furthermore, it is believed that EGC consumes less power than MRC, although at the expense of being less adaptable to changing path interference, resulting in a less robust performance. Thus, it is within the scope of the present invention to initially use EGC as the diversity combining technique, and upon discovery that the performance is inadequate, increment the processing technique to MRC. If MRC was determined to be inadequate, then IRC is implemented. 
     The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the spirit and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.