Abstract:
A transmitting device ( 100 ) transmits a signal on a forward channel ( 106 ) at a predetermined power level. Upon receipt of the signal, a receiving device ( 102 ), estimates a signal quality for the signal. If the signal quality is below a threshold, the receiving device transmits a power control message on at least a portion of a single reverse channel ( 104 ), wherein the power control message requests an increase in transmit power for subsequently received signals, and wherein the single reverse channel is shared by a plurality of receiving devices. The transmitting device adjusts the transmit power level based on observing the reverse channel.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to a method and apparatus for transmit power control during a group call to a plurality of devices.  
       BACKGROUND OF THE INVENTION  
       [0002]     Current two-way radio systems do not have a power control mechanism for a group dispatch type of call in a talk around mode (i.e., a call without a repeater or a base station), which leads to high power consumption and more interference to the adjacent channels.  
         [0003]     Periodic transmission of link quality information back to the transmitting device helps to control transmission power in the case of a single transmitter single receiver (“STSR”) radio frequency (“RF”) call. In the case of a single transmitter  100  multiple receivers  102  (“STMR”) group call in talk around mode as illustrated in  FIG. 1 , however, power control becomes difficult due to bandwidth constraints in a narrowband system which make it impractical to receive independent feedback from every receiving device  102  in the group call. Moreover, the transmitting device  100  has no knowledge of the number and/or identity of the receiving devices  102  involved in the group dispatch call.  
         [0004]     Thus, there exists a need for transmit power control during a dispatch call to multiple devices. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0005]     A preferred embodiment of the invention is now described, by way of example only, with reference to the accompanying figures in which:  
         [0006]      FIG. 1  illustrates a system topology comprising a transmitting device and a plurality of receiving devices in accordance with the present invention;  
         [0007]      FIG. 2  illustrates a flowchart of a power control algorithm for the receiving device in accordance with the present invention;  
         [0008]      FIG. 3  illustrates a power control threshold and hysteresis loop for power control adaptation for the receiving device in accordance with the present invention;  
         [0009]      FIG. 4  illustrates a first example of the format of the reverse channel in accordance with the present invention;  
         [0010]      FIG. 5  illustrates a second example of the format of the reverse channel in accordance with the present invention;  
         [0011]      FIG. 6  illustrates a third example of the format of the reverse channel in accordance with the present invention;  
         [0012]      FIG. 7  illustrates a state machine in the transmitting device for transmit power control in accordance with the present invention; and  
         [0013]      FIG. 8  illustrates an example of the state machine of  FIG. 7  in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]     It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate identical elements.  
         [0015]     The present invention allows multiple receiving devices  102  to provide periodic feedback regarding its individual received signal quality to the transmitting device  100  during a group/dispatch call by allowing the receiving devices  102  to transmit power control (“PC”) messages on the reverse channel  104  when its link quality falls below a threshold. The reverse channel  104  is a common channel for all receiving devices  102  in a group/dispatch call. The present invention further enables the transmitting device  100  to reach an optimum/desired transmit power level and enables the receiving devices  102  to receive signals from the transmitting device  100  having at least threshold link quality by allowing the transmitting device  100  to dynamically adjust the transmit power level based on an observation of the common reverse channel  104 .  
         [0016]     Let us first discuss the present invention from the perspective of the receiving device  102 . The receiving device  102  estimates a received signal quality (“RXQUAL”) of a signal received on a forward channel  106 . The estimation of the RXQUAL is a function of one or more parameters, such as, but not limited to, bit error rate (“BER”), message error rate (“MER”), frame error rate (“FER”), received signal strength indicator (“RSSI”), symbol error rate (“SER”), waveform eye opening, frequency/time lock, or the like. If an increase in power is desired based on the RXQUAL measurements, the receiving device  102  schedules the transmission of a PC message on the reverse channel  104 ; if an increase in power is not desired based on the RXQUAL measurements, the receiving device  102  does not schedule the transmission of a PC message on the reverse channel  104 . The PC message is a request from the receiving device  102  to the transmitting device  100  to increase the transmit power level. Thus, on the reverse channel  104 , multiple receiving devices  102  involved in the group/dispatch call that require a power boost transmit the same PC message on the reverse channel  104  around the same time.  
         [0017]     A flowchart of the PC algorithm at the receiving device  104  is illustrated in  FIG. 2 . As illustrated, the receiving device  102  continuously estimates the RXQUAL of the signals received on the forward channel  106  (at step  200 ). Based on each RXQUAL measurement, the receiving device  102  determines whether an improvement is desired in the link quality (at step  202 ). If an improvement in the link quality is desired, the receiving device  102  transmits a PC message to the transmitting device  100  on the reverse channel  104  requesting a power increase and continues to estimate the RXQUAL of the signals received from the transmitting device  100  on the forward channel  106  (at step  204 ). If an improvement in the link quality is not needed or desired, the receiving device  102  does not transmit a PC message to the transmitting device  100  on the reverse channel  104 , but rather just continues to estimate the RXQUAL of the signals received from the transmitting device  100  on the forward channel  106  (at step  206 ).  
         [0018]     Determining whether an improvement is desired in the link quality by the receiving device  102  is better illustrated in  FIG. 3 . A hysteresis loop  300  in the receiving device  102 , as illustrated in  FIG. 3 , helps the transmitting device  100  in reducing oscillations near the optimum power boundaries; the hysteresis (PC_HYST) loop  300  is included in a power control threshold  302  for the receiving device  102  to dampen the power control oscillations of the transmitting device  100 . As illustrated in  FIG. 3 , if the forward channel measurement indicates a poor RXQUAL measurement, then the receiving device  102  continuously requests an increase in power by transmitting the PC message on the reverse channel  104  until RXQUAL&gt;(PC_THRESH+PC_HYST). Once RXQUAL&gt;(PC_THRESH+PC_HYST), the receiving device  102  does not transmit the PC message on the reverse channel  104  until the RXQUAL falls below the PC threshold  302 . In the preferred embodiment, the PC threshold  302  is set to such a value that the speech quality just starts to degrade and that a FER of preferably less that 1% is observed.  
         [0019]     The PC message transmitted by the receiving device  102  on the reverse channel  104  can be designed in multiple ways. For example, the PC message may comprise a PC preamble message and a control message as illustrated in  FIG. 4 . Alternatively, the PC message may just comprise the PC preamble message as illustrated in  FIG. 5 . In all cases, the PC message contains at least the PC preamble message. The two-way radio system may support more than one PC message format but the position of the PC preamble message is the same for all supported PC messages. The PC preamble message is a predetermined known message and for example can be a function of known bits, known symbols or known waveform patterns or the like. Receiving devices  102  involved in the group/dispatch call that desire a transmit power increase transmit the same PC preamble message. In one embodiment, the design of the reverse channel format may comprise a dedicated PC slot for transmission of the PC message resulting in time division multiplexing of the PC message on the PC slot and other reverse channel messages on the reverse channel slot as illustrated in  FIG. 6 . Any message may be transmitted on the reverse channel slot, and for example may include one or more of synchronization message, control message, or data message. The transmitting device  100  may also schedule a transmission on at least a portion of the reverse channel slot.  
         [0020]     There is a likelihood of collisions on the reverse channel as more than one device may transmit on the reverse channel. For example, collisions on the PC slot may occur as more than one receiving device  102  may transmit a PC message requesting a transmit power increase. As the same PC preamble message is transmitted, the composite received PC preamble signal can be considered as the PC preamble message being propagated through a multipath channel due to different propagation delays between each pair of transmitting and receiving devices  100 ,  102 . A PC detector (not shown) in the transmitting device  100  can be appropriately designed to detect the presence or absence of the PC preamble signal on the reverse channel  104 .  
         [0021]     In an alternative embodiment, receiving devices  102  that desire an increase in transmit power may transmit the PC message on at least a portion of the reverse channel  104  that may also be used by other receiving devices  102  that do not desire an increase in transmit power. For example, the design of the reverse channel format may include a common PC/synchronization slot. Receiving devices  102  that desire a transmit power increase may transmit the PC message on the common PC/synchronization slot, while receiving devices  102  that do not desire a transmit power increase but need to transmit some information on the reverse channel  104  may use the PC/synchronization slot to transmit a synchronization message. In such a case, the PC message and the synchronization message may be selected such that the transmitting device  100  can reliably detect them. For example, the messages may be selected from the family of sequences that have good auto-correlation and possibly good cross-correlation properties such as orthogonal Walsh sequences. In addition to the PC/synchronization slot, the reverse channel  104  may have other slots such as the reverse channel slot to transmit other messages, as mentioned above.  
         [0022]     Let us now discuss the present invention from the perspective of the transmitting device  100 . The device transmitting on the forward channel  106  initially sets the transmit power level to a predetermined level such as the maximum power level or to a power level where a communication is guaranteed to all intended recipients in a group dispatch call. Thereafter, for power control in a group/dispatch call, the transmitting device  100  executes the power control state machine as illustrated in  FIG. 7 . The state machine  700  in the transmitting device  100  makes a decision to increase, decrease, or maintain a transmit power level depending on the current power state; thus, the state machine  700  has four power states: INITIALIZE  702 , MAINTAIN  704 , DECREASE  706  and INCREAESE  708 .  
         [0023]     The decision to transition from the MAINTAIN power state  704  to either the DECREASE or INCREASE power states  706 ,  708  includes a voting process. The voting parameters used in the voting process comprise K 1 , N 1 , K 2  and N 2 : K 1  is the number of PC messages detected in the observation window (i.e., time interval) of N 1  reverse channel intervals; K 2  is the number of PC messages not detected in the observation window of N 2  reverse channel intervals. A PC message may be declared to be detected by the transmitting device  100  if at least the PC preamble message is detected. The voting parameters, K 1 , N 1 , K 2 , N 2  determine the nimbleness of the power control algorithm in the transmitting device  100 . The state machine  700  at the transmitting device  100  also comprises a power oscillation counter (osc_cntr; not shown). In the preferred embodiment, a power oscillation is considered when the transmitting device  100  transitions from a DECREASE power state  706  to an INCREASE power state  708  and the power oscillation counter is set to a value greater than zero, as will be explained below. The voting process along with the use of the minimum (smallest) power change step size when a power oscillation is detected damps the power control oscillations about the minimum required transmit level for reliable communications. The state machine  700  dynamically adjusts the step size by which the power changes (increases or decreases). The power change step size may be selected from a predetermined set of values or may be calculated based on a function of one or parameters, such as power control state, previous step size value, PC detection, power oscillation counter, or the like.  
         [0024]     Referring to  FIG. 7 , the transmitting device  100  starts in the INITIALIZE power state  702  when it initiates a call. The transmit power level is set at the predetermined level while in the INITIALIZE power state  70 ; in the preferred embodiment, the transmitting device  100  transmits with maximum power during call initiation. The transmitting device  100  transitions to the MAINTAIN power state  704  with the power oscillation counter set to zero and a minimum power change step size set to a predetermined value (example, 0.5 dB).  
         [0025]     First, let us discuss the actions of the transmitting device  100  while operating in the MAINTAIN power state  704  in the present example. While in the MAINTAIN power state  704 , if the transmitting device  100  detects a PC message on the reverse channel  104  K 1  times within the predefined observation window N 1  (i.e., if K 1  successful PC detections have occurred within N 1 ) and the transmit power level is not at a predetermined maximum power level, the transmitting device  100  increases the transmit power level by the minimum power change step size, sets the power oscillation counter to zero, if not already set to zero, and transitions from the MAINTAIN power state  704  to the INCREASE power state  708 . If the transmitting device  100  does not detect a PC message on the reverse channel  104  K 2  times within the predefined observation window N 2  (i.e., if K 2  unsuccessful PC detections have occurred within N 2 ) and the current transmit power level is not at a predetermined minimum power level, the transmitting device  100  transitions decreases the transmit power level by the minimum power change step size and transitions from the MAINTAIN power state  704  to the DECREASE power state  706 . It should be noted that when either K 1  or K 2  is greater than one, if less than K 2  unsuccessful PC detections have occurred within the observation window N 2  or if less than K 1  successful PC detections have occurred within the observation window N 1 , the transmitting device  100  maintains the current power level and remains in the MAINTAIN power state  704 . Delaying the decision to decrease the transmit power level (by a least K 2  frames) during the initial few frames may be advantageous for late-entry receiving devices  102  who have missed the initial call setup signal.  
         [0026]     Next, let us discuss the actions of the transmitting device  100  while operating in the DECREASE power state  706 . While in the DECREASE power state  706 , if the transmitting device  100  does not detect a PC message on the reverse channel  104  and the transmit power level is not already at a predetermined minimum power level, the transmitting device  100  checks the power oscillation counter to see if the power oscillation condition has been met. If no power oscillation is determined (osc-cntr=0), the transmitting device  100  remains in the DECREASE power state  706 , increases the power change step size (for example, in a linear or logarithmic fashion), decreases the transmit power level by the current power change step size, and keeps the power oscillation counter value set to zero. If, however, power oscillation is determined (osc_cntr&gt;0), the transmitting device  100  remains in the DECREASE power state  706 , decreases the transmit power level by the minimum power change step size, and decrements the power oscillation counter value (preferably by one). On the other hand, while in the DECREASE power state  706 , if the transmitting device  100  detects a PC message on the reverse channel  104 , the transmitting device  100  transitions from the DECREASE power state  706  to the INCREASE power state  708 , maintains the current power change step size, sets the power oscillation counter value to the current power change step size divided by the minimum power change step size, and increases the transmit power level by the current power change step size. It should be noted that a transition from the DECREASE power state  706  to the INCREASE power state  708  is considered a power oscillation about the desired optimum transmit power level and hence a power oscillation flag is set by initializing the oscillation counter to a non-zero value=(step size)/(min. step size).  
         [0027]     Finally, let us discuss the actions of the transmitting device  100  while operating in the INCREASE power state  708 . While in the INCREASE power state  708 , if the transmitting device  100  is still detecting a PC message on the reverse channels  104 , the transmitting device  100  remains in the INCREASE power state  708 , and if the power level is not already at a predetermined maximum power level, continues to increase the power change step size (for example, in a linear or logarithmic fashion), and resets the power oscillation counter value to zero on each PC message detection. If, however, a PC message is not detected on the reverse channels  104  while currently operating in the INCREASE power state  708 , the transmitting device  100  sets the power change step size to the minimum power change step size and transitions from the INCREASE power state  708  to the MAINTAIN power state  704 .  
         [0028]     Thus, in a group/dispatch call in a narrow band system, the power oscillations cannot be avoided since the bandwidth is not available for the transmitting device  100  to receive feedback regarding received signal quality from all the receiving devices  102 . The voting process along with the use of the minimum (smallest) power change step size when a power oscillation is detected by the transmitting device  100  dampens the power control oscillations about the minimum required transmit level.  
         [0029]      FIG. 8  illustrates an example of the PC algorithm for the transmitting device  100  for voting parameters K 1 , N 1 , K 2  and N 2 . In this example, assume that K 1 =1, N 1 =1, K 2 =2 and N 2 =3. The PC decisions, current power state of the algorithm, and the value of the power oscillator counter are also illustrated in  FIG. 8 . It can be seen from  FIG. 8  that the transmit power level first (y-axis) decreases with increasing step size until it is below the desired power level at which time the power oscillation condition is detected and the power level is increased. Further, the power level changes occur in the minimum power change step size around the desired transmit power level.  
         [0030]     In the above example, the values of the voting parameters, K 1 , N 1 , K 2 , N 2 , have been fixed for the entire call duration. It should be noted, however, that it could be changed dynamically when a condition is detected, such as detection of a power control oscillation. The power change step sizes also may be changed dynamically with possibly different step sizes for power level increase and decrease.  
         [0031]     Thus, to describe the example in detail, the transmitting device  100  initially sets the transmit power level to a power level P, which is typically much higher than the desired minimum transmit power level (shown as a dotted line), and the transmit state machine transitions from the INITIALIZE power state  702  to the MAINTAIN power state  704  with the power oscillation counter (osc_cntr) set to zero. During the first K 2 =2 reverse channel intervals, the transmitting device does not detect the PC message. As a result, the state machine  700  at the transmitting device  100  transitions to the DECREASE power state  706  and the transmitting device  100  decreases the transmit power level by the minimum power change step size.  
         [0032]     During the third through the sixth reverse channel intervals, the transmitting device  100  does not detect a PC message on the reverse channel  104 , and thus decreases the transmit power level with an increasing power change step size (in this example, in a linear fashion) while the state machine  700  remains in the DECREASE power state  706 . It should be noted that after the power level adjustment based on the sixth reverse channel interval, the transmit power level is below the desired minimum transmit power level.  
         [0033]     During the seventh reverse channel interval, the transmitting device  100  detects a PC message on the reverse channel  104  that passes the voting process (K 1 =1 and N 1 =1). As such, the transmitting device  100  increases the transmit power level by the current (most recent) power change step size and sets the power oscillation counter to five (current step size/min step size).  
         [0034]     During the eighth reverse channel interval, the transmitting device  100  does not detect a PC message on the reverse channel  104 , and so the state machine  700  transitions to the MAINTAIN power state  704  with the power change step size set to the minimum value; the transmitting device  100  does not change the transmit power level at this time.  
         [0035]     During the ninth reverse channel interval, the transmitting device  100  does not detect a PC message on the reverse channel  104  and the state machine  700  transitions to the DECREASE power state  706  as it passes the voting process (K 2 =2 unsuccessful detections in observation window of N 2 =3). The transmitting device  100  decreases the transmit power level by the minimum power change step size.  
         [0036]     During the tenth through twelfth reverse channel intervals, the transmitting device  100  again does not detect a PC message on the reverse channel  104 , thus the state machine  700  remains in the DECREASE power state  704 . However, since the power oscillation counter is greater than zero, the transmitting device  100  decreases the transmit power level only by the minimum power change step size for each reverse channel interval. The power oscillation counter is also decreased (by 1 in this example) and its value after detection during the twelfth reverse channel interval is two. It should be noted that the transmit power level after the power adjustment based on twelfth reverse channel interval is again below the desired minimum transmit power level.  
         [0037]     During the thirteenth reverse channel interval, the transmitting device  100  detects a PC message on the reverse channel  104 , and the state machine  700  transitions to the INCREASE power state  708  with the transmit power level being increased by the current power change step size, which now is the minimum power change step size resulting in the power oscillation counter set to one.  
         [0038]     During the fourteenth reverse channel interval, the transmitting device  100  does not detect a PC message on the reverse channel  104 , and the state machine  700  transitions to the MAINTAIN power state  704  with the power change step size set to the minimum value; the transmitting device  100  does not change the transmit power level at this time.  
         [0039]     During the fifteenth reverse channel interval, the transmitting device  100  does not detect a PC message on the reverse channel  104 , and the state machine  700  transitions to the DECREASE power state  706  as it passes the voting process (K 2 =2 unsuccessful detections in observation window of N 2 =3). As a result, the transmitting device  100  decreases the transmit power level by the minimum power change step size. It should be noted that the transmit power level after the power adjustment based on fifteenth reverse channel interval is again below the desired minimum transmit power level.  
         [0040]     The conditions resulting from the transmitting device  100  detecting PC messages during the sixteenth through eighteenth reverse channel intervals (and future reverse channel intervals in groups of three) are similar to those conditions described above during the thirteenth through fifteenth reverse channel intervals, and the transmit power levels and the state transitions are similar. As a result, the transmit power level is thus reduced close to the desired minimum power level and the transmit power oscillations have been minimized to minimum step size change in power level.  
         [0041]     While the invention has been described in conjunction with specific embodiments thereof, additional advantages and modifications will readily occur to those skilled in the art. The invention, in its broader aspects, is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Various alterations, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Thus, it should be understood that the invention is not limited by the foregoing description, but embraces all such alterations, modifications and variations in accordance with the spirit and scope of the appended claims.