Abstract:
A mobile terminal is controlled via over-the-air feedback so as to enable its data transmissions to be independently and successfully decoded at each of the base stations in its active set absent a transmit power limitation or data retransmission limit. Using the decoded data, the channel is re-estimated and the waveform received from the mobile terminal is reconstructed and subtracted from the total interference at each base station in the active set where decoding has been successful. As a result, transmissions from other mobile terminals, which have yet to be successfully decoded at such a base station, will experience a higher signal-to-noise ratio and thus an increased likelihood of being successfully decoded.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application is a Continuation of U.S. application Ser. No. 11/409,161 filed on Apr. 21, 2006, to Shirish Nagaraj, et al., now issued as U.S. Pat. No. 8,493,941, commonly assigned with the present invention and incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to wireless communications. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a wireless system, mobile terminals transmit and receive data over bi-directional wireless links from one or more base stations. The mobile terminal-transmit direction is known as the uplink and the mobile terminal-receive direction is known as the downlink. The set of base stations with which a mobile terminal is communicating is known as the active set of base stations for that mobile terminal, or that mobile terminal&#39;s active set. During normal conditions when a mobile terminal is within a base station&#39;s coverage area, the active set for that mobile terminal would generally be a single base station so that the active set includes only that one base station, which is that mobile terminal&#39;s serving station. When a mobile terminal, however, is within the range of multiple base stations and as such is in a handoff state, the active set includes the multiple base stations, which are each monitoring the signal from that mobile terminal and decoding it when able to do so. Only one of the base stations, however, is the serving base station for that mobile terminal and the other base stations are non-serving base stations.  FIG. 1  shows three base stations  101 ,  102  and  103  and their respective cell coverage areas  104 ,  105  and  106 . Mobile terminal  107  is shown within the coverage area  104  of base station  101 , which is its serving base station. Non-serving base stations  102  and  103 , however, in addition to serving base station  101  constitute the active set for mobile terminal  107 . 
         [0004]    In a data system, on the downlink, the mobile terminal  107  receives data from only one base station but has the option of reselecting the serving base station in order to receive data from any other base station in its active set depending on from which base station the mobile terminal receives a signal with the highest signal-to-noise ratio. On the uplink, the serving and the non-serving base stations each attempts to demodulate and decode transmissions from the mobile terminal. 
         [0005]    The capacity of a wireless system refers either to the number of mobile terminals that can simultaneously transmit or receive data, or the aggregate date rate of these mobile terminals, either expressed in mobile terminals/sector, erlangs/sector or data throughput/sector. The uplink capacity of the system can be different from its downlink capacity. For symmetric services, such as voice, (i.e., required throughput/data rate for a mobile terminal on the uplink is equal to that on the downlink), the overall system capacity is limited by the lower of uplink and downlink capacity. In current wireless systems specified by standards such as CDMA2000 1 x, EV-DO Rev 0 and Rev A, HSDPA/EDCH, and WiMAX, the uplink has a substantially lower capacity than the downlink. This imbalance needs to be remedied for full use of downlink capacity and to maximize the number of mobile terminals than can operate symmetric services on the system. 
         [0006]    In wireless systems that are based on direct spread or multi-carrier (optionally with precoding) CDMA, a plurality of mobile terminals within a sector (and across sectors) re-use a spreading sequence or a set of frequency tones to communicate with their respective active sets, while being differentiated by mobile terminal-specific codes. A mechanism for increasing uplink sector capacity is to perform successive interference cancellation on these transmissions at the base station transceiver.  FIG. 2  illustrates base station noise rise components. As shown, at a base station receiver  200 , the total rise over thermal noise in a sector consists of the composite signal  201  from the mobile terminals within that sector for which that base station is the serving base station, and the out-of-cell interference  202  caused by mobile terminals transmitting in adjacent sectors. The latter includes interference from those mobile terminals in the adjacent sectors for which base station receiver  200  is within these mobile terminals&#39; active set but for which base station  200  is non-serving, plus the interference caused by other transmitting mobile terminals in other sectors that do not include base station  200  within each such mobile terminal&#39;s active set. 
         [0007]    An illustrative method of interference cancellation is disclosed in U.S. Pat. No. 7,385,944. Using such an interference cancellation method, if the decoding of any mobile terminal is successful, its signal is reconstructed and subtracted from the composite received signal at the base station.  FIG. 3  shows a successive interference cancellation scheme at an exemplary base station receiver  300  that is the serving base station for four mobile terminals  301 ,  302 ,  303  and  304  within a sector of that base station receiver  300 . The received power at base station receiver  300  from mobile terminals  301 ,  302 ,  303 , and  304  is respectively P — 1, P — 2, P — 3, and P — 4. In addition, base station receiver  300  receives a composite signal power as the result of out-of-cell interference (IOC) caused by transmissions from mobile terminals out of the sector. When a particular transmission from a mobile terminal from within the sector is successfully decoded by the base station receiver, the transmission is reconstructed and subtracted from the composite signal at the base station, after which another received signal is demodulated, decoded, reconstructed and subtracted from the remaining composite signal. This process is repeated for each of the remaining signals. Advantageously, the signals from the mobile terminals that are decoded later in the demodulation and decoding process do not “see” the interference from transmissions from the mobile terminals that were decoded earlier in the sequence.  FIG. 3  shows a successive calculation of the signal-to-noise ratios (Snr — 1-Snr — 4) of the four mobile terminals  301 - 304 , respectively. Starting with mobile terminal  301 , Snr — 1 is calculated as P — 1/(P — 2+P — 3+P — 4+IOC+N), where N is the measurable thermal noise. The contribution from each is successively subtracted off from the received composite signal at the base station receiver, so that, for the last mobile terminal  304 , Snr — 4 is calculated as P — 4/(IOC+N). Since the mobile terminals that are decoded later see a higher signal-to-noise ratio, they are capable of supporting either a higher rate of transmission and/or increased reliability. 
         [0008]    In the above-described scenario, it is not possible for the base station receiver to successfully decode the transmissions of all the mobile terminals that have this base station sector in their active set. As a result, most of the out-of-cell interference received by a base station receiver cannot be deducted. Thus, as noted above, the signal-to-noise ratio for station  304  is still limited by this out-of-cell interference IOC. 
         [0009]    Typically, the power control rule followed by mobile terminals is to either (i) follow power control commands from the serving sector in its active set, or (ii) follow a rule known as the or-of-the downs, whereby the mobile terminal lowers its power if any of the base stations in the active set instructs it to do so via a down power control command. While an or-of-the-downs power control ensures successful reception of the mobile terminal&#39;s transmission at [[at]] least one base station (presumably the one with the best uplink connection from the mobile terminal), it also ensures that the mobile terminal&#39;s transmission is not received with adequate signal-to-noise ratio to be successfully decodable at all of the base stations in the active set. Thus, this undecodable interference limits the capacity gain from a system employing successive interference cancellation. Even as mobile terminals within a sector transmit with ever increasing powers, in order to increase their signal-to-noise ratios at the base station receiver (and hence achieve higher data rates), their interference to adjacent sectors grows proportionately, thereby limiting the rates that can be achieved by mobile terminals in those sectors. In turn, the interference from the mobile terminals in adjacent sectors marginalizes the gains for the mobile terminals with the sector under consideration that increased their power in the first place. 
         [0010]    A methodology is thus desired that enables a base station receiver to reconstruct and subtract the out-of-cell interference from the composite received signal so that the signal-to-noise ratio can be improved for all in sector mobile terminals. 
       SUMMARY OF THE INVENTION 
       [0011]    In an embodiment, a device, e.g. a mobile terminal, includes a receiver and a transmitter. The receiver is configured to receive commands from a plurality of base stations. The transmitter is configured to send, when the a mobile terminal is in handoff status, data packets to the plurality of base stations in the mobile terminal&#39;s active set of base stations, wherein the active set including a serving base station and at least one non-serving base station. The transmitter is further configured to increase the transmit power of the transmitter, in response to the receiver receiving a command to increase the transmit power from any of the base stations, unless the transmit power is at its maximum power or the transmit power exceeds a power targeted by the serving base station by a predetermined amount. 
         [0012]    Another embodiment is a method, e.g. for interference management in a wireless communication system. The method includes sending, at a mobile terminal in handoff status, data packets to a plurality of base stations in the mobile terminal&#39;s active set of base stations, wherein the active set includes a serving base station and at least one non-serving base station. The method further includes increasing the mobile terminal&#39;s transmit power, in response to receiving a command to increase the transmit power from any of the base stations in the active set, unless the transmit power is at its maximum power or the transmit power exceeds a power targeted by the serving base station by a predetermined amount. 
         [0013]    In any embodiment of the aforementioned device or method, the transmit power may be increased by a predetermined up-step. In any embodiment, the transmitter may be configured to increase or decrease the transmit power only in response to commands received from the serving base station, on the condition that: the transmit power is at its maximum power; the mobile terminal receives a command to increase its transmit power from any of the base stations in the active set, and the transmit power exceeds a power targeted by the serving base station by a predetermined amount. 
         [0014]    In any embodiment the transmitter may be configured to increase the transmit power only if the receiver determines the received command is determined to be reliable. In such embodiments the receiver may determine that the command to increase the transmit power is reliable on the condition that a power of the received command to increase the power is greater than a predetermined threshold. 
         [0015]    In another embodiment a device, e.g. a mobile terminal, includes a receiver and a transmitter. The receiver is configured to receive commands from a plurality of base stations. The transmitter is configured to send, when the mobile terminal is in handoff status, data packets to a plurality of base stations in an active set of base stations, the active set including a serving base station and at least one non-serving base station. The receiver is further configured to increase a transmit power in response to the receiver receiving a negative acknowledgment (NACK) of a successful decoding of a given packet received from any of the base stations at the end of that packet&#39;s retransmission limit, unless the transmit power is at its maximum power or the transmit power exceeds a power targeted by the serving base station by a predetermined amount. 
         [0016]    Yet another embodiment is a method, e.g. for interference management in a wireless communication system. The method includes sending, at a mobile terminal in handoff status, data packets to a plurality of base stations in its active set of base stations, wherein the active set includes a serving base station and at least one non-serving base station. The method further includes increasing the mobile terminal&#39;s transmit power, in response to receiving a negative acknowledgment (NACK) of a successful decoding of the packet from any of the base stations in the active set at the end of a given packet&#39;s retransmission limit, unless the transmit power is at its maximum power or the transmit power exceeds a power targeted by the serving base station by a predetermined amount. 
         [0017]    In any embodiment of the device or method described immediately above, the transmitter may be configured to increase the transmit power by a predetermined up-step. In any such embodiment, if the transmit power is at its maximum power or if the transmit power exceeds a power targeted by the serving base station by a predetermined amount, and the receiver receives from any of the base stations in the active set a command to increase the transmit power, then for a subsequent data transmission the transmitter is configured to increase the transmit power only if the receiver receives a NACK from the serving base station and to decrease the transmit power if the receiver receives an ACK from the serving base station. In any embodiment of the device or method described immediately above, the transmitter may be configured to increase the transmit power only if the receiver determines a NACK received from any of the base stations in the active set is reliable. In any embodiment of the device or method described immediately above the receiver may determine the NACK is reliable by comparing a power of the received NACK with a predetermined threshold. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0018]    The present invention will be better understood from reading the following description of non-limiting embodiments, with reference to the attached drawings, wherein below: 
           [0019]      FIG. 1  shows a prior art arrangement of a mobile terminal and the serving base station and non-serving base stations in its active set; 
           [0020]      FIG. 2  illustrates base station noise rise components; 
           [0021]      FIG. 3  shows a prior art successive interference cancellation scheme at an exemplary base station receiver that is the serving base station for four mobile terminals within a sector of that base station receiver; 
           [0022]      FIG. 4  shows the steps at a mobile terminal in accordance with a first embodiment that employs an or-of-the ups rule in controlling its transmit power during handoff; 
           [0023]      FIG. 5  shows the steps at a mobile terminal in accordance with a second embodiment that employs an or-of-the-NACKS rule in controlling its transmit power during handoff; and 
           [0024]      FIG. 6  shows the steps at a mobile terminal in accordance with a third embodiment that employs an or-of-the-NACKS rule in controlling its retransmissions of data during handoff. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    With reference to the flowchart in  FIG. 4 , the methodology employed at a mobile terminal operating in accordance with an or-of-the-ups rule is shown. At step  401 , a determination is made whether the mobile terminal is in handoff (i.e., whether or not the active set is greater than one). In addition, but not shown, an optional additional determination may be made of whether the mobile terminal&#39;s average path losses to the base stations in its active set differ from each other by more than a predetermined amount. If it is not in handoff, at step  402 , the mobile terminal follows power control commands received from its one serving base station. If it is in handoff (and if the optional determination of average path losses indicates that such average path losses differ from each other by less than the predetermined amount), then, at step  403 , the mobile terminal monitors power control commands from the base stations in its active set and accumulates power control commands (the sequence of +1 s and −1 s) from the serving base station to determine the power targeted by the base station. Specifically, by accumulating these +1 s and −1 s, the power level at which the serving base station would like the mobile terminal to operate can be determined. At step  404 , a determination is made whether any of the power commands received by any of the base stations in the active set is an “up” (an or-of-the ups rule). If not, indicating that the received power level at all of that base stations in the active set is at or above the target (and in turn that the transmission is likely to have been successful at all of the base stations), then, at step  405 , the mobile terminal decreases its transmit power by a predetermined down-step. If, at step  404 , any of the power commands received from any of the base stations in the active set is an “up” (indicating that transmission in unlikely to have been successful at the base station(s) from which the up is received), then the mobile terminal transmit power may be increased. Before increasing its transmit power, however, a determination is made, at step  406 , whether the mobile terminal is already at its maximum transmit power, or whether the mobile terminal transmit power exceeds the power targeted by the serving base station by more than a predetermined amount. If either is the case, then, at step  407 , the mobile terminal reverts to an or-of-the-downs power control rule (i.e., the mobile terminal lowers its power if any base station in the active set instructs it to do so). Alternatively, but not shown, the mobile terminal reverts to following power control from only the serving base station. If, at step  406 , the mobile terminal is not at its maximum transmit power and does not exceed the power targeted by the serving base station by the predetermined amount, then, at step  408 , the mobile terminal increases its transmit power by a predetermined up-step. 
         [0026]    With reference to the flowchart in  FIG. 5 , the methodology employed at a mobile terminal operating in accordance with an or-of-the-NACKS rule is shown. At step  501 , a determination is made whether the mobile terminal is in handoff. As in the first embodiment, an optional addition determination may be made of whether the mobile terminal&#39;s average path losses to the base stations in its active set differ from each other by more than a predetermined amount. If it is not in handoff, at step  502 , the mobile terminal follows acknowledgment feedback (ACKS/NACKS) from its one serving base station. If it is in handoff, (and if the optional determination of average path losses indicates that such average path losses differ from each other by less than the predetermined amount), then, at step  503 , at the end of the packet transmission limit (i.e., after the mobile terminal has transmitted a data packet its maximum allowed number of times), the mobile terminal monitors acknowledgments from all base stations in the active set. In addition, the transmit power of the mobile terminal that is required for a successful reception at the serving base station is tracked. At step  504 , a determination is made whether at the end of the packet transmission limit any of the acknowledgments is a NACK, indicating an unsuccessful reception (an or-of-the-NACKS determination) at the base station(s) from which the NACK(s) is received. It should be noted that in certain systems a NACK is indicated by base station silence, i.e., the absence of the base station transmitting an ACK. If none of the responses is a NACK, then the packet has been successfully received at each of the base stations in the active set and, at step  505 , the mobile terminal power is decreased by a down-step for a next packet transmission. If, however, at step  504 , any of the base station responses is a NACK, then the mobile terminal power may be increased. Before increasing its transmit power, however, a determination is made, at step  506 , whether the mobile terminal is already at its maximum transmit power, or whether the mobile terminal transmit power exceeds the power targeted by the serving base by more than a predetermined amount. If either is the case, then, at step  507 , the mobile terminal reverts to an or-of-the-ACKS power control rule for subsequent packet transmissions (i.e., the mobile terminal increases its transmit power only if doesn&#39;t receive an ACK from at least one base station in its active set). Alternatively, but not shown, the mobile terminal reverts to following power control based on the ACK/NACK received from only the serving base station. If, at step  506 , the mobile terminal is not at its maximum transmit power and does not exceed the power targeted by the serving base station by the predetermined amount, then, at step  508 , the mobile terminal increases its transmit power by a predetermined up-step. 
         [0027]    With reference to the flowchart in  FIG. 6 , another methodology employed at a mobile terminal operating in accordance with an or-of-the-NACKS rule is shown. At step  601 , a determination is made whether the mobile is in handoff. As in the previously discussed embodiments, in addition, but not shown, an additional optional determination may be made of whether the mobile terminal&#39;s average path losses to the base stations in its active set differ from each other by more than a predetermined amount. If it is not in handoff, at step  602 , the mobile terminal follows acknowledgment feedback from its one serving base station. If it is in handoff, (and if the optional determination of average path losses indicates that such average path losses differ from each other by less than the predetermined amount), then, at step  603 , the mobile terminal monitors acknowledgments from all the base stations in its active set. At step  604 , a determination is made whether any of the base station responses to a packet transmission is a NACK. If not, at step  605 , the mobile terminal prepares to transmit the next data packet. If, however, at step  604 , it does receive a NACK from one or more base stations in its active set, the mobile terminal may retransmit the packet. If, at step  606 , the retransmission limit has not been reached for this packet, then, at step  607 , the mobile terminal retransmits the data packet. If, however, at step  606  the retransmission limit has been reached for this packet, then, at step  608 , retransmission for subsequent packets is performed based on an or-of-the-ACKS rule wherein the packet is not retransmitted if an ACK is received from any base station in the active set. Alternatively, but not shown, if the retransmission limit has been reached, retransmission for subsequent packets may be based on the ACK/NACK acknowledgments received only from the serving base station. 
         [0028]    In each of the embodiments described above, the mobile terminal may perform a test to determine the reliability of the ACKS or NACKS, or power control bits that it receives from the base stations in its active set in order to determine whether or not to raise its transmit power or to retransmit a packet. Specifically, the received power of the received ACKS/NACKS or power control bits can be compared against a threshold. In those systems in which ACKS and NACKS are both positively transmitted, then if the power is greater than the threshold, then the received ACKS/NACKS or power control bits can be assumed to be reliable and used to make the appropriate determination of increasing/decreasing the transmit power or retransmitting the packet. On the other hand, if the power is below the threshold, a decision to increase the power or retransmit the packet is made only on the basis of those ACKS/NACKS or power control bits that are deemed to be reliable. For those systems in which a NACK is indicated by base station silence a NACK would be deemed to be reliable if its power was below a threshold. 
         [0029]    Upon a successful decoding of a mobile station&#39;s data transmission the received signal from the mobile terminal is reconstructed and subtracted off from the composite received signal prior to decoding other mobile terminal&#39;s transmissions with a concomitant improved signal-to-noise ratio for these other mobile terminals&#39; transmissions. For mobile terminals that have adopted the fallback position for power control or ACK/NACK feedback, the amount of interference presented to the non-serving base stations is likely to be decreased and the penalty due to not being able to successfully decode those transmissions is reduced. 
         [0030]    As previously discussed, the described methodology can be used in any system where out-of-cell interference limits the signal-to-noise ratio for one or more in-cell mobile terminals. Examples of such system include a system employing successive interference cancellation or an OFDMA system that allows only one in-cell transmission over a given set of frequency tones. 
         [0031]    The above-described embodiment is illustrative of the principles of the present invention. Those skilled in the art can devise other embodiments without departing from the spirit and scope of the present invention.