Abstract:
A system and method reduce power consumption in a multi-hop wireless communications network. A signal is received from a source node in an intermediate node. The signal includes a request to relay a message to a destination node. The request includes information on power requirements to relay the message either in a relay mode or a regeneration mode. If the available power at the intermediate node exceeds the power requirements, then the request is accepted. Either the relay mode or the regeneration mode is selected, based on criteria for retransmitting the message, and the message is then relayed to the destination node using the selected mode.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to the field of wireless communications, and more particularly to power efficient multi-hop data transmission in wireless communications networks.  
         BACKGROUND OF THE INVENTION  
         [0002]    In an ad hoc wireless network, transceivers or “nodes” are arranged to communicate with each other without any network infrastructure or centralized administration. The arrangement can be static or dynamic, or combinations thereof. The nodes can be cellular telephones, portable computing devices, or special purpose devices such as sensors. The nodes in the network establish routing among themselves to form their own network. Due to a limited transmission range of the transceivers, messages from a source node may have to pass through one or more intermediate routing nodes before reaching a destination node.  
           [0003]    In many ad hoc wireless networks, most, if not all of the nodes are battery powered. Therefore, minimizing power consumption is a primary concern. Some techniques for reducing power decrease transcoder complexities, use low power circuits and low signaling-cost routing protocols. Other techniques attempt to exploit the network topology to reduce power.  
           [0004]    Heinzelman et al., in “ Energy - efficient Communication Protocol for Wireless Micro-sensor Networks,”  Proc. of the IEEE Hawaii Int. Conf. on System Sciences, pp. 3005-3014, January, 2000, describe communication protocols for power reduction in a wireless network. They describe a clustering based protocol that utilizes randomized rotation of local cluster heads to evenly distribute the power load among the nodes in the network. They also indicate that when the distance between two nodes is short, direct transmission is more efficient than multiple hop transmission.  
           [0005]    Chang et al, in “ Energy Conserving Routing in Wireless Ad - hoc Networks,”  Proc. of IEEE INFOCOM 2000, March, 2000, describe methods for selecting routes and corresponding power levels in a static wireless network so that power consumption is reduced.  
           [0006]    Catovic et al, in “ A new approach to minimum energy routing for next generation multi - hop wireless networks,”  Journal of Communications and Networks, Special Issue on “ Evolving from  3 G deployment to  4 G definition,”  December 2002, describe a technique for transmitting data over two different channels at different power levels. A rake receiver is used to reconstruct the original data by combining the two received signals.  
           [0007]    Chen et al., “ Energy Efficient System Design with Optimum Transmission Range for Wireless Ad - hoc Networks ,” Proc of IEEE Int. Conf. on Communications, ICC&#39;02, pp. 945-952, May, 2002, determine optimum transmission range and hop distances in wireless ad-hoc networks.  
           [0008]    The prior art has not addressed the problem of bit error propagation through the multi-hop paths. To eliminate error propagation, the transmit signal power levels must be increased on multi-hop paths. As a result, overall power consumption in the network increases. On the other hand, at each intermediate node, a received signal can be decoded and re-encoded, and then forwarded to the destination or to the next intermediate node. This can prevent error propagation, at the cost of increasing power consumption due to complexities of the transcoding process.  
           [0009]    Most important, deciding whether to simply amplify and transmit the data or to regenerate the data depends on the position of the intermediate node relative to the source and destination nodes, and the level of power loss. Therefore, there is a need for a system and method that can reduce power consumption in a wireless ad hoc network.  
         SUMMARY OF THE INVENTION  
         [0010]    The system and method according to the invention reduces power consumption in a multi-hop wireless communications network. A signal from a source node is received at an intermediate node. The signal including a relay request, a message, a relay power requirement and a regeneration power requirement. The request is accepted if available power at the intermediate node exceeds an amount of power required to relay the message to a destination node. A relay mode or a regeneration mode is selected based on local criteria at the intermediate node and the message is relayed to the destination node using the selected mode.  
       
    
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a block diagram of a wireless communications network according to the invention;  
         [0012]    [0012]FIG. 2 is a flow diagram of a method for reducing power consumption in a multi-hop wireless communications network according to the invention;  
         [0013]    [0013]FIG. 3A is a flow diagram of relay mode operation according to the invention;  
         [0014]    [0014]FIG. 3B is a flow diagram of regeneration mode operation according to the invention; and  
         [0015]    [0015]FIG. 4 is a block diagram of a wireless node according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    System Structure  
         [0017]    [0017]FIG. 1 shows a wireless communications network  100  according to the invention. The network  100  includes a source node  110 , and an intermediate node  130  and a destination node  120 . The output of the source node is a signal  111 . The signal includes a relay-request  112 , a message  113 , a relay mode power requirement  114 , and a regeneration mode power requirement  115 .  
         [0018]    A power savings is expressed as a difference between the amount of power required to transmit the message over an indirect path  116  and a direct path  117 .  
         [0019]    System Operation  
         [0020]    [0020]FIG. 2 shows a method  200  for reducing power consumption in a multi-hop wireless communications network  100  according to the invention. The signal  111  is received  210  by the intermediate node  130  from the source node  110 .  
         [0021]    During step  220 , the intermediate node  130  decides whether to accept or deny the request  112  from the source node  110  to relay the message  113  to the destination node  120 . Depending on the decision of step  220 , the intermediate node sends an indication  221  of acceptance (Ack) or rejection (Nack) of the request. If the request is denied or not accepted within a predetermined amount of time, then the source node  110  can seek another node to act as the intermediate node, or the source node can attempt to send the message  113  directly to the destination node  120 .  
         [0022]    If the indication  221  is an Ack, the intermediate node selects  230  a mode for relaying the message  113  to the destination node  120 . The selected mode can be either a regeneration mode or a relay mode, both of which are discussed in detail below.  
         [0023]    [0023]FIG. 3A shows the steps for the relay mode. Here, the intermediate node  130  amplifies  310  the signal  111  from the source node  110 , and retransmits  320  the amplified signal  321  to the destination node  120 .  
         [0024]    [0024]FIG. 3B shows the steps for regeneration mode. In this mode, the signal received at the intermediate node  130  is first demodulated  330  then decoded  340 . The decoding  340  process corrects any bit errors that occurred. The decoded data  341  are then encoded  350 , amplified  360  and transmitted  370  to the destination node  120 .  
         [0025]    Regeneration Mode  
         [0026]    The source node A  110  determines the amount of power required to transmit the signal  111  directly to destination node B  120  by Equation (1) as:  
         α t     e     AB =α r     e     d   ab   γ 10 x     ab     /10 , watts,  (1)  
         [0027]    where the required power α re  meets a bit error rate requirement Pr e , a value γ is a path-loss exponent, and a value x ab , is a shadowing loss, in decibels, on the path from the source node A  110  to the destination node B  120 . The value α re  depends on what type of radio modulation is being used, e.g., PAM, QPSK, QAM.  
         [0028]    The source node A  110  determines the power savings that could be obtained if intermediate node C  130  acts as a regenerator relay. The power savings is the difference between power used on the relay path A→C→B  116  and the power used on the direct path A→B  117 . To find this difference, the source node A  110  determines the power to transmit to intermediate node C  120  and the power used at the intermediate node to transmit to the destination node B  120 . These two powers, denoted (α t     e     A , α t     e     C ), depend on distances (d ac ,d cb ) and the required bit error rate (BER) Pr e . The power reduction due to regeneration is then determined by Equation (2) as:  
         Δα regen =α t     e     A +α t     e     C −α t     e     AB   +P   c .  (2)  
         [0029]    Equation 2 gives the optimum power saving, neglecting shadowing losses, in the network  100  where the intermediate node acts as a regenerator and relays the signal the source node  110  to the destination node  120 .  
         [0030]    [0030]FIG. 3A shows the regenerator relay method with the computed transmit powers at the source node A  110  and intermediate node C  130  (α t     e     A , α t     e     C ). Depending on the distances between the nodes. The power reduction Δα regen  can be positive or negative.  
         [0031]    If the power reduction Δα regen  is positive, then the source node does not send a relay request to the intermediate node. If the power reduction Δα regen  is negative, then the source node sends the relay request to the intermediate node.  
         [0032]    Relay Mode  
         [0033]    In relay mode, the intermediate node  130  forwards the message  113  to the destination node B  120  without correcting any errors that occurred in the transmission from the source node A  110  to the intermediate node C  120 . The transmit powers at the source and intermediate nodes are (α t     e     A , α t     e     C ) and the optimal bit error rates at the intermediate node and the destination node are (*Pr e   ac , *Pr e   cb ). Because relaying allows bit errors to propagate from intermediate node to destination node, Equation (3)  
           Pr   e   ac   +*Pr   e   cb   =Pr   e   (3)  
         [0034]    is satisfied by  
               ln        (         d     a                 c     γ          10       x     a                 c       /   10             d   cb   γ          10       x   cb     /   10           )       =              ln        (       Q     -   1            (           log   2        M       (     1   -     1   /   M       )            (       Pr   e   ab     -     Pr   e     a                 c         )       )       )       +                          (           erf     -   1            (     1   -         2                   log   2        M       (     1   -     1   /   M       )            (       Pr   e   ab     -     Pr   e     a                 c         )         )       2     -                                (       erf     -   1            (     1   -         2                   log   2        M       4        (     1   -     1   /   M       )              Pr   e     a                 c           )       )     2     -                          ln        (       Q     -   1            (         2        log   2        M       4        (     1   -     1   /   M       )              Pr   e     a                 c         )       )                                   
 
         [0035]    and *Pr e   cb  is found by *Pr e   cb =Pr e −*Pr e   ac .  
         [0036]    From these optimal bit error rates, the optimal transmit powers (α t     e     A , α t     e     C ) are determined. After the values (α t     e     A , α t     e     C ) are known, the source node A  110  can then determine the power savings according to  
         Δα relay =α t     e     A +α t     e     C −α t     e     AB   +P   c .  
         [0037]    The above described method finds (*Pr e   ac , *Pr e   cb , α t     e     A , α t     e     C ) for any rectangular M-ary modulation scheme, however, the same analysis technique applies to other modulation schemes as well.  
         [0038]    The value Δα relay  is always less than the value Δα regen  because, for a fixed Pr e , the relay method requires additional power.  
         [0039]    However, this additional power is only a fraction of a decibel. Therefore, the intermediate node  130  can determine whether to select relay mode or regenerator mode based on local criteria, such as processing load. The intermediate node  130  can then perform relaying and the overall system still achieves power reduction.  
         [0040]    Node Structure  
         [0041]    [0041]FIG. 4 shows a node  400  according to the invention. Each node includes an antenna  405  connected to a transmit block  410  and a receive block  450 . The transmit and receive blocks are coupled to each other by switches  491 - 492 . In addition, the node includes a processor  480  and a mode selector  490 .  
         [0042]    The transmit block  410  includes an encoder  440 , a transmitter  420 , and an amplifier  430 . The receive block includes a receiver  460  and a decoder  470 .  
         [0043]    Node Operation  
         [0044]    The transmit block  410  encodes, modulates, and amplifies the signal  111 . The signal can originate locally or from another node.  
         [0045]    The receive block chain  450  demodulates and decodes a received signal  411 . The output of the receiver block  460  is a bitstream including the message  113 , that is directed to the processor  480 , and to either the transmitter for relaying when the switch  491  is in the “0” (off) position, or to the decoder  470  when the switch  491  is in the “1” (on) position. The decoder  470  decodes the bit stream  461  and corrects bit errors.  
         [0046]    After decoding, the message  113  is reconstructed and is either transmitted by the transmit chain  410  if the switch  492  is in the “0” position for regeneration mode, or is passed to the local node  495  if the processing unit  480  determines the message  113  is intended for the local node.  
         [0047]    The processor  480  continuously processes received messages and determines if the messages are relay control messages, relay data, or data destined for the local node. The processor also determines the setting of the switches  491 - 492 . If the data are relay data, the decision of what mode, e.g., regenerator or relay, to forward the data is made by the mode selector  490 . The decision is made as a function of the savings, and the switches  491  and  492  are set accordingly.  
         [0048]    Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications may be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention