Patent Publication Number: US-7218680-B1

Title: Retransmission techniques for enhanced performance in fading wireless communication channels

Description:
This application claims the priority under 35 U.S.C. 119(e)(1) of copending U.S. provisional application No. 60/185,780, filed on Feb. 29, 2000. 

   FIELD OF THE INVENTION 
   The invention relates generally to wireless communications and, more particularly, to wireless communications over fading channels. 
   BACKGROUND OF THE INVENTION 
   Present telecommunication system technology includes a wide variety of wireless networking systems associated with both voice and data communications. An overview of several of these wireless networking systems is presented by Amitava Dutta-Roy,  Communications Networks for Homes , IEEE Spectrum, pg. 26, December 1999. Therein, Dutta-Roy discusses several communication protocols in the 2.4 GHz band, including IEEE 802.11 direct-sequence spread spectrum (DSSS) and frequency-hopping (FHSS) protocols. A disadvantage of these protocols is the high overhead associated with their implementation. A less complex wireless protocol known as Shared Wireless Access Protocol (SWAP) also operates in the 2.4 GHz band. This protocol has been developed by the HomeRF Working Group and is supported by North American communications companies. The SWAP protocol uses frequency-hopping spread spectrum technology to produce a data rate of 1 Mb/sec. Another less complex protocol is named Bluetooth after a 10 th  century Scandinavian king who united several Danish kingdoms. This protocol also operates in the 2.4 GHz band and advantageously offers short-range wireless communication between Bluetooth devices without the need for a central network. 
   The Bluetooth protocol provides a 1 Mb/sec data rate with low energy consumption for battery powered devices operating in the 2.4 GHz ISM (industrial, scientific, medical) band. The current Bluetooth protocol provides a 10-meter range and an asymmetric data transfer rate of 721 kb/sec. The protocol supports a maximum of three voice channels for synchronous, CVSD-encoded transmission at 64 kb/sec. The Bluetooth protocol treats all radios as peer units except for a unique 48-bit address. At the start of any connection, the initiating unit is a temporary master. This temporary assignment, however, may change after initial communications are established. Each master may have active connections of up to seven slaves. Such a connection between a master and one or more slaves forms a “piconet.” Link management allows communication between piconets, thereby forming “scattenets.” Typical Bluetooth master devices include cordless phone base stations, local area network (LAN) access points, laptop computers, or bridges to other networks. Bluetooth slave devices may include cordless handsets, cell phones, headsets, personal digital assistants, digital cameras, or computer peripherals such as printers, scanners, fax machines and other devices. 
   The Bluetooth protocol uses time-division duplex (TDD) to support bi-directional communication. Spread-spectrum technology or frequency diversity with frequency hopping permits operation in noisy environments and permits multiple piconets to exist in close proximity. The frequency hopping scheme permits up to 1600 hops per second over 79 1-MHZ channels or the entire ISM spectrum. Various error correcting schemes permit data packet protection by ⅓ and ⅔ rate forward error correction. Further, Bluetooth uses retransmission of packets for guaranteed reliability. These schemes help correct data errors, but at the expense of throughput. 
   The Bluetooth protocol is specified in detail in  Specification of the Bluetooth System , Version 1.0A, Jul. 26, 1999, which is incorporated herein by reference. 
   In wireless communication systems such as mentioned above, the well-known disadvantageous phenomenon of fading is encountered. 
   In a Bluetooth SCO (Synchronous Connection-Oriented) link, essentially used for voice communications, a packet of type HV1 can be used. This packet has 80 bits of data that are encoded using a ⅓ repetition code to produce 240 bits of coded data. In the receiver a majority decoding scheme is applied to decode the 80 bits. Since the communications link here is a wireless link and hence subject to fading, most of the errors will occur in packets subjected to severe fading. 
   It is therefore desirable to improve the quality of communication provided by a wireless communication channel that is subject to fading. 
   According to the invention, a bit sequence is transmitted over a wireless communication channel a plurality of times, and the receiving end can determine the transmitted bit sequence (1) by making a majority logic decision with respect to the received bit sequences or (2) based on the received bit sequences and corresponding quality information associated with the respective transmissions. Quality indicators associated with the respective transmissions can be compared or otherwise used in combination to determine the received bit sequence. The invention advantageously applies the effect of repetition coding across a plurality of transmissions, and thereby provides more gain in fading channels than prior art schemes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  diagrammatically illustrates pertinent portions of exemplary embodiments of a wireless packet transmitting station according to the invention. 
       FIG. 2  diagrammatically illustrates pertinent portions of exemplary embodiments of a wireless packet receiving station according to the invention. 
       FIG. 3  illustrates exemplary operations which can be performed by the receiving station of  FIG. 2 . 
       FIG. 4  diagrammatically illustrates pertinent portions of exemplary embodiments of another wireless packet receiving station according to the invention. 
       FIG. 5  illustrates exemplary operations which can be performed by the receiving station of  FIG. 4 . 
       FIG. 6  diagrammatically illustrates pertinent portions of exemplary embodiments of another wireless packet receiving station according to the invention. 
       FIG. 7  illustrates exemplary operations which can be performed by the receiving station of  FIG. 6 . 
       FIG. 8  diagrammatically illustrates pertinent portions of exemplary embodiments of another wireless packet receiving station according to the invention. 
       FIG. 9  illustrates exemplary operations which can be performed by the receiving station of  FIG. 8 . 
       FIG. 10  diagrammatically illustrates pertinent portions of exemplary embodiments of a wireless packet receiving station according to the invention. 
       FIG. 11  illustrates exemplary operations which can be performed by the embodiments of  FIG. 10 . 
       FIG. 12  illustrates simulation results achieved according to the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  diagrammatically illustrates exemplary embodiments of a wireless packet transmitting station according to the invention. The transmitting station of  FIG. 1  transmits each packet N times (an original transmission+N−1 retransmissions). In the example of  FIG. 1 , packet  1  is initially loaded into a buffer  11 , and is sequentially transmitted by a wireless transmitter  13  over a wireless communications link  15  for a total of N transmissions. When the transmitter  13  has completed the Nth transmission, a signal  17  is output from the transmitter  13  in order to load the next packet, namely packet  2  into the buffer  11 . Thereafter, packet  2  is transmitted N consecutive times, and the process is repeated for each packet in the packet sequence designated generally at  14 . Also as shown in  FIG. 1 , each retransmission of a given packet, such as packet  1 , can be performed at a different transmit frequency (f 1 , f 2  . . . fN) thereby advantageously achieving diversity gain. As one example, the transmitting station of  FIG. 1  can be a Bluetooth master device wherein, for example, all transmissions of packet  1  are directed to the same slave device during respective time slots of a conventional Bluetooth SCO link. As another example, the transmitting station of  FIG. 1  could be a Bluetooth slave device wherein, for example, all transmissions of packet  1  are directed to an associated master device during respective time slots of a Bluetooth SCO link. 
     FIG. 10  diagrammatically illustrates pertinent portions of exemplary embodiments of a wireless packet receiving station according to the invention. The packet receiving station of  FIG. 10  includes a transmitted bit sequence determiner  100  which receives (via an unillustrated wireless communications interface) N received bit sequences which each correspond to a transmitted bit sequence that has been included in each of N packets transmitted, for example, by the transmitting station of  FIG. 1 . The transmitted bit sequence determiner also receives communication quality information respectively corresponding to the N packet transmissions (and thus to the N received bit sequences). The transmitted bit sequence determiner  100  then makes a determination as to the transmitted bit sequence, based on the N received bit sequences and the corresponding communication quality information. In some embodiments the determiner  100  compares the communication quality information associated with the N received bit sequences, and thereby makes the determination of the transmitted bit sequence. In other embodiments the determiner  100  uses the communication quality information associated with the N received bit sequences to combine the N received bit sequences and thereby make the determination of the transmitted bit sequence. In still other embodiments, the determiner can include majority logic that applies a majority logic operation to the received sequences on a per bit basis. The bit-by-bit decisions of the majority logic operation constitute the determination of the transmitted bit sequence. The communication quality information is therefore not used in majority logic embodiments. 
     FIG. 11  illustrates exemplary operations which can be performed by the wireless packet receiving station illustrated in  FIG. 10 . At  110 , the received bit sequences are produced. The communication quality information associated with the received bit sequences is obtained at  111 . At  112 , a determination of the transmitted bit sequence is made based on the received bit sequences and the communication quality information. The use of majority logic to produce the determination of the transmitted bit sequence is illustrated at  113 . 
   As will be evident hereinbelow, further exemplary embodiments of the present invention, described relative to  FIGS. 2–9 , advantageously incorporate inventive features described above with respect to  FIGS. 10 and 11 . 
   Assume that a packet transmitted over a wireless communication link includes a predetermined bit sequence. For each bit of the transmitted sequence, if the probability of receiving that bit in error is known, then the optimal receiver can be derived from principles of information theory. In particular, the goal is to maximize the a priori probability of receiving the transmitted sequence correctly. Thus, the optimal receiver would be: 
                   max   x     ⁢     p   ⁡     (     r   ❘   x     )               (   1   )               
where r represents the received bit sequence and x represents the transmitted bit sequence (r and x can each include one or more bits) and p(r|x) represents the probability that the transmitted sequence x results in the received sequence r at the receiving end.
 
   Because the respective probabilities associated with N transmissions of a packet (e.g., packet  1  in  FIG. 1 ) are independent, Expression 1 above can be written as follows: 
   
     
       
         
           
             
               
                 
                   
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   where r 1 , r 2  . . . r N  represent the received sequences respectively associated with packet transmissions  1  through N. The desired transmitted sequence x is that which maximizes the product of probabilities in Expression 2. 
   The probabilities in Expression 2 above would generally depend on the respective signal-to-noise ratios (SNRs), after fading, of the respective packet transmissions. If the SNRs can be estimated, then the probabilities in Expression 2 can be stored in a look-up table (in a suitable memory device) indexed against SNR. These probabilities can be determined, for example, empirically based on experimentation. 
     FIG. 2  diagrammatically illustrates pertinent portions of an exemplary embodiment of a wireless packet receiving station which can implement the optimal receiver represented by Expression 2 above. In  FIG. 2 , a conventional receiver at  20  uses conventional techniques to receive incoming packets transmitted over the wireless communication link  15  by the packet transmitting station of  FIG. 1 . A packet decoder is coupled to the receiver  20 , and can use conventional techniques to decode N transmissions of a given packet. The N decoded packets, respectively including the N received sequences r 1 –r N , are stored in a buffer  22 . The receiver  20  includes a SNR estimator  21  which can use conventional techniques to estimate the SNR associated with each of the N received packets. The estimated SNRs, designated SNR 1 –SNR N , are also input to the buffer  22 . The received sequences r 1 –r N  and their corresponding SNR estimates SNR 1 –SNR N  are input to a look-up table  23 . Also input to the look-up table  23  are all possible transmitted bit sequences, designated as x(i) in  FIG. 2 . If the transmitted sequence is known to be K bits long, then there are 2 K  possible transmitted sequences. Thus the index i in x(i) can take integer values from 1 through 2 K , one value for each possible transmitted sequence. For each of the 2 K  possible transmitted sequences represented by x(i), N corresponding probabilities are (stored in and) obtained from the look-up table  23 , one probability for each of the N packets. 
   For example, given a received sequence such as r 1  and its associated SNR estimate SNR 1 , and given a selected one of the 2 K  possible transmitted sequences x(i), for example x(4), a predetermined probability associated with the received sequence r 1  and its corresponding estimate SNR 1  and the possible transmitted sequence x(4) can be retrieved from the look-up table  23 . The remaining N−1 probability values associated with x(4) correspond to r 2 –r N  and their respective estimates SNR 2 –SNR N . The N probability values obtained from table  23  are multiplied by multiplier  24  to produce a product P(i) (P(4) in this example) of the N probabilities associated with the particular possible transmitted sequence x(i) (see also Expression 2). This product P(i) is stored in a buffer  25  along with products of probabilities associated with the other possible transmitted sequences x(i). Thus, the buffer  25  includes 2 K  products, one for each possible transmitted sequence x(i). These 2 K  products P(1) . . . P(2 K ) in the buffer  25  are input to a maximum determiner  26  that determines which of the 2 products is the largest, and outputs a signal  27  indicative of the largest product. This signal  27  is applied to a selector  28  to select the corresponding one of the 2 K  possible transmitted sequences x(1), . . . x(2 K ). The sequence selected by the selector  28  is taken to be the transmitted sequence x, and can be provided for use by a communication application. 
     FIG. 3  illustrates exemplary operations which can be performed by the receiving station of  FIG. 2 . After a packet is received at  31 , it is decoded at  32 , and its associated SNR is estimated. The operations at  31  and  32  are repeated until it is determined at  33  that all transmissions of the packet have been received. Thereafter at  34  the product of probabilities P(i) is determined based on the received sequences (r 1 –r N ), their estimated SNRs (SNR 1 –SNR N ), and the i th possible transmitted sequence (x(i)). As shown at  35  and  36 , the operations at  34  are repeated until all of the 2 K  possible transmitted sequences x(i) have been considered. Thereafter at  37 , the possible transmitted sequence x(i) having the largest associated product P(i) is selected. 
     FIG. 4  diagrammatically illustrates pertinent portions of a further exemplary embodiment of a wireless packet receiving station according to the invention. The packet receiving station of  FIG. 4  includes a conventional receiver  41  which can use conventional techniques to receive packets transmitted over the wireless communication link  15  by the packet transmitting station of  FIG. 1 . A conventional packet decoder  42  is coupled to the receiver  41  for decoding each of the N packets received from the transmitting station. The N decoded packets, respectively including received bit sequences r 1 –r N , are stored in a buffer  44  coupled to an output of the decoder  42 . 
   The N packets received from the transmitting station are also input to a correlator  43  coupled to the receiver  41 . The correlator  43  can use conventional techniques to provide respective correlation values (e.g., maximum correlation values) for the N received packets. These correlation values, designated as α 1 –α N  in  FIG. 4 , are stored in a buffer  45  coupled to an output of the correlator  43 . The correlator  43  can correlate with the longest known part of the received packets, for example a known header portion. In Bluetooth embodiments, the correlator can perform conventional sync word correlation to produce the correlation values α 1 –α N . The correlation values stored in the buffer  45  are input to a maximum determiner  46  which determines the largest of the correlation values α 1  and α N  and outputs a signal  47  indicative thereof. The signal  47  controls a selector  48  appropriately to select the one of the buffered bit sequences r 1 –r N  that corresponds to the largest of the correlation values α 1 –α N . The selected received bit sequence is taken to be the transmitted sequence x, and can be provided to a communication application. 
   The wireless packet receiving station illustrated in  FIG. 4  can be, for example, a Bluetooth packet receiving station such as a Bluetooth master device or a Bluetooth slave device. In Bluetooth embodiments, the sequence x output from the selector  48  can be produced by applying the sign function to the selected one of the received bit sequences r 1 –r N , that is, sign (selected one of bit sequences r 1 –r N ). 
     FIG. 5  illustrates exemplary operations which can be performed by the wireless packet receiving station of  FIG. 4 . At  51 – 53 , each of the N transmitted packets is received and decoded, and the corresponding N correlation values are obtained. Thereafter at  54 , the received bit sequence that has the largest associated correlation value is selected. 
     FIG. 6  diagrammatically illustrates pertinent portions of further exemplary embodiments of a wireless packet receiving station according to the invention. The packet receiving station of  FIG. 6  can produce and buffer the received bit sequences r 1 –r N  and the correlation values α 1 –α N  in generally the same fashion as described above with respect to  FIG. 4 . The received bit sequences r 1 –r N  and the correlation values α 1 –α N  are input to a combiner  61  which combines the received bit sequences and associated correlation values as follows: 
                   ∑     j   =   1     N     ⁢              α   j          2     ⁢     r   j               (   3   )               
In this combining operation, the correlation values α 1 –α N  are essentially used as estimates of the fading amplitudes respectively associated with the N transmitted packets. In some embodiments, |α j | can be used in Expression 3 instead of |α j | 2 . The output of the combiner  61  is coupled to a conventional packet decoder  63 , which decodes the output of combiner  61  to produce the receiving station&#39;s determination of the desired transmitted sequence x.
 
     FIG. 7 , when considered in conjunction with  FIG. 5 , illustrates exemplary operations which can be performed by the packet receiving station of  FIG. 6 . As can be seen from  FIGS. 5 and 7 , after the N packets have been received, decoded and correlated at  51 – 53  in  FIG. 5 , the correlation values are combined at  71  with the received bit sequences from the corresponding decoded packets. Thereafter at  72 , the result of the combining operation  71  is decoded, after which operations can return to  51  in  FIG. 5 . 
     FIG. 8  diagrammatically illustrates pertinent portions of further exemplary embodiments of a wireless packet receiving station according to the invention. In the packet receiving station of  FIG. 8 , the correlation values α 1 –α N  can be obtained and buffered in generally the same fashion as described above with respect to  FIGS. 4 and 6 . However, in the packet communication station of  FIG. 8 , the N packets received by receiver  41 , including bit sequences y 1 –y N  corresponding to the transmitted bit sequence x, are buffered at  81  without decoding. The bit sequences y 1 –y N  are then provided to a combiner  82  along with the correlation values α 1 –α N . The combiner  82  combines the bit sequences y 1 –y N  with the correlation values α 1 –α N  as follows: 
                   ∑     j   =   1     N     ⁢              α   j          2     ⁢       y   j     .               (   4   )               
In some embodiments, |α j | can be used in Expression 4 instead of |α j | 2 . The combiner  82  includes an output coupled to a conventional packet decoder  83 . The packet decoder  83  decodes the output of the combiner  82 , thereby producing the receiving station&#39;s determination of the desired transmitted sequence x.
 
     FIG. 9  illustrates exemplary operations which can be performed by the packet receiving station of  FIG. 8 . At  91 – 93 , all N packets are received, and their corresponding correlation values are obtained. Thereafter at  94 , the correlation values are combined with the bit sequences received in the corresponding packets. Thereafter at  95 , the result of the combining operation  94  is decoded. 
   The exemplary wireless packet receiving stations illustrated in  FIGS. 6 and 8  can be, for example, Bluetooth master and slave devices. In such Bluetooth embodiments, the outputs of the combiners  61  and  82  can represent the sign function applied respectively to Expressions 3 and 4 above, namely, sign 
           (       ∑     j   =   1     N     ⁢              α   j          2     ⁢     r   j         )         
and sign
 
   
     
       
         
           
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     FIG. 12  illustrates simulation results which show examples of the performance of the embodiments of  FIGS. 4 and 5  ( 101 ), the embodiments of  FIGS. 6–9  ( 102 ), and majority logic embodiments ( 103 ). As shown in  FIG. 10 , the embodiments of  FIGS. 4–9  generally provide 1.5 dB of gain at a bit error rate of 10 −3 . 
   It will be recognized by workers in the art that the embodiments of  FIGS. 1–11  can be readily implemented, for example, by suitable modifications in software, hardware, or a combination of software and hardware, in conventional wireless packet transmitting and receiving stations, for example Bluetooth master and slave devices. 
   Although exemplary embodiments of the invention are described above in detail, this does not limit the scope of the invention, which can be practiced in a variety of embodiments.