Abstract:
Space time transmit diversity ( 9, 14, 17, 19 ) is applied at the block level to an original block of bits ( 12 ) in order to reduce the effects of fading in wireless communication systems that use nonlinear modulation schemes ( 13, 33 ). At the receiving end, fading parameters (α 1 , α 2 ) are estimated (α E1 , α E2 ) and the properties of complex conjugates are utilized ( 28, 29, 201, 202 ) to produce a result (r 1 , r 2 ) that is representative of the original block of bits.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     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 10th 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.  
         [0002]     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 a maximum asymmetric data transfer rate of 723 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 “scattemets.” 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.  
         [0003]     The Bluetooth protocol uses time-division duplex (TDD) to support bi-directional communication. 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 2.4 GHz 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.  
         [0004]     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.  
         [0005]     In wireless communication systems such as described above, the well-known disadvantageous phenomenon of fading is encountered. Conventional transmit diversity techniques can provide several dB&#39;s of gain to thereby at least partially overcome the fading problem. Some known transmit diversity schemes require an estimate of the channel at the transmitter, which estimate can be made from previous receptions at the same frequency. However, because the operating environment is not totally static, such estimates are sometimes not very accurate.  
         [0006]     Another known technique for overcoming fading is antenna space time transmit diversity. An example of this technique is disclosed in U.S. Ser. No. 09/205,029 (Attorney Docket No. TI-28441), filed on Dec. 3, 1998 and incorporated herein by reference. The space time transmit diversity disclosed therein is bit level space time transmit diversity for use with linear modulation schemes such as QPSK modulation. However, bit level space time transmit diversity cannot be used in wireless communication systems that utilize non-linear modulation schemes, for example the GFSK modulation scheme utilized in Bluetooth systems.  
         [0007]     It is therefore desirable to apply space time transmit diversity techniques in wireless communication systems that utilize nonlinear modulation.  
         [0008]     The present invention applies space time transmit diversity to achieve diversity gains in wireless communication systems that utilize nonlinear modulation schemes. In particular, space time transmit diversity (STTD) is applied at the block level to an original block of bits advantageously to reduce the effects of fading in wireless communication systems that use nonlinear modulation schemes. At the receiving end, fading parameters are estimated and the properties of complex conjugates are utilized to produce a result that is representative of the original block of bits.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  diagrammatically illustrates pertinent portions of exemplary embodiments of a transmitting station that implements block level space time transmit diversity according to the invention.  
         [0010]      FIGS. 2-4  diagrammatically illustrate pertinent portions of exemplary embodiments of a receiving station which implements block level space time transmit diversity according to the invention.  
         [0011]      FIG. 5  diagrammatically illustrates pertinent portions of further exemplary embodiments of a transmitting station according to the invention.  
         [0012]      FIG. 6  illustrates exemplary operations which can be performed by the receiving station of  FIGS. 2-4 .  
         [0013]      FIG. 7  illustrates exemplary operations which can be performed by the transmitting station of  FIG. 1 .  
         [0014]      FIG. 8  illustrates exemplary operations which can be performed by the transmitting station of  FIG. 5 .  
         [0015]      FIG. 9  illustrates the communication performance of a nonlinear modulation wireless communication system with (and without) block level space time transmit diversity according to the invention.  
     
    
     DETAILED DESCRIPTION  
       [0016]      FIG. 1  diagrammatically illustrates pertinent portions of exemplary embodiments of a transmitting station according to the invention. For example, the transmitting station of  FIG. 1  could be a Bluetooth master or slave device. In  FIG. 1 , input digital information bits are applied to a block formatter  11  which establishes from the input bits an original block of bits having two parts x 1  and x 2 , shown generally at  12 . Parts x 1  and x 2  each include a plurality of bits. The two-part block of bits  12  is input to a nonlinear modulator such as an FSK (or GFSK) modulator  13  which uses conventional techniques to modulate a carrier signal with the block of bits to produce at  14  a modulated block of information including a first part a and a second part b which respectively correspond to the parts x 1  and X 2  at  12 .  
         [0017]     The two-part modulated block at  14  is input to an STTD encoder  15  which outputs at  9  a re-ordered two part block including a first part −b *  which represents the negative of the complex conjugate of the second part b of the modulated block at  14 , and also including a second part a *  which is the complex conjugate of the first part a of the modulated block at  14 . Complex conjugation as described herein can be performed in any desired conventional manner. For example, if m= cos (w c τ−φ(t)), then the complex conjugate m * = cos (w c τ−φ(t)). As another example, the part a *  of  FIG. 1  can be produced by using the negative of the bits that were used to produce the modulated part a in  FIG. 1 , that is, by also modulating −x 1  at  13 . In such embodiments, the modulator  13  can provide both a *  and b * , as shown by broken line in  FIG. 1  (i.e., the modulator  13  performs the complex conjugate function of the encoder  15 ), so the encoder at  15  need only perform the re-ordering and negation operations.  
         [0018]     The block output at  9  from the STTD encoder  15  is applied to a transmit processing section  17 , and the modulated block  14  output from the modulator  13  is applied to another transmit processing section  19 . The transmit processing sections  17  and  19  utilize conventional transmit processing techniques to effect transmission of the blocks  9  and  14  across a wireless communication link  18  (e.g., a Bluetooth link) via respective antennas  10  and  16 . In the example of  FIG. 1 , transmission of the part a  *  of block  9  corresponds in time with transmission of the part b of block  14 , and transmission of the part −b *  of block  9  corresponds in time with transmission of the part a of block  14 , thereby providing space and time transmit diversity. Also as shown in  FIG. 1 , the wireless communication channel associated with antenna  10  has a fading parameter designated as α 1 , and the wireless communication channel associated with antenna  16  has a fading parameter designated as α 2 .  
         [0019]      FIGS. 2 and 3  diagrammatically illustrate pertinent portions of exemplary embodiments of a receiving station according to the invention. For example, the receiving station could be a Bluetooth master device or a Bluetooth slave device. As shown in  FIG. 2 , the wireless communication signals transmitted by antennas  10  and  16  of  FIG. 1  are received at an antenna  20  of a wireless communication interface. The antenna  20  is coupled to a receive processing section  21  of the wireless communication interface, which utilizes conventional receive processing techniques to produce from the received antenna signals a block of information including a first part c and a second part d, as designated generally at  22 . The block at  22  is input to a separator  23  which separates the block into its constituent parts c and d. Recalling from  FIG. 1  that blocks  9  and  14  were transmitted via the respective antennas  10  and  16  having the respective fading parameters α 1  and α 2  associated therewith, the parts c and d in  FIG. 2  can be expressed as follows: 
   c=α   1   a−α   2   b   *   Equation 1    d=α   1   b+α   2   a   * ,  Equation 2         where the superscript “*” denotes the complex conjugate.          
         [0021]     The receiving station can use conventional techniques to produce estimates α E1  and α E2  of the respective fading parameters α e1  and α e2 . For example, a fading parameter estimater shown generally at  200  can be a conventional linear receiver, which provides fading parameter estimates in its normal operation. The fading parameter estimates α E1  and α E2  can be determined, for example, based on earlier transmissions received individually from the respective antennas  10  and  16 , and can be stored in a suitable database (not explicitly shown). Using the estimated fading parameters, the following two signals can be formed: 
 
 r   1 =α E1   *   c+α   E2   d   *   Equation 3 
 
 r   2 =−α E2   c   * +α *   E1   d .  Equation 4 
 
         [0022]     In  FIG. 2 , the multiplier pairs at  24  and  26 , the adders at  25  and  27 , and the complex conjugators at  201 ,  202 ,  28  and  29  form a combiner that combines the parts c and d with the estimates α E1  and α E2  to produce the combined result signals r 1  and r 2  as defined by Equations 3 and 4 above. Combining Equations 3 and 4 with Equations 1 and 2, utilizing the symmetry of complex conjugates, and assuming that α E1 ≈α 1 , and α E2 ≈α 2  the signals r 1  and r 2  can be expressed as follows: 
 
 r   1 =(|α E1 | 2 +|α E2 | 2 ) a   Equation 5 
 
 r   2 =(|α E1 | 2 +|α E2 | 2 ) b   Equation 6 
 
         [0023]     As illustrated in  FIG. 3 , the signals r 1  and r 2  are input to a block formatter  31  which formats the signals r 1  and r 2  into a block of information at  32  having a first part r 1  and a second part r 2 . This two-part block at  32  is input to a nonlinear (such as an FSK or GFSK) demodulator  33  which uses conventional demodulation techniques to produce at  34  the receiving station&#39;s determination of the original block of bits (see also  12  in  FIG. 1 ).  
         [0024]      FIG. 4  diagrammatically illustrates further exemplary embodiments of a receiving station according to the invention. In the embodiment of  FIG. 4 , the signals r 1  and r 2  from  FIG. 2  are input to respective parallel nonlinear demodulators  41  which use conventional demodulation techniques to produce the receiving station&#39;s determination of the first and second parts x 1  and X 2  of the original block of bits (see also  12  in  FIG. 1 ).  
         [0025]      FIG. 5  diagrammatically illustrates pertinent portions of further exemplary embodiments of a transmitting station according to the invention, which can be used in conjunction with the receiving station of  FIG. 4 . In the embodiment of  FIG. 5 , the input bits are applied to a separator  51  which produces therefrom the first and second parts x 1  and x 2  (see  12  in  FIG. 1 ) in parallel format, thus providing the original block of bits in parallel format. The parts x 1  and x 2  are applied to respective parallel nonlinear modulators  54 , which utilize conventional modulation techniques to produce a modulated block in parallel format, including the parts a and b which respectively correspond to the parts x 1  and x 2  (see also  FIG. 1 ). The parts a and b are input to a block formatter  55 , which produces at  56  a block of information including the parts a and b, which block can then be input to the STTD encoder  15  and the transmit processing section  19  of  FIG. 1 . Thus, the embodiment of  FIG. 5  provides an alternative arrangement for producing the two-part modulated block illustrated generally at  14  in  FIG. 1 .  
         [0026]      FIG. 6  illustrates exemplary operations which can be performed by the receiving stations of  FIGS. 2-4 . After a block of information is received at  61 , the block is divided (separated) into two parts at  62 , and the signals r 1  and r 2  are formed at  63 . Thereafter, the signals r 1  and r 2  are demodulated (and downscaled) at  64  to produce the receiving station&#39;s determination of the original block of bits.  
         [0027]      FIG. 7  illustrates exemplary operations which can be performed by the transmitting station of  FIG. 1 . At  71 , a two-part block of bits is established (see  12  in  FIG. 1 ). At  72 , the two-part block of bits is modulated to produce a two-part original modulated block (see  14  in  FIG. 1 ). At  73 , the two parts of the original modulated block are re-ordered and complex-conjugated, and at  74  one of the re-ordered, complex-conjugated parts is negated, thereby producing an STTD block (see also  9  in  FIG. 1 ). At  75 , the original modulated block and the STTD block are transmitted (in the timewise relationship described above) using respective first and second antennas.  
         [0028]      FIG. 8  illustrates exemplary operations which can be performed by the transmitting station of  FIG. 5 . At  81 , the input bits are separated into two parts (original block in parallel format). At  82 , the two parts are respectively modulated to produce two corresponding modulated parts (modulated block in parallel format). At  83 , the modulated parts are formatted into a two-part modulated block. From this point, operations can proceed to  73  in  FIG. 7  (see broken line in  FIG. 7 ) and, after the operations at  75  in  FIG. 7 , operations can return to  81  (see broken line in  FIG. 7 ).  
         [0029]      FIG. 9  illustrates exemplary simulation results for a conventional nonlinear-modulated wireless communication system at  92  and for a nonlinear-modulated wireless communication system which utilizes space time transmit diversity according to the invention at  94 . The curve  94  demonstrates better performance.  
         [0030]     It will be evident to workers in the art that the embodiments of  FIGS. 1-8  can be readily implemented, for example, by suitably modifying software, hardware, or a combination of software and hardware, in conventional wireless transmitting and receiving stations that support plural transmit antennas, for example Bluetooth master and slave devices.  
         [0031]     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.