Abstract:
Antenna reception diversity is provided for wireless communications such that a received signal (r) can be produced by combining antenna signals (v i ) with their associated fading amplitudes (α i ) as estimated by a linear receiver ( 32 ). Also, antenna signals (v 1   i ) can be combined with their associated correlation values (α 1   i ) in place of estimated fading amplitudes. Further, inherent characteristics of a non-linear wireless communication receiver can be exploited such that a received signal (r 2 , r 3 ) can be produced without any additional overhead that would otherwise be needed to provide estimated fading amplitudes.

Description:
This application claims the priority under 35 U.S.C. 119(e)(1) of U.S. provisional application No. 60/184,634, filed on Feb. 24, 2000. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to wireless communications and, more particularly, to antenna reception diversity in wireless communications. 
     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 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 “scatternets.” 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. 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 1/3 and 2/3 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. 
     Fading is a well known problem in wireless communications systems such as Bluetooth systems. Antenna reception diversity techniques are conventionally used to overcome fading in wireless communications. With antenna reception diversity, a communication signal is received by a plurality of antennas, and the associated antenna signals are then suitably combined to produce the desired communication signal for the receiver. Antenna reception diversity techniques can therefore improve communication quality in the presence of fading. 
     It is therefore desirable to provide for improved antenna reception diversity in wireless communications systems such as Bluetooth systems. 
     The present invention provides antenna reception diversity wherein the received signal can be produced by combining the antenna signals with their associated fading amplitudes as estimated by a linear receiver. Also according to the invention, the antenna signals can be combined with their associated correlation values in place of estimated fading amplitudes. Further according to the invention, inherent characteristics of the receiver can be exploited such that the received signal is produced without any additional overhead that would otherwise be needed to provide estimated fading amplitudes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 diagrammatically illustrate pertinent portions of a conventional Bluetooth receiver. 
     FIG. 3 diagrammatically illustrates pertinent portions of an exemplary embodiment of a wireless communication receiver according to the invention. 
     FIG. 4 diagrammatically illustrates pertinent portions of a further exemplary embodiment of a wireless communication receiver according to the invention. 
     FIG. 5 illustrates exemplary operations which can be performed by the receivers of FIGS. 3 and 4. 
     FIG. 6 diagrammatically illustrates pertinent portions of a further exemplary embodiment of a wireless communication receiver according to the invention. 
     FIG. 7 diagrammatically illustrates pertinent portions of a further exemplary embodiment of a wireless communication receiver according to the invention. 
     FIG. 8 illustrates exemplary operations which can be performed by the receivers of FIGS. 6 and 7. 
     FIG. 9 illustrates simulation results obtained according to the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 diagrammatically illustrates pertinent portions of a conventional non-linear wireless communication receiver, for example a Bluetooth receiver. As illustrated in FIG. 1, the signal, for example a Bluetooth FSK (frequency shift keying) signal, from a given antenna is input to an intermediate frequency (IF) filter  11 , and the resulting filtered signal is input to a limiter  12 . The output of the limiter is applied to a discriminator  13 , whose output is coupled to a low pass filter (LPF)  14 . 
     FIG. 2 illustrates the discriminator  13  of FIG. 1 in more detail. In the example of FIG. 2, the discriminator  13  is implemented as a delay and multiply circuit which multiplies the limiter output signal by a delayed version of the limiter output signal (see also FIG.  1 ). 
     Optimal antenna selection diversity according to the invention can be achieved by combining the radio frequency (RF) signals (received by a plurality of antennas) after the IF filter  11  and before the limiter  12 . According to one exemplary embodiment of the invention, a linear receiver can be inserted after the IF filter  11  as illustrated generally in the exemplary embodiment of FIG.  3 . All receiver examples described herein assume the use of N antennas and N corresponding RF front ends. 
     The example of FIG. 3 (taken in conjunction with FIG. 1) illustrates at  31  N antenna signals which have been received from N separate antennas and have each passed through an associated IF filter such as shown at  11  in the non-linear receiver of FIG.  1 . The signals at  31  are input to a conventional N-channel linear receiver (or N linear receivers)  32 . The linear receiver  32  can use conventional techniques to estimate fading amplitudes associated with each of the N antennas. For each of the N antenna signals at  31 , the linear receiver can use conventional techniques to estimate fading amplitudes from training symbols, for example the symbols of the Bluetooth synchronization word. The fading amplitudes for each antenna signal are output from the linear receiver  32  to an averager  35  which can average the estimated fading amplitudes associated with each of the antenna signals, and thereby can output N average estimated fading amplitudes. These average estimated fading amplitudes are designated as α i  in FIG.  3 . These average estimated fading amplitudes are input to a combiner  36  along with N corresponding antenna signals v i  produced by N associated non-linear receivers such as the one shown in FIG.  1 . The combiner  36  combines α i  and v i  as follows              r   =       ∑     i   =   1     N                   α   i          2          v   i                 Equation  1                                
     in order to produce the received signal r. The signals v i  are thus ratiometrically combined with respect to the squares of the signals α i . In some embodiments, |α i | 2  is replaced in Equation 1 by |α i |. 
     FIG. 4 diagrammatically illustrates pertinent portions of another embodiment of a wireless communication receiver according to the invention. In the receiver of FIG. 4, N antenna signals which have each passed through an associated IF filter  11 , limiter  12  and discriminator  13  as illustrated in FIG. 1, are input to a bank of N low pass filters  41 . The signals output from the filters  41  can be input to a conventional thresholder  43 , and are also input to a combiner  42 . These signals are designated as v 1   i  in FIG.  4 . The thresholder  43  can be omitted in some embodiments, as shown by broken line. The signals output from the thresholder  43  are applied to a conventional correlator  44  which can use conventional techniques to correlate with any known part of the received signals. In a Bluetooth example, the correlator  44  can correlate with the Bluetooth synchronization word. As another example, the correlator can correlate with a larger part of the received signal, for example a packet header, in situations when the header is fixed (i.e., known). The maximum value of the correlation for each antenna signal is output from the correlator  44  to the combiner  42 . These maximum values, designated as α 1   i  in FIG. 4, can be used as an estimate of the fading amplitude. The combiner  42  combines the signals v 1   i  and α 1   i  as follows              r1   =       ∑     i   =   1     N                     α      1     i          2          v1   i                 Equation  2                                
     to produce the received signal r 1 . The signals v 1   i  are thus ratiometrically combined with respect to the squares of the signals α 1   i . In some embodiments, |α i   1 | 2  is replaced in Equation 2 by |α i   1 |. 
     FIG. 5 illustrates exemplary operations which can be performed by the receivers of FIGS. 3 and 4. The signals from the antennas are received at  51 , and the corresponding fading amplitude information is obtained at  52 . Thereafter at  53 , the fading amplitude information is combined with the antenna signals (e.g., using Equation 1 or 2) to produce the received signal. 
     The present invention recognizes that, if the limiter  12  is removed from the conventional receiver of FIG. 1, it can be shown analytically that the output of the delay and multiply discriminator  13  (see also FIG. 2) has already been multiplied by the square of the fading amplitude associated with that antenna. The exemplary receiver of FIG. 6 exploits this characteristic by coupling the output of the IF filter bank  61  directly to a bank of delay and multiply discriminators (or an N-channel discriminator) at  62 . The discriminator outputs are applied to an LP filter bank  63 , and the resulting LP-filtered signals, designated as v 2   i  in FIG. 6, are applied to a combiner  64 . The combiner combines the v 2   i  signals as follows              r2   =       ∑     i   =   1     N          v2   i               Equation  3                                
     to produce the received signal r 2 . Because the discriminator outputs in FIG. 6 are already multiplied by the square of the corresponding fading amplitude, there is no need to estimate the fading amplitudes in the embodiment of FIG.  6 . However, the multipliers (or N-channel multiplier) at  65  of FIG. 6 need to be real number multipliers, which can be relatively complex to implement. 
     FIG. 7 diagrammatically illustrates pertinent portions of an exemplary embodiment of a wireless communication receiver (e.g., a Bluetooth receiver) which exploits the aforementioned fading amplitude multiplication property of a delay and multiply discriminator, and which also avoids the necessity of implementing a real number multiplier. The embodiment of FIG. 7 is generally similar to the embodiment of FIG. 6, except that limiters (or an N-channel limiter)  71  are inserted into the delay and multiply discriminators between the IF filters  61  and the delay elements (or N-channel delay element)  66 . With this arrangement, the multiplier  65 A is just a real number adder, which is easily implemented, for example, using charge/discharge capacitors. A combiner  72  combines the outputs v 3   i  of the LP filters  63  as follows              r3   =       ∑     i   =   1     N          v3   i               Equation  4                                
     to produce the received signal r 3 . 
     FIG. 8 illustrates exemplary operations which can be performed by the receivers of FIGS. 6 and 7. The antenna signals are received at  81 , and are applied to delay and multiply discriminators at  82 . At  83 , the fading amplitude multiplication characteristic of the discriminators is exploited to combine the antenna signals with the corresponding fading amplitude information. Thereafter at  84 , the discriminator outputs are combined, for example, according to Equation 4. 
     FIG. 9 illustrates exemplary simulation results associated with the receivers of FIGS. 3,  4  and  7  with N=2 antennas. In FIG. 9, the curve  91  is optimum reception diversity with N=2 antennas, the curve  92  is obtained using one antenna, and the curve  93  is obtained using two selectively switched antennas which share a common RF front end. The curve  94  corresponds to the receiver of FIG. 3, the curve  95  corresponds to the receiver of FIG.  4  and the curve  96  corresponds to the receiver of FIG.  7 . 
     It will be evident to workers in the art that the embodiments described above with respect to FIGS. 2-8 can be implemented, for example, by suitable modifications in software, hardware, or a combination of software and hardware, in conventional radio receivers that use antenna reception diversity, for example Bluetooth receivers.