Abstract:
A mobile handset is able to receive and transmit to base stations operating in compliance with different standards, for example CDMA, TDMA or AMPS. The mobile handset converts the received signal down to baseband and digitizes only a single channel at a time rather than the whole band. The channel chosen is the biggest of the various transmission systems used. For example, the channel widths for CDMA, TDMA and AMPS are, respectively 1.25 MHz, 30 KHz and 30 KHz. Accordingly to deal with these three systems a 1.25 MHz channel is digitized. Thereafter a digital signal processor determines which of the three systems is being used and appropriate processing and demodulation is carried out.

Description:
FIELD OF THE INVENTION 
     This invention relates to wireless and particularly but not exclusively to radio receivers used in mobile communication systems. 
     BACKGROUND OF THE INVENTION 
     There are currently many different radio standards in existence in North America and elsewhere. For example, in North America the frequency band 824-894 MHz (824 to 849 MHz for handset transmit and 869 to 894 MHz for handset receive) is reserved for cellular communication systems among which are AMPS (Analog Mobile Phone System) analog cellular defined by the standard EIA/TIA-553 (Electronic Industry Association/Telecommunications Industry Association Standard 553), TDMA (Time Division Multiple Access) digital cellular defined by the standard EIA/TIA/IS-136 (where IS means Interim Standard) and CDMA (Code Division Multiple Access) digital cellular defined by the standard EIA/TIA/IS-95. The frequency band 1850-1990 MHz (1850 to 1910 MHz for handset transmit and 1930 to 1990 MHz for handset receive) is, on the other hand, reserved for PCS (Personal Communications Systems) and the two main standards operating in this band are J-Std-009 which defines upband TDMA and J-Std-008 which defines upband CDMA. 
     A radio receiver designed to operate in compliance with one of the standards is not generally able to operate with any of the other standards. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a single radio receiver which is able to communicate with a plurality of different radio systems. 
     According to one aspect, the present invention provides a radio receiver for reception of radio signals encoded according to a plurality of different protocols but transmitted within a common frequency band, the receiver comprising: at least one downconversion stage for deriving baseband information in a selected frequency band having a width substantially equal to the maximum channel width used in the different protocols; an A/D conversion stage for converting the baseband information in the selected frequency band to digital information; and a digital signal processor for processing the digital information corresponding to the baseband information in the selected frequency band, wherein the digital signal processor first determines whether the digital information corresponds to a single channel of a first particular one of the protocols, if the digital signal processor determines that the digital information corresponds to a single channel of the first protocol the digital signal processor processes the digital information according to the first protocol, if the digital signal processor determines that the digital information does not correspond to a single channel of the first protocol the digital signal processor subsequently filters out a single channel of another particular one of these protocols, determines whether the digital information in the filtered single channel corresponds to the other protocol and, if it does correspond, processes the digital information in the filtered single channel according to the other protocol. 
     In a preferred embodiment the common frequency band is 869-894 MHz and the different protocols are AMPS, CDMA and TDMA, and the selected frequency bandwidth of the baseband information is 1.25 MHz. 
     Alternatively or additionally the common frequency band is 1930-1990 MHz and the different protocols are CDMA and TDMA, and the selected frequency bandwidth of the baseband information is 1.25 MHz. 
     The invention also provides a radio receiver comprising a first receiver portion for reception of radio signals encoded according to a plurality of different protocols but transmitted within a first common frequency band and a second receiver portion connected in parallel to the first receiver portion for reception of radio signals encoded according to a plurality of different protocols but transmitted within a second common frequency band, the first receiver portion comprising: at least one downconversion stage for deriving baseband information in a selected frequency band having a width substantially equal to the maximum channel width used in the different protocols; the second receiver portion comprising: at least one downconversion stage for deriving baseband information in a selected frequency band having a width substantially equal to the maximum channel width used in the different protocols; the radio receiver further comprising: an A/D conversion stage connected to outputs of the first and second receiver portions for converting the baseband information in the selected frequency band to digital information; and a digital signal processor for processing the digital information corresponding to the baseband information in the selected frequency band, whereby the particular protocol used can be discerned and the digital information processed accordingly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of this invention will now be described with reference to the attached drawings in which: 
     FIG. 1 is a block diagram of a mobile transceiver constructed according to the present invention; 
     FIG. 2 is a block diagram showing details of the front end of the mobile transceiver of FIG. 1; 
     FIG. 3 illustrates an important aspect of the technique used in the invention; and 
     FIGS.  4   a  and  4   b  are flowcharts illustrating the steps carried out in the DSP algorithm. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a mobile handset  10  has an antenna  11  to which are connected a first transceiver portion  12  and a second transceiver portion  13 . Transceiver portion  12  has a signal input  14  from the antenna, receive outputs  15   a  and  15   b  and transmit inputs  16   a  and  16   b . Similarly transceiver portion  13  has a signal input  18  from the antenna, receive outputs  19   a  and  19   b  and transmit inputs  20   a  and  20   b.    
     Receive outputs  15   a  and  15   b  are connected to respective inputs  22   a  and  22   b  of high speed A/D (analog/digital) converters  24  having an output  25  connected to an input  26  of a high speed DSP (digital signal processor)  27 . The A/D converters  24  incorporate a parallel to series converter (not shown). Receive outputs  19   a  and  19   b  are also connected respectively to inputs  22   a  and  22   b.    
     Transmit inputs  16 a and  16 b are connected to respective outputs  30   a  and  30   b  of high speed D/A (digital/analog) converters  32  having an input  33  connected to an output  34  of the DSP  27 . The D/A converters  32  incorporate a series to parallel converter (not shown). Transmit inputs  20   a  and  20   b  are also connected respectively to outputs  30   a  and  30   b.    
     The DSP  27  has an output  35  connected through an audio D/A converter  36  to an input  37  of the handset audio transducer driver circuit  38 . Audio circuit  38  is connected to a speaker  39  and a microphone  40 . Audio circuit  38  has an output  41  connected through an audio A/D converter  42  to an input  44  of DSP  27 . 
     The handset  10  also has a display  46 , user interface (keypad)  47  and a microcontrol circuit  48  which is interconnected to the display  46  and the user interface  47  so as to control them. The microcontrol circuit is also connected to the DSP  27 . 
     The details of the transceiver portion  12  are not critical to the operation of the invention. Any transceiver which is capable of converting the radio signals received at input  14  down to baseband with the correct channel bandwidth is acceptable. A single conversion type is shown but a dual conversion type could be used instead. In the specific embodiment shown the transceiver comprises a receive side  52  and a transmit side  53  both connected through a duplexer  54  to the signal input  14 . 
     Referring now to FIG. 2, the receive side  52  includes a low noise amplifier  55  connected between the duplexer  54  and a surface acoustic wave (SAW) passband filter  56  the output of which is connected to an input  57  of a mixer  58  which has another input  59  connected to a programmable RF synthesizer  60 . An output  62  of mixer  58  is connected to a SAW channel filter  63  which is in turn connected to an IF amplifier  64 . The output of the IF amplifier  64  is split and connected to the inputs  66  and  67  of two mixers  68  and  69  respectively. The mixers  68  and  69  each have another input  70  and  71  respectively. A phase shifter  73  driven by a local oscillator (LO)  74  supplies an in-phase component of the LO frequency to input  70  of mixer  68  and a quadrature component of the LO frequency to input  71  of mixer  69 . Output  75  of mixer  68  and output  76  of mixer  69  are respectively connected through low-pass filters  77  and  78  which have outputs  79  and  79 ′. A Baseband Inphase Rx signal as will be explained below is derived at output  79  and a Baseband Quadrature Rx signal is derived at output  79 ′ as will also be explained below. Outputs  79  and  79 ′ correspond respectively to outputs  15   a  and  15   b  of FIG.  1 . 
     Continuing to refer to FIG. 2, the transmit side includes two mixers  80  and  81  each of which has an input  82  and  83  which correspond respectively to inputs  16   a  and  16   b  shown in FIG. 1. A Baseband Inphase Tx signal is supplied to input  82  of mixer  80  and a Baseband Quadrature Tx signal is supplied to input  83  of mixer  81  as will be described below. The programmable RF synthesizer is connected to an input  85  of a phase shifter  86  connected to supply an inphase component of the frequency supplied by the RF synthesizer  60  to a second input  87  of mixer  80  and to supply a quadrature component to a second input  88  of mixer  81 . The output  89  of mixer  80  and the output  90  of mixer  81  are both connected to a SAW passband filter  92  which is in turn connected through a power amplifier  93  to the duplexer  54 . 
     In operation, signals received in antenna  11  are fed through the duplexer  54 , amplified in amplifier  55  and filtered in SAW filter  56 . Assuming the passband of filter  56  is 869-894 MHz, the full receive band of the cellular communication band is received. The filtered signal is then mixed in mixer  58  with a signal generated by the programmable RF synthesizer  60 . The RF synthesizer is programmed to a particular frequency required to select a desired 1.25 MHz channel from the filtered signal passed to input  57  of mixer  58 . Assume the desired channel is the first 1.25 MHz band in the full mobile receive band of 869 to 894 MHz. This means that the first channel lies between 869 MHz and 870.25 MHz within the full band. In this case the synthesizer  60  would be programmed to generate a frequency of 969 MHz. The output signal of mixer  58  is a combination of the sum and difference of the two signals being mixed, i.e. 969+869 MHz=1838 MHz and 969−869 MHz=100 MHz. The SAW filter  63  is centered on the intermediate frequency of 100 MHz and has a bandpass function which will pass only 1.25 MHz. Thus, the 100 MHz signal is passed and the 1838 MHz signal is rejected. Since the SAW bandwidth is 1.25 MHz, only one channel is passed. 
     The 100 MHz intermediate frequency is amplified in amplifier  64  and the amplified signal is mixed in mixer  68  with a 100 MHz signal from oscillator  74 . The difference signal which is obtained at the output  75  of mixer  68  is a 1.25 MHz baseband channel, i.e., a channel extending essentially from DC to 1.25 MHz. The sum signal is removed by the low-pass filter  77 . 
     The 100 MHz intermediate frequency signal is also mixed down to a 1.25 MHz baseband channel in mixer  67  using the quadrature component of the 100 MHz LO signal from phase shifter  73 . Again, the sum signal is removed by filter  78  leaving the difference signal. Thus, at output  79  there is derived a 1.25 MHz Baseband Inphase Rx signal and at output  79 ′ there is derived a 1.25 MHz Baseband Quadrature Rx signal. 
     This above described process of mixing down to baseband can be used to select any one of the 1.25 MHz channels simply by programming the synthesizer  60  to a different frequency. 
     The significance of the 1.25 MHz value may be ascertained from a consideration of FIG. 3. A single channel for the CDMA digital cellular standard is 1.25 MHz wide while for each of the AMPS analog cellular and TDMA digital cellular standards a single channel is 30 KHz wide. FIG. 3 shows that a 1.25 MHz wide channel from the 25 MHz passband is converted down to baseband. Transceiver portion  13  is identical to transceiver portion  12  except that it has a filter passband of 1930-1990 MHz and, if the received signals are in that range, again as shown in FIG. 3, a 1.25 MHz wide channel is converted down to baseband. 
     The 1.25 MHz wide signal is passed through the A/D converters  24  to reproduce the encoded digital information and this is then processed by the DSP  27  to derive the decoded digital data which is passed through the audio D/A converter  36  to audio driver circuit  38  causing an audio signal to be reproduced by speaker  39 . 
     The actual processing steps carried out in DSP  27  will depend on the particular requirements of the cellular operating company. Each cellular operating company will have a preferred sequence of modes that the handset will operate in. For example, a particular cellular operating company may have CDMA service in one area, TDMA service in another area and AMPS in other areas. This company would prefer that the user use CDMA or TDMA if it is available and only use AMPS as a third choice or when roaming into another cellular operating company&#39;s territory. This is the example illustrated in the flowchart of FIGS.  4   a  and  4   b.    
     Referring to FIG.  4   a , with the handset on as indicated at step  100  the program moves to step  101  where a subroutine determines whether or not the received signal is a CDMA signal. As indicated in step  102  the result for the CDMA test may be valid or invalid. If it is valid, the program steps to block  103  for CDMA baseband processing as per EIA/TIA/IS-95, abbreviated to IS-95. 
     If the result of the CDMA test is invalid, the program steps instead to block  104  where another subroutine determines whether or not the received signal is a TDMA signal. Block  105  represents the outcome of this test, i.e., there is a valid or invalid result of the TDMA test. If the test result is valid, the program steps to block  106  for TDMA baseband processing as per EIA/TIA/IS-136, abbreviated to IS-136. 
     If the result of the TDMA test is invalid, the program steps instead to block  107  where a further subroutine determines whether or not the received signal is an AMPS signal. The result of this test is indicated in block  108 . If the test indicates a valid AMPS signal, the program steps to block  109  where AMPS baseband processing in accordance with EIA/TIA-553, abbreviated to EIA-553, is carried out. Otherwise, the program steps to block  110  which indicates that the user is alerted that no service is available. 
     Referring now to FIG.  4   b , the subroutines referred to in blocks  101 ,  104  and  107  will now be described. The subroutine TryCDMA begins by setting the radio front end to the CDMA control channel frequency as indicated at block  112 . The subroutine then steps to block  113  where baseband processing on the received control channel is carried out as per IS-95. The subroutine then steps to block  114  where the control channel is decoded as per IS-95. A decision is then made in block  115  as to whether or not there is valid information on the control channel as per IS-95. This results either in a Set Mode=Valid outcome as shown in block  116  or a Set Mode=Invalid outcome as indicated in block  117 . The subroutine then returns as indicated in block  118  to the main program. 
     The subroutine TryTDMA begins by setting the radio front end to capture the TDMA control channel within the selected 1.25 MHz band as indicated in block  120 . Then, as indicated in block  121 , DSP filtering is performed to separate the TDMA 30 KHz control channel from the incoming 1.25 MHz band. The next step to be carried out is illustrated in block  122 , i.e., baseband processing on the received control channel as per IS-136 is performed. The next step is the decoding of control channel information as per IS-136. A decision is then made in block  124  as to whether or not there is valid information on the control channel as per IS-136. The outcome is either a Set Mode=Valid outcome  125  or a Set Mode=Invalid outcome  126  after which the subroutine returns to the main program. 
     The subroutine TryAMPS begins by setting the radio front end to capture the AMPS control channel within the selected 1.25 MHz band as illustrated at block  130 . Thereafter, DSP filtering is performed to separate the AMPS 30 KHz control channel from the incoming 1.25 MHz band as shown at block  131 . Next, baseband processing is performed on the received control channel as per EIA-553 as shown at block  132 . The next step, illustrated by block  133 , is to decode the control channel information as per EIA-553. A decision is then made in block  134  as to whether or not there is valid information on the AMPS control channel as per EIA-553. If there is, the outcome is a Set Mode=Valid outcome  135  and, if there is not, the outcome  136  is a Set Mode=Invalid. After that the subroutine returns to the main program. 
     It is noted that 14 bits of resolution is chosen for the A/D converters  24  at a sampling rate of 3.2 Mega bits per second. This sampling rate is greater than twice the signal bandwidth, so there will be no loss of information on A/D conversion. The 14 bits of resolution are required in order to handle a reasonably large dynamic range on the incoming baseband information. 
     The data from the A/D converters is then applied to the DSP  27 . Normally the bitstream from the A/D converters  24  is applied to the DSP  27  in a serial bitstream fashion. The bitrate of this serial data stream will be 44.8 Mega bits per second. (A 14 bit parallel interface running at 3.2 MHz to the DSP  27  can also be considered in order to reduce the data rate to the DSP  27 ; however the parallel interface will increase the number of interface pins required). 
     The multiple mode capable radio receiver device will require a very powerful DSP device. Each 14 bit sample at the chosen 3.2 Mb/sec rate will occur every 312 nano seconds. 
     To separate AMPS or TDMA information from the 1.25 MHz channel requires a DSP implementation of a bandpass filter with approximately 70 db&#39;s of rejection 30 KHz removed from the passband. This level of filtering will require a minimum of a 10th order infinite impulse response (IIR) filter or a 256 tap finite impulse response (FIR) filter. The finite impulse response filter has the advantage that it has linear phase response which may be required by the application. 
     If an IIR filter is used, it requires approximately 5 DSP instructions per order for a total of 50 instructions. These 50 instructions must be executed continuously between each sample. Since a sample of 14 bits arrives every 312 nano seconds the minimum baseline DSP performance is 160 MIPS (million instructions per second). This level of performance is within reach of single chip DSP processors in the next few years. For example, the Texas Instruments C54 family of DSP devices has 50 MIPS now and is projected to have approximately 100 MIPS of processing power by the end of 1997. 
     If an FIR filter of 256 taps is used, it would require 256 DSP cycles between each data sample. This would require a minimum baseline performance of 820 MIPS. This level of DSP power would require multiple DSP devices or alternately a hardwired implementation of the FIR filter. 
     In TDMA or AMPS mode, the DSP must first perform the 30 KHz channel separation filtering and the proceed with the baseband processing. In CDMA mode, the channel separation step is bypassed and the DSP proceeds directly to the baseband processing. This baseband processing is specific depending on the mode of operation (AMPS, TDMA or CDMA). The baseband processing stage includes demodulation of the incoming baseband signals. For CDMA, TDMA and AMPS this involves demodulating respectively QPSK (Quadrature Phase Shift Keying) DQPSK (Differential Quadrature Phase Shift Keying) and FM (Frequency Modulation) modulated signals. The total approximate baseband DSP processing requirements are on the order of 5 MIPS for AMPS, 40 MIPS for TDMA, and 60 MIPS for CDMA. 
     According to the invention, A/D conversion and DSP processing are carried out on the maximum channel of interest, and not a wider band. This is in contrast to other software radio techniques which convert a very large section of radio bandwidth (for example the full 25 MHz band) to digital. By performing the A/D conversion on only the relevant information expected, the power consumption of the radio is much lower. (High speed A/D converters and high speed DSP have high power consumption). This reduced power consumption is of prime importance in a handset application. 
     The operation of the transmit side of the mobile handset will now be described. Once the handset has determined whether the base station is transmitting CDMA, TDMA or AMPS as described above, the DSP will determine the required modulation scheme for transmission to the base station. Assuming the base station is CDMA then as indicated above the modulation scheme is QPSK. Thus, audio input from microphone  40  results in digital data being supplied from audio A/D  42  to DSP  27  where it is encoded and together with D/A converters  32  produces Baseband Inphase Tx and Quadrature TX signals at inputs  16   a  and  16   b  of Transceiver  12  of FIG.  1 . 
     Referring now to FIG. 2, the inphase and quadrature transmit signals are mixed with a radio frequency carrier signal generated by synthesizer  60 . 
     The RF carrier is in the transmit range 824-849 MHz and will be selected according to the desired channel. The modulated carrier is then passed through filter  92  and amplified in power amplifier  93  and passed through duplexer  54  to antenna  11 .