Abstract:
A wireless communication arrangement includes a transmitter that transmits a signal having a carrier that repeatedly and sequentially hops through a first sequence of frequencies. A receiver includes a mixer having an antenna signal input for receiving an antenna signal, and a local oscillator for generating a local oscillator signal and providing the local oscillator signal to a local oscillator input of the mixer. The local oscillator signal repeatedly and sequentially hops through a second sequence of frequencies having fewer members than the first sequence of frequencies and the repetition frequency with which the local oscillator signal hops through the second sequence of frequencies is substantially equal to the repetition frequency with which the carrier hops through the first sequence of frequencies. Preferably, the receiver includes an ADC that is sampled at a rate greater than twice the bandwidth of the antenna signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims benefit of U.S. Provisional Application No. 60/949,300 filed Jul. 12, 2007, the entire disclosure of which is hereby incorporated herein by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The subject matter disclosed in this application relates to a multi-band OFDM receiver. 
         [0003]    The WiMedia Alliance has been established to promote wireless multimedia connectivity and interoperability between devices in a personal area network. As part of this mission, the WiMedia Alliance has specified a multi-band OFDM (Orthogonal Frequency Division Multiplexing) radio transmitter that transmits a bit stream at 320 Mbps, 400 Mbps or 480 Mbps using a frequency spreading scheme by which the spectrum from 3.168 GHz to 10.560 GHz is divided into 14 bands each 528 MHz wide and having center frequencies at 3.432 GHz, 3.96 GHz, etc. up to 10.296 GHz. Bands  1 - 12 , having center frequencies from 3.432 GHz to 9.240 GHz are allocated to four band groups, each containing three bands, whereas bands  13  and  14  are allocated to a band group containing just two bands. The carrier hops through the bands sequentially and repeatedly, remaining in each band for an interval of 312.5 ns. Within each band, the carrier is modulated by 100 subcarriers or tones that are sufficiently spaced in frequency to be orthogonal. Each tone has two components in quadrature (I, J) and the two components are modulated in amplitude by four consecutive bits of the incoming bit stream in accordance with a 16-ary quadrature amplitude modulation (16QAM) scheme. Two hundred consecutive bits (50 sets of four consecutive bits) modulate both the 50 lower tones in each band and the 50 higher tones, so that tone k+50 (k=1-50) conveys the same information as tone k. The transmitter is also able to transmit at lower data rates (for example, 200 Mbps, in which case the two components of each tone are modulated in amplitude by two consecutive bits of the bit stream employing QPSK modulation. 
         [0004]    In principle, the receiver ( FIG. 1 ) may recover the signal information from the antenna signal using a mixer  10  that receives a local oscillator signal that hops synchronously with the frequency hopping of the transmitter to downconvert the antenna signal to an intermediate frequency, a low pass filter  11  to remove spurious modulation products, and an analog-to-digital converter  12  for sampling the signal at 1.056 GHz and generating a baseband bitstream. A baseband digital signal processing block  13  recovers the data and provides a control signal that is used to synchronize operation of the local oscillator  14 . Although the arrangement shown in  FIG. 1  is functional, the need to hop the receiver&#39;s local oscillator signal synchronously with the transmitter&#39;s carrier contributes significant complexity to the RF receiver design. 
       SUMMARY OF THE INVENTION 
       [0005]    According to a first aspect of the disclosed subject matter there is provided a wireless communication arrangement comprising a transmitter that transmits a signal having a carrier that repeatedly and sequentially hops through a first sequence of frequencies, and a receiver including a mixer having an antenna signal input for receiving an antenna signal, and a local oscillator for generating a local oscillator signal and providing the local oscillator signal to a local oscillator input of the mixer, wherein the local oscillator signal repeatedly and sequentially hops through a second sequence of frequencies having fewer members than the first sequence of frequencies and the repetition frequency with which the local oscillator signal hops through the second sequence of frequencies is substantially equal to the repetition frequency with which the carrier hops through the first sequence of frequencies. 
         [0006]    According to a second aspect of the disclosed subject matter there is provided a method of operating a wireless transmitter and receiver comprising employing the transmitter to transmit a signal having a carrier that repeatedly and sequentially hops through a first sequence of frequencies, and employing the receiver to mix a received antenna signal with a local oscillator signal that repeatedly and sequentially hops through a second sequence of frequencies having fewer members than the first sequence of frequencies, and wherein the repetition frequency with which the local oscillator signal hops through the second sequence of frequencies is substantially equal to the repetition frequency with which the carrier hops through the first sequence of frequencies. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
           [0008]      FIG. 1  is a schematic block diagram of a receiver suitable for receiving the WiMedia multi-band OFDM signal, 
           [0009]      FIG. 2  is a schematic block diagram of a receiver embodying the subject matter disclosed in this application, 
           [0010]      FIG. 3  is a graph illustrating waveforms that are used in explaining operation of the receiver shown in  FIG. 2 , 
           [0011]      FIG. 4  is a schematic block diagram of a second receiver embodying the subject matter disclosed in this application, 
           [0012]      FIG. 5  is a graph illustrating waveforms that are used in explaining operation of the receiver shown in  FIG. 4 , 
           [0013]      FIG. 6  is a schematic block diagram of a third receiver embodying the subject matter disclosed in this application, and 
           [0014]      FIG. 7  is a graph illustrating waveforms that are used in explaining operation of the receiver shown in  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Referring to  FIGS. 2 and 3 , during each of the four lower band groups, for which the center frequency of the middle band (F 2 ) is Fc and the center frequencies of the lower and upper bands (F 1  and F 3 ) are Fc−528 MHz and Fc+528 MHz respectively as shown by waveform A in  FIG. 3 , the local oscillator signal remains at the frequency Fc rather than hopping with the center frequency of the individual bands. 
         [0016]    Considering first the band F 2 , having a frequency range from Fc−264 MHz to Fc+264 MHz, mixing with the local oscillator signal at Fc translates the antenna signal to the range from −264 MHz to +264 MHz as shown by waveform B in  FIG. 3 . Referring to  FIG. 2 , the analog output signal of the mixer  20  is amplified by a controllable gain element  21  and is supplied to the ADC  22 . The bandwidth of the signal (528 MHz) is such that the signal can be digitized by the ADC using a sampling clock at 1.056 GHz. 
         [0017]    When the analog mixer output signal is converted to digital form by the ADC  12 , by sampling at 1.056 GHz and quantizing the samples, the spectrum of the analog signal is replicated in the digital domain at intervals of 528 MHz as indicated by the dashed line portions of the waveform E in  FIG. 3 . 
         [0018]    The mixer translates the band F 1 , having a frequency range from Fc−792 MHz to Fc−264 MHz, to the range from −792 MHz to −264 MHz and the bandwidth of the signal is still 528 MHz (waveform C). By digitizing the analog mixer output signal, the ADC replicates the spectrum of the analog signal in the digital domain at intervals of 528 MHz. Thus, the ADC replicates the spectrum in the band from −792 to −264 MHz in the band from −264 MHz to +264 MHz as shown by the dashed line portions of waveform F. Similarly, the mixer also translates the band F 3 , having a frequency range from Fc+792 MHz to Fc+264 MHz, to the range from +264 MHz to +792 MHz (waveform D) and the ADC replicates the spectrum in the band from +264 MHz to +792 MHz in the band from −264 MHz to +264 MHz (waveform G). 
         [0019]    By employing a local oscillator signal at Fc for all three bands and sampling at 1.056 GHz, signal power for all three bands is in the analysis range from −264 MHz to +264 MHz and can be processed by the digital portion of the receiver. 
         [0020]    The output signal of the ADC  22  is a bit stream at 1.056 Gb/s and is supplied to a sync detector  23 , which monitors the bit stream for a sync sequence, and to a packet data processor  24 , which recovers payload data packets from the bit stream when the sync detector identifies the sync sequence. In addition, the output signal of the ADC is supplied to an automatic gain control circuit  25  for controlling the gain element  21  in order to normalize the signal amplitude. 
         [0021]    The sync detector  23  supplies a control signal to a mix frequency controller  26 , which controls the frequency of the local oscillator signal so that the frequency of the local oscillator signal matches the center frequency of the middle band in the current band group (containing three bands) or another suitable frequency in the event that the current band group contains a different number of bands. 
         [0022]      FIG. 4  illustrates a development of the receiver shown in  FIG. 2 . In the case of the receiver shown in  FIG. 4 , the ADC  22  oversamples the amplified mixer signal by sampling at twice the Nyquist rate (i.e. at 2.112 GHz). 
         [0023]    The ADC&#39;s sampling rate of 2.112 GHz corresponds to 1.056 MHz complex, which may be considered to be −1.056 GHz and +1.056 GHz, having corresponding Nyquist frequencies of −528 MHz and +528 MHz. 
         [0024]    Referring to both  FIG. 4  and  FIG. 5 , and considering first the band F 2 , sampling at +1.056 GHz detects signal power in the band from 0 (DC) to +528 MHz and sampling at −1.056 GHz detects signal power in the band from −528 MHz to 0. By digitizing the analog mixer output signal using a sampling clock at 1.056 GHz complex, the ADC replicates the spectrum of the analog signal at intervals of 1.056 GHz, as shown by the dashed line portions of waveform B in  FIG. 5 . Similarly, considering the bands F 1  and F 3 , the ADC replicates the spectra of the analog signals at intervals of 1.056 GHz (waveforms C and D). Consequently, the frequency ranges from −528 MHz to −264 MHz and from +264 MHz to +528 MHz contain signal power from both band F 1  and band F 3 . 
         [0025]    Referring to  FIG. 4 , the output signal of the ADC is split into two paths A and B. The signal on path A is supplied via a digital low pass filter  27 A having a cutoff frequency of 528 MHz to one input of a maximum power detector  28 . The signal on path B is translated by +528 MHz by a mixer  29  and the output signal of the mixer is supplied to a digital low pass filter  27 B having a cutoff frequency of 528 MHz. The output of the low pass filter  27 B is supplied to a second input of the maximum power detector  28 . 
         [0026]    If the receiver is currently processing band F 2 , the signal received by the maximum power detector on path A contains signal power over the range from −264 MHz to +264 MHz (waveform E 1 ) and the signal received on path B contains signal power over the range from 0 to +528 MHz (waveform E 2 ). However, the range from +264 MHz to +528 MHz is outside the analysis range of the maximum power detector and consequently the maximum power detector interprets the signal on path A as having greater signal power than that on path B. 
         [0027]    If the receiver is currently processing band F 1  or F 2 , the signal received by the maximum power detector on path A contains signal power over the range from −528 MHz to −264 MHz and from +264 MHz to +528 MHz (waveform F 1 ) and the signal received on path B contains signal power over the range from −264 MHz to +264 MHz (waveform F 2 ). However, the ranges from −528 MHz to −264 MHz and from +264 MHz to +528 MHz are outside the analysis range of the maximum power detector and consequently the maximum power detector interprets the signal on path B as having greater power than that on path A. 
         [0028]    Based on whether the signal on path A or on path B has greater signal power, the maximum power detector is able to distinguish between band F 2  and bands F 1  and F 3 , and detect the transitions from band F 1  to band F 2  and from band F 2  to band F 3 . In this manner, the maximum power detector is able to keep track of the hopping by the transmitter. 
         [0029]    The maximum power detector selects the signal of greater power and supplies that signal to the sync detection block, the packet data processor and the automatic gain control circuit. 
         [0030]      FIG. 6  illustrates a development of the receiver shown in  FIG. 4 . In the case of  FIG. 6 , the ADC samples the output signal of the gain element at 3.168 GHz (corresponding to 1.584 GHz complex), having Nyquist frequencies of −792 MHz and +792 MHz. 
         [0031]    Referring to both  FIG. 4  and  FIG. 6 , and considering first the band F 2 , sampling at +1.584 GHz detects signal power in the band from 0 to +792 MHz and sampling at −1.584 GHz detects signal power in the band from −792 MHz to 0. By digitizing the analog mixer output signal using a sampling clock at 1.584 GHz complex, the ADC replicates the spectrum of the analog signal at intervals of 1.584 GHz, as shown by the dashed line portions of the waveform B in  FIG. 7 . Similarly, considering the bands F 1  and F 3 , the ADC replicates the spectra of the analog signals at intervals of 1.584 GHz (waveforms C and D). 
         [0032]    Referring to  FIG. 6 , the output signal of the ADC is split into three paths A, B and C. The signal on path A is supplied via the digital low pass filter  27 A having a cutoff frequency of 528 MHz to one input of the maximum power detector  28 . The signal on path B is translated by +528 MHz by a mixer  29 B and the output signal of the mixer is supplied to a digital low pass filter  27 B having a cutoff frequency of 528 MHz. The output of the low pass filter  27 B is supplied to a second input of the maximum power detector  28 . The signal on path C is translated by −528 MHz by a mixer  29 C and supplied via a digital low pass filter  27 C to a third input of the maximum power detector  28 . 
         [0033]    If the receiver is currently processing band F 2 , the signal received by the maximum power detector on path A contains signal power over the range from −264 MHz to +264 MHz and the signal received on paths B and C contains no signal power. If the receiver is currently processing band F 1 , the signal received by the maximum power detector on path A contains no signal power, the signal received on path B contains signal power over the range from −264 MHz to +264 MHz (waveform D, where the asterisk indicates frequency translation) and the signal received on path C contains no signal power. Similarly, if the receiver is currently processing band F 3 , the signal received by the maximum power detector on path A contains no signal power, the signal received on path B contains no signal power and the signal received on path C contains signal power over the range from −264 MHz to +264 MHz (waveform F). Thus, the maximum power detector  28  is able to determine, based on which path currently provides the signal of maximum power, whether band F 1 , F 2  or F 3  is currently being received. The maximum power detector selects the signal having the maximum power and directs that signal to the AGC, sync detector and packet data processor. 
         [0034]    It can be shown that in the case of sampling at 1.056 GHz, the noise level is three times that of single band, whereas with sampling a 2.112 GHz, the noise level is 1.67 times that of a single band and when sampling at 3.168 GHz, there is no increase in noise. 
         [0035]    A receiver having the topology shown in  FIG. 6  may employ an ADC that is sampled at 4.224 GHZ. In this case, the transition bands are outside the hopping bands due to additional oversampling. 
         [0036]    It will be appreciated that the invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims, as interpreted in accordance with principles of prevailing law, including the doctrine of equivalents or any other principle that enlarges the enforceable scope of a claim beyond its literal scope. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated. The word “comprise” or a derivative thereof, when used in a claim, is used in a nonexclusive sense that is not intended to exclude the presence of other elements or steps in a claimed structure or method.