Abstract:
Narrow band interference in a received pulsed radio signal is removed by first shifting the narrow band interference to a lower frequency using a non-linear filter, and the passing the shifted signal through a high pass filter before detecting energy in the pulsed radio signal. The pulsed radio signal can be an ultra-wideband signal, and the non-linear filter can use a Teager-Kaiser energy operator.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to interference cancellation, and more particularly to cancelling narrow band interference in pulse radio signals. 
       BACKGROUND OF THE INVENTION 
       [0002]    In pulse radio systems, the transmitted signal encodes information as sharp carrier pulses at center frequencies in logical connex. One form of pulsed radio signals uses ultra-wide band UWB signaling. UWB has traditional applications in non cooperative radar imaging. Other applications include wireless communication, target sensing, sensor networks, precision locating and tracking. 
         [0003]    UWB can have a frequency bandwidth greater than 500 MHz. Instead of being a continuous sinusoidal carrier wave as in conventional radio signals, UWB uses very short pulses. One valuable aspect of pulse-based UWB is that the pulse are very short in space, e.g., less than 23 cm for a 1.3 GHz bandwidth pulse, so most signal reflections do not overlap the original pulse. 
         [0004]    In target sensing applications, signals with a wide bandwidth carry more information when compared to a narrow band system. This enables a finer time resolution, and a higher target and range resolution. As for wireless communications, wide band and ultra-wide band signals enable higher data rates. 
         [0005]    In wireless wide band and ultra-wide band (UWB) systems, performance can be degraded due to co-existing high power narrow band signals. Because UWB systems operate over extremely wide frequency bands, they have to co-exist with other narrow band systems, which may operate at much higher power levels, e.g., see 47 C.F.R 15, SubPart F: Ultra-wideband, and the IEEE 802.15.4a standard. 
         [0006]    In addition, intentional interferers can use high power narrow band signals to jam UWB systems. Even though narrow band interferers operate on a very small fraction of the bandwidth utilized by the wide band systems, they can still saturate front end amplifiers and other circuits of wide band receivers due to their relatively high operating power. Therefore, it is necessary to provide pulsed radio transceivers that can decrease narrow band interference (NBI). 
         [0007]    Conventional systems that deal with NBI can either avoid NBI at the transmitter, or cancel the NBI at the receiver. Up to now, most NBI techniques do not provide an optimum solution, and they all have severe limitations for short pulse radio signals, such as used in UWB systems. 
         [0008]    Most practical NBI avoidance techniques, such as multiband schemes, multicarrier approaches and pulse shaping methods, require accurate information about the center frequency of the NBI, which is not known a priori in most of the situations and needs to be estimated. 
         [0009]    Although a number of digital NBI cancellation techniques are known for wide band systems, none of those approaches are practical for pulsed radio signals systems, because of an extremely high sampling rates and larger dynamic ranges for analog-to-digital converters to resolve the NBI. Conventional techniques are mostly based on employing adaptive filters, which behave similar to notch filters, to adaptively estimate and decrease NBI in the digital domain. Analog notch filtering can only be effective for canceling NBI in pulsed radio signals if exact information about the NBI is known, i.e., the center frequency and power, and that the NBI is fixed and always exists. This is impractical. 
         [0010]    Another method uses a bank of analog notch filters arranged as rake filters at the receiver, which results in receivers that are complex and costly. The complexity increases depending on the number of rake fingers employed. Furthermore, the NBI may exist as a transient signal in the system which makes most of the NBI avoidance and cancellation techniques ineffective. 
         [0011]      FIG. 1  shows a conventional receiver  100  with a matched-filter. A transmitter transmits a signal  111  via an antenna  110 . The signal  113  is received at an antenna  115  of the receiver. The channel has narrow band interference  112  and additive white Gaussian noise (AWGN)  114 . 
         [0012]    The received signal  113  is first passed through a low noise amplifier (LNA)  120 , and a band-pass filter (BPF)  130  to filter out any out-of-band interference and noise. The output of the BPF  130  is correlated  140  with a stored signal template  150 . The correlator output is integrated  160  over a certain interval, typically equal to signal duration, and digitized  170  by sampling. Hypothesis testing  180  is applied onto the digitized correlator output to recover the signal  111 . 
         [0013]    However, if the narrow band interference  112  is present in the received signal  113 , then the matched-filter receiver  100  has to be much more complex in order to estimate the power spectral density (PSD) of the interference, and to use a pre-whitening filter before the matched-filtering operation. 
         [0014]    Moreover, if the interference  112  is sinusoidal, nonlinear receivers, that perform maximum likelihood formulation, need to be implemented to obtain optimal performance, which are also highly complex and do not have any general structure. 
         [0015]    If there is transient narrow band interference in the system with unknown parameters, then none of the above approaches are effective. Another challenge is that the received waveform shape must be perfectly known for the matched-filter to give optimum performance. However, in UWB systems, the waveform of the received signal is completely different from that of the transmitted signal, due to numerous reasons inherent in UWB signals. 
         [0016]      FIG. 2  shows an alternative  200  receiver, which uses self-heterodyne energy-detectors (ED). The transmitter  205  feeds its antenna  210  with the signal  211  to be transmitted. In the ED receiver  200 , the received signal  213  is received at the receiver antenna  215  and then passed through an LNA  220  and a BPF  230 . The output of the BPF  230  is squared  240 , integrated  260 , and converted to digital via sampling  270 . Hypothesis testing  280  is applied onto the digital signal to recover the signal  211 . 
         [0017]    This receiver enables sampling at sub-Nyquist rates without any significant loss in the performance. However, sampling at lower rates introduces a trade-off between receiver complexity and target/range resolution in a target detection application. In the ED receiver, the energy measure of the received signal is 
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         [0000]    where r(t) and r demote the received signal and a length of the integration window, respectively. 
         [0018]    Equation (1) corresponds to the received signal energy normalized to the window length. The received signal r(t) is sampled with a sampling rate t s  to obtain the samples y k , which are compared to a previously defined threshold γ. If the threshold is exceeded, then the signal is present. This can be handled as a binary hypothesis testing problem, in which the interference-plus-noise hypothesis H 0  is tested against a signal-plus-interference-plus-noise hypothesis H 1  as 
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         [0000]                  1   :y   k   =S   k   +i   k   +n   k ,  (2) 
         [0000]    where s k , i k , and n k  denote the unknown signal  211 , the interference  212  and the noise  214  present in the system, respectively. The AWGN noise samples n k  are modeled as a Gaussian random variable with a variance σ n   2 . 
         [0019]    It should be noted that that for an AWGN channel and completely unknown signal parameters, the ED provides the uniformly most powerful (UMP) test, i.e., it gives the optimal detection performance. However, if there is an unknown NBI  212  in the system, ED is not UMP anymore, and signal energy needs to be increased in order to compensate for the performance degradation caused by the NBI. 
       SUMMARY OF THE INVENTION 
       [0020]    The embodiments of the invention provide a method and apparatus for decreasing narrow band interference (NBI) in a pulse radio receiver. The receiver uses energy detection. The receiver is capable of suppressing NBI in the analog domain at a very low additional cost, which makes the receiver practical to implement for transient signal detection in the presence of NBI in ultra-wide band (UWB) systems. 
         [0021]    The invention can be used in both active and passive sensing systems to detect the presence of targets, as well as in wireless communications such as demodulation of on-off keyed (OOK) UWB signals. 
         [0022]    The receiver uses a cascaded non-linear energy operator and a high pass filter (HPF). The non-linear energy operator is a Teager-Kaiser energy operator (TKEO. With the TKEO, the NBI is converted to a low frequency signal, which is then suppressed by a high-pass filter before energy is detected. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a block diagram of a conventional matched-filter based receiver; 
           [0024]      FIG. 2  is a block diagram of a conventional energy detection based receiver; 
           [0025]      FIG. 3  is a block diagram of a receiver with non-linear filtering according to an embodiment of the invention; 
           [0026]      FIG. 4  is a block diagram of a non-linear filter according to an embodiment of the invention; 
           [0027]      FIG. 5  is a schematic of a spectrum of narrow band interference with respect to an ultra-wide band spectrum; and 
           [0028]      FIG. 6  is a graph comparing a performance of the receiver according to the invention to the performance of conventional receivers. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]      FIG. 3  shows a Teager-Kaiser Energy Detector (TKED) receiver  300  according to an embodiment of the invention. The TKED receiver can decrease narrow band interference (NBI) in pulsed wide band and ultra-wide band (UWB) radio signals. 
         [0030]    The receiver  300  include a low noise amplifier  320  connected to a band pass filter (BPF)  330 . 
         [0031]    The output of the BPF is fed to a non-linear filter  400  according to an embodiment of the invention as described in detail below. 
         [0032]    The non-linear filter is followed by a square-law device  350 . The output of the device  350  is integrated  360  and sampled, and then followed by hypothesis testing  380 . The operation of the non-linear filter is described below. 
         [0033]      FIG. 4  shows the non-linear filter  400 . The filter includes a Teager-Kaiser energy operator (TKEO)  401  and a high-pass filter  402  connected sequentially between the antenna and the energy detector. The TKEO operator  401  acts as a spectrum-shifter and ‘moves’ the NBI component of the received signal towards very low frequencies. These low-frequency components resulting from the NBI are then eliminated by the high pass filter  402  before energy detection. The mechanism of the TK operator is discussed in the next section. 
         [0034]    Teager-Kaiser Energy Operator (TKEO) 
         [0035]    The Teager-Kaiser energy operator (TKEO) is a non-linear differential energy operator which was first describe by F. Kaiser, “On a simple algorithm to calculate the ‘energy’ of a signal,” IEEE International Conference on Acoustics, Speech and Signal Processing, vol. 1, pp. 381-384, April 1990, incorporated herein by reference. The TKEO has been used for transient signal detection, but not for NBI cancellation in pulsed radio signal, such as UWB signals. 
         [0036]    Let φ(s(t)) denote a continuous time TK energy operation on an input signal s(t). Then, φ(s(t)) is 
         [0000]      φ( s ( t ))={dot over ( s )}( t ) 2   −s ( t ){umlaut over ( s )}( t ),  (3) 
         [0000]    where {dot over (s)}(t) indicates the first derivative of the signal s(t), and {umlaut over (s)}(t) is the second derivative of the signal s(t). 
         [0037]    If the signal s(t) is a single sinusoidal pulse with amplitude A and frequency f c , then for 
         [0000]        s ( t )= A  cos(2 πf   c   t +θ), and 
         [0000]      φ( s ( t ))= A   2 (2π f   c ) 2 ,  (4) 
         [0000]    where θ is a random phase of a sinusoid uniformly distributed within [0,2 πpi]. 
         [0038]    Thus, the TKEO functions as a frequency-to-DC converter. The output of the TKEO is a DC signal, which is proportional to the square of both the amplitude and the frequency of the pulse, see Equation 4. 
         [0039]    The pulse s(t) is sampled at frequency f s  to obtain discrete-time samples s(n). In discrete time, the TKEO is 
         [0000]      φ( s ( n ))={dot over ( s )}( n ) 2   −s ( n− 1) s ( n+ 1)  (5) 
         [0040]    Note that the TKEO is almost an instantaneous operator because it only needs two adjacent samples. Thus, the TKEO can applied to UWB signals, which have extremely short pulses, on the order of nanoseconds. If s(n)=A cos(Ωn+θ), then 
         [0000]      φ( s ( t ))= A   2 (sin(Ω)) 2 ,  (6) 
         [0000]    where Ω=2πf c /f s . 
         [0041]    TKED Receiver 
         [0042]    As shown in  FIG. 3 , the transmitter  305  feeds its antenna  310  with a pulsed radio signal  311 , e.g., an ultra-wide band signal (UWB). The signal  311  passes through a channel, and arrives at the receiver antenna  315 . The received signal  313  can include NBI  312  and AWGN  314 . 
         [0043]    The received signal  313  is first passed through the LNA  320  and the BPF  330  to remove out-of-band interference and pass only the frequency band of the signal of interest  311 . 
         [0044]    The non-linear filter  400  is applied to the output of the BPF  330 . The output of the filter  400  is then fed into the square-law operator  350 , integrated  360  and converted to discrete signals by sampling at rate t s    370 . 
         [0045]    The binary hypothesis testing  380  is applied on the discrete signal by comparing the test statistics to a threshold γ. 
         [0046]    As shown in  FIG. 4 , the non-linear filter  400  includes the TKEO  401  and the high pass filter (HPF)  402 . 
         [0047]    Effect of the TK Operator on Wide Band Signals and NBI 
         [0048]      FIG. 5  shows a UWB signal  540  and NMI  530  as a function of signal power  510  and frequency  511 . The UWB signal  540  has a low cutoff frequency of f L    521  and a high cutoff frequency of f H    523 . The NBI  530  has a center frequency f i    522 . 
         [0049]    The power level of die NBI is substantially higher than the power level of the UWB signal  540 . Therefore, if the NBI is not suppressed, the received NBI can easily saturate the front end of a conventional UWB receiver. 
         [0050]    The NBI can be modeled in two ways: first, as a single tone interferer of the form s(t)=A i  cos(2πf i t+θ i ), where A i , f i , and θ are the amplitude, frequency, phase of the NBI, respectively; and second as band-limited interference, which is modeled as a zero mean Gaussian random process with power spectral density of 
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         [0000]    where B nbi  and f i  are the bandwidth and the center frequency of the interferer, respectively, and P i  is the power spectral density. In the analysis of the TKED receiver performance, the single tone interference is considered for simplicity of this description. 
         [0051]      FIG. 6  compares signal detection probability (PD)  620  as a function of signal energy to interference plus noise energy ratio (EINR)  610  for a matched-filter receiver  660 , an ED receiver  650 , a TKED receiver  649  without a high-pass filter, and the TKED receiver  630  according to embodiments of invention. 
         [0052]    As stated before, the matched filter design is impractical for decreasing narrow band interference in pulsed radio signals. Clearly, our TKED receiver  300 , also referred to as ‘TK+HPF+ED receiver, outperforms the ED receiver and the ‘TK+ED only’ receiver, which does not include the high pass filter (HPF). 
         [0053]    Although the invention has been described with reference to certain preferred embodiments, it is to be understood that various other adaptations and modifications can be made within the spirit and scope of the invention. Therefore, it is the object of the append claims to cover all such variations and modifications as come within the true spirit and scope of the invention.