Abstract:
In accordance with the present invention, novel methods for adaptive receiver design and related parameter estimation techniques for efficient and non-coherent reception of ultrawideband signals are presented. Efficient estimation of maximum excess delay of the channel for enabling many useful adaptation techniques is additionally provided. Also, noise power estimation which significantly improves the performance of the receivers is presented.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to U.S. provisional patent application No. 60/736,591, entitled: “Efficient and Adaptive Received Design for Ultrawideband Communication,” filed Nov. 14, 2005, by the same inventor, the contents of which are herein incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   As wireless communication systems are making the transition from wireless telephony to interactive internet data and multi-media types of applications, the desire for higher data rate transmission is increasing tremendously. As more and more devices go wireless, future technologies will face spectral crowding and coexistence of wireless devices will be a major issue. Considering the limited bandwidth availability, accommodating the demand for higher capacity and data rates is a challenging task, requiring innovative technologies that can coexist with devices operating at various frequency bands. Ultrawideband (UWB), which has been considered primarily for use with radar applications in the past, offers attractive solutions for many wireless communication areas, including wireless personal area networks (WPANs), wireless telemetry and telemedecine, and wireless sensors networks. With its wide bandwidth, UWB offers a capacity much higher than the current narrowband systems. While conventional narrowband communication systems employ radio frequency (RF) carriers of much higher frequency than the information rate required for transmitting baseband signals, UWB is a carrierless (baseband) transmission system. As such, UWB does not require the necessary up/down conversion of conventional communication systems, resulting in a much simpler solution and less expensive circuitry. Other benefits of UWB include immunity to multi-path effects, high resolution (sub-decimeter range), robustness against eavesdropping, and easier material penetration. 
   Simple, power efficient, low cost, and small sized UWB transceiver design is a challenging task. There are several receivers proposed for UWB communication. Fully coherent receivers, like rake receivers, and correlator receivers perform well but at the expense of extremely high computational and hardware complexity. In general, a coherent receiver requires several parameters regarding the received signal and radio channel. The multipath delays, the channel coefficients for each delayed multipath component and the distortion of the pulse shape need to be estimated for optimal coherent reception. Note that in UWB, the number of multipath components is very large and it is not uncommon for UWB to comprise hundreds of multipath components. Note also that given the total constant transmitted power, the power in each of these multipath components will be very low. Therefore, estimating the delays and coefficients from the received multipath components is an extremely challenging task. Therefore, receivers that relax these estimation requirements would be preferable. 
   Non-coherent receiver designs in UWB relax the amount of information that needs to be estimated accurately for the detection of the transmitted bits. In other words, the synchronization, channel estimation, and pulse shape estimation is not as stringent as in the case of the fully coherent receivers. Some of the non-coherent transceiver designs known in the art include a transmitted reference (TR) based UWB transceivers, an energy detector, and a differential detector. Common to all these non-coherent transceiver designs is that the channel estimation and received pulse estimation are not necessary. Also, the timing estimation is easier and the receiver performance is more immune to the timing mismatch. 
     FIG. 1  illustrates a coherent rake receiver  10  that is well known in the art. With a coherent rake receiver, the received signal energy over several multipath components  20  are captured with the correlators  15  at each finger and these fingers, or correlator outputs, are combined  25  to make a bit decision  30 . The correlator  15  at each finger requires the templates  35  for the received pulse to match to the received signal&#39;s pulse. The correlators  15  in each finger need to be synchronized to the exact received multipath position. In the case of a coherent rake receiver  10  the optimal combining requires the channel coefficient and/or signal-to-noise-ratio estimates for efficient combining. 
     FIG. 2  illustrates three different non-coherent receivers currently known in the art.  FIG. 2(   a ) illustrates TR-based receiver.  FIG. 2(   b ) illustrates an energy detector and  FIG. 2(   c ) illustrates a differential detector. In all these types, the integration time  40  is larger than the duration of the pulse. Often the integration time is on the order of the maximum excess delay of the channel. Note that none of these receivers require a local template to correlate the received signal. The TR-based receiver and differential detector correlate the signal with a delayed version of itself  45 . In a TR-based receiver the delayed version of the signal is the known reference signal. In the differential detector, both of the signals are data, but, the decision is made on the difference of the correlated signal. Note that in non-coherent receivers the analog received signal is sampled after the integrator, often utilizing integrate-dump circuitry. 
   While the integration time in a non-coherent receiver should be large enough to accumulate all the signal energy, the integration should not be too large as to receive excess noise. The optimal integration time depends upon the maximum excess delay of the channel and the noise power. Accordingly, an efficient, accurate and low complex delay spread and/or maximum excess delay estimation is needed in the art to provide a non-coherent receiver that maximizes the data rate for a user while also reducing the inter-pulse interference of the signal. 
   SUMMARY OF INVENTION 
   In accordance with the present invention is provided a method of non-coherent ultrawideband communication, the method including the steps of, receiving an ultrawideband signal comprising a plurality of transmitted symbols at a plurality of parallel correlators, the signal having a predetermined pulse-to-pulse duration, correlating the ultrawideband signal with a delayed version of the ultrawideband signal using the plurality of parallel correlators, integrating the correlated signal utilizing a plurality of parallel integrators to establish a plurality of integrator outputs, each of the plurality of parallel integrators having an predetermined adaptable integration interval, sampling the integrator outputs and averaging the sampled outputs to provide a plurality of power estimates of the signal, estimating a noise power of the ultrawideband channel from the power estimates of the signal, estimating a maximum excess delay of the ultrawideband channel from the power estimates of the signal and combining the sampled integrator outputs based upon the estimated maximum excess delay and the estimated noise power of the channel to identify the transmitted symbols. 
   In an additional embodiment, the ultrawideband signal comprises a multipath delay and the adaptable integration intervals of the plurality of parallel integrators are adapted to cover a portion of the multipath delay. 
   The step of estimating the noise power of the channel may be accomplished utilizing one of two possible methods. A first method applicable when the maximum excess delay of the channel is always less than the pulse-to-pulse duration of the signal. The first method including the steps of integrating the signal utilizing an integration time interval that integrates the signal power from the maximum excess delay to the beginning of the next pulse to allow integration of the noise power of the channel only and repeating the integration step over a plurality of transmitted symbols and pulses and averaging the results of the integration steps to provide an estimate of the noise power. The second method applicable when the pulse-to-pulse duration of the transmitted signal is adapted and on-off keying modulation is employed to transmit the signal. The second method including the steps of inserting a plurality of zero training symbols into the transmitted signal and integrating the signal during the zero training symbols and estimating the noise power from the integrated signal. 
   In a specific embodiment, the estimate of the noise power is used to assist in the step of estimating a maximum excess delay of the channel to improve the accuracy of the maximum excess delay estimate. 
   With the method of the present invention, the pulse-to-pulse duration may be adapted based on the maximum excess delay of the channel to improve the performance and the data rate of the receiver. 
   Combining the sampled integrator outputs is based upon the estimated maximum excess delay and the estimated noise power of the channel further and may additionally comprise coherently combining the sampled integrator outputs. Various methods for combining the sampled integrator outputs are within the scope of the present invention, including, but not limited to, equal gain combining, maximal ratio combining, interference rejection combining, narrowband combining and multi-access interference rejection combining. An additional step may also be included whereby the combining weights for the integrator outputs are determined. 
   As a result of the method of the present invention, the receiver is inherently synchronized to the beginning and to the end of the received transmitted symbol. 
   Accordingly, the present invention provides a method of non-coherent ultrawideband communication, including the steps of receiving an ultrawideband signal having at least one transmitted symbol, correlating and integrating the received signal utilizing a plurality of parallel correlators and a plurality of parallel integrators, each of the plurality of correlators and integrators to capture a portion of the signal and coherently combining the captured portions of the signal to make a decision on the transmitted symbol. 
   The non-coherent ultrawideband receiver employed in accordance with the present invention may be based upon a transmitted-reference type receiver, an energy detector receiver, a differential detector receiver, or another non-coherent ultrawideband receiver known in the art. 
   Due to the high sampling rate requirements and other complexity constraints, many current practical UWB receiver designs are based on non-coherent receivers. These receivers employ analog front-end circuitry before the analog-to-digital-converter (ADC) and baseband signal processing. For example, analog multipliers (for correlating the signal with its delayed version) followed by analog integrate-and-dump circuitry are commonly used for TR-based and differential detectors. In an energy detector, an analog square device followed by analog integrate-and-dump circuit is used. 
   The integration interval in non-coherent receivers should be large enough to accumulate all the signal energy. On the other hand, the integration interval should not be too large to receive excess noise. The optimal integration interval depends on the maximum excess delay of the channel and noise power. Thus providing motivation to develop efficient, accurate and low complex delay spread and/or maximum excess delay estimation methods. 
   In addition, the maximum excess delay affects the pulse-to-pulse duration and if care is not taken inter-pulse-interference (IPI) can degrade the performance of the transceiver performance significantly. A simple IPI scenario is described with reference to  FIG. 4 . The present invention proposes adaptive pulse-to-pulse duration (adaptive frame size) based on the maximum excess delay estimate. This reduces the IPI while maximizing the data rates for a given user. 
   In addition, the present invention includes the use of multiple parallel integrators with shorter integration times instead of using a single integrator with a fixed (non-adaptive) integration time. These multiple integrator outputs will be used to make a decision on the transmitted symbols. The combining of multiple parallel integrators will be based on the maximum excess delay of the channel and noise variance estimation.  FIG. 3  is a graphical illustration of a noisy and a noiseless received signal. Also, these multiple parallel integrator outputs are used for improving the timing synchronization. The parallel integrator outputs track the timing changes and adaptively fine tune to the accurate timing position for improving the performance of the receiver. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which: 
       FIG. 1  is an illustration of a simple rake receiver structure currently known in the art. 
       FIG. 2  is an illustration of three different receiver structures currently known in the art, a TR-based receiver, an energy detector and a differential detector. 
       FIG. 3  is a graphical illustration of a noisy and a noiseless received signal. 
       FIG. 4  is a representation of Inter-pulse interference (IPI) scenario. 
       FIG. 5  is a flow diagram illustrating the method in accordance with the present invention. 
       FIG. 6  is a flow diagram illustrating an embodiment of noise power estimation in accordance with the present invention. 
       FIG. 7  is a flow diagram illustrating an additional embodiment of noise power estimation in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference to  FIG. 5 , in accordance with an embodiment of the present invention a method of non-coherent ultrawideband communication is provided, the method including the steps of, receiving an ultrawideband signal comprising a plurality of transmitted symbols at a plurality of parallel correlators, the signal having a predetermined pulse-to-pulse duration  50 , correlating the ultrawideband signal with a delayed version of the ultrawideband signal using the plurality of parallel correlators  60 , integrating the correlated signal utilizing a plurality of parallel integrators to establish a plurality of integrator outputs, each of the plurality of parallel integrators having an predetermined adaptable integration interval  70 , sampling the integrator outputs and averaging the sampled outputs to provide a plurality of power estimates of the signal  80 , estimating a noise power of the ultrawideband channel from the power estimates of the signal  90 , estimating a maximum excess delay of the ultrawideband channel from the power estimates of the signal  100  and combining the sampled integrator outputs based upon the estimated maximum excess delay and the estimated noise power of the channel to identify the transmitted symbols  110 . 
   The output of a single integrator is not sufficient to estimate maximum excess delay of channel and noise variance. In accordance with the present invention, multiple parallel integrators (with shorter integration times) can be employed to estimate the maximum excess delay of the channel and the noise variance. The integrators are adapted to cover different parts of the multipath delays. Also, an integrator is allowed to cover beyond the maximum excess delay of the channel (mainly for noise variance estimation). From these integrator outputs, decision is made about the maximum excess delay of the channel and noise variance. Note that these decisions are specific to the receiver type. For example, the decision will be different for TR-based scheme than the energy detector. However, the idea can be applied to various types of non-coherent receivers. 
   To make a decision on maximum access delay of the channel, the sampled outputs of the parallel integrators are averaged over several pulses and possible symbols. The averaging will reduce the effect of noise and the integrator outputs will have distributions (different distribution depending on the receiver implementation) with different means (nonzero means). The means are identified by averaging. The mean values provide information about the energy/power of the signal over the parallel integrators. These power/energy estimates are then used to make a decision on noise variance and maximum excess delay estimate. 
   For noise power estimation, two possible embodiments are provided in accordance with the present invention. In one approach, illustrated in  FIG. 6 , where the maximum excess delay of the channel is always less than the pulse-to-pulse duration, an integrator is used that integrates the signal power from the maximal excess delay (at the point where the signal is no longer exists) to the beginning of the next pulse  91 . This way, only the noise power is integrated. It is necessary to average this over many pulses and for many symbols to obtain a more reliable estimate of the noise power  92 . 
   In a second approach for estimating the noise power, illustrated in  FIG. 7 , if the pulse-to-pulse duration is adapted and on-off keying modulation is used at the transmitter, then regularly inserted training symbols of zero (off) can be inserted to the transmitted signal  95 . When the transmitter is OFF (i.e. transmitting zero), then the outputs of the integrators over this symbol time can be used to estimate the noise power  96 . 
   Once the noise power is determined, this information can also be used to help the calculation of the maximum excess delay of the channel. The maximum excess delay can be determined based solely from all the integrator outputs without the knowledge of the noise power estimate. However, a more reliable decision can be made if the noise power estimation is also used in the decision. Once a decision on maximum excess delay of the channel is made, the performance and data rate of the transceiver can be improved by adapting the pulse-to-pulse duration. If the pulse-to-pulse duration is shorter than the maximum excess delay of the channel IPI (inter-pulse interference) will be observed. If it is too large, then the maximal data rate for a user will not be high. Adaptive design will make sure that high data rates are obtained while keeping the IPI at a minimum. 
   The symbol decisions will be based on the combination of these integrator outputs. Note that since digital samples are being used for the combining, optimal combining techniques can be employed, such as maximal ratio combining, interference rejection combining, etc. The power differences at the correlator outputs will be used for efficient combining. Note that as well as the total power, the noise power over all these correlator outputs can be estimated by using 00K modulation and the second noise variance estimation technique mentioned above. This way, the signal-to-noise-ratio in each correlator output can be estimated and maximal ratio combining can be employed efficiently. Taking this one step further, the noise correlation can be estimated across the parallel correlator outputs if the noise also includes interference. The noise correlation can be used for interference rejection combining. 
   The present invention also provides for the tracking of the timing position using the parallel integrator approach. The single integrator approach does not allow for tracking of the timing position. However, a parallel integration approach that includes additional short time integrations beyond the multipath components of the received signal period (i.e. the total combined integration time is larger than the maximum excess delay of channel) will be able to track the fine timing position. Note that additional integrators “early” (before the estimated first multipath component) and “late” (after the last multipath component) are required for efficient tracking. 
   In accordance with the present invention, adaptation is inherent on the collection of multipath components. The multiple energy components, due to the integration of the multipath, are combined adaptively depending on the energy on each of these components and also depending upon the noise as well as the interference power. In general terms, instead of having a single correlator and integrator for the whole maximum excess delay of the channel, the present invention provides for multiple correlators and integrators. Each of these correlator/integrators will try to capture part of the signal that is being received. Then, the multiple signal contributions are coherently combined to arrive at the decision result. 
   In addition, the present invention also adapts the maximum excess delay of the channel. The overall integration is adapted and with the present invention even if the location where the energy block starts and ends in the received signal is unknown, the proposed method in accordance with the present invention automatically adjusts the location. As such, the proposed method also automatically synchronizes with the signal. 
   Also, the interference rejection combining of the integrator outputs provides additional benefits. Interference rejection combining has been applied to multiple antenna systems. The outputs of the integrators are interpreted as antenna elements (which is a very realistic assumption), similar approaches can be implemented here as well. As a result, multiple access interference and narrowband interference capabilities can be introduced. 
   It will be seen that the advantages set forth above, and those made apparent from the foregoing description, are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
   It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween. Now that the invention has been described.