Method and apparatus for improving signal reception in a receiver

A method and apparatus for improving signal reception in a receiver (100) by performing all-channel and/or on-channel estimations on a received signal so as to predict future RF environments. The prediction is achieved through the use of one or more detector systems (122, 124) positioned to sample and detect predetermined signal metrics of the received signal (103) prior to analog-to-digital conversion (112) and subsequent post-processing (114). Future estimations of the channel condition are thus generated prior to the arrival of the actual samples (115) at a controller section (116). The detectors (122, 124) provide triggers (123, 125) to the controller (116) so that active stages (130) within the receiver (100) can be adjusted and scaled as needed via a serial port interface (SPI) (126) based on signal conditions.

FIELD OF THE INVENTION

The present invention relates generally to communication devices and more particularly to improving signal reception in portable radios.

BACKGROUND

Portable communication devices, such as hand-held two-way radios, cell phones, mobile vehicular radios and the like, must operate in very dynamic radio frequency (RF) environments. Signals received by such devices are often subjected to fading and multi-path envelope variations that can corrupt the received signal, increasing bit error rate (BER) and reducing channel efficiency. Today's error correction strategies utilize protocol centric redundancies or post demodulation error correction to mitigate these problems. Both of these mitigation strategies however, encumber the communication device design with increased protocol complexity and/or demodulator processing requirements, thus making implementation more complex.

Accordingly, there is a need to improve receive signal capability in a portable communication device.

DETAILED DESCRIPTION

In accordance with the present invention, there is provided herein a method and apparatus for improving signal reception in a receiver of a portable or mobile communication device by performing off-channel and on-channel estimations of a received signal so as to predict future RF environments. The prediction is achieved through the use of one or more detector systems positioned to sample and detect predetermined signal metrics of the received signal prior to analog-to-digital conversion and subsequent post-processing. At least two detectors are contained each detector system. Future estimations of the channel condition are thus generated prior to the arrival of the actual samples at a controller section. The detectors provide triggers to the controller so that active stages within the receiver can be adjusted and scaled as needed.

FIG. 1is a block diagram of a receiver100operating in accordance with the present invention. Receiver block diagram100generally includes an antenna102, a pre-selector filter104, a low noise amplifier (LNA)106, a mixer108, an intermediate frequency (IF) filter110, and an analog to digital converter (ADC)112all under control of controller section116, formed here of a digital signal processor118and host microprocessor120. For the purposes of this application and for most receivers in general, the analog hardware located prior to ADC112is generally referred to as the receiver's front-end130. Post processing circuitry114, found in typical receivers after the ADC112, performs such functions as decimation, filtering and formatting of the digital signal, but also creates a latency in the receive signal path.

In accordance with the present invention, receiver100further includes a channel estimator132formed of at least one detector system, shown here as first and second detector systems122,124for detecting all-channel and on-channel signal metrics respectively. The all-channel signal metrics detected by the first detector system122may include both off-channel and on-channel metrics. First detector system122includes at least two “n” detectors for verifying whether the all-channel metrics exceed one or more thresholds. Second detector system124includes at least two “k” detectors for determining whether the on-channel signal exceeds another set of one or more thresholds. The channel estimator132provides scalable thresholds generating metrics for the received signal modulation and/or general telemetry indicative of channel dynamics.

In operation, antenna102receives RF signal103for filtering through preselector filter104and presenting a filtered RF signal105to low noise amplifier106. Low noise amplifier106generates amplified signal107which is mixed at mixer108with a local oscillator (LO) signal. Mixer108produces intermediate frequency (IF) signal109which is filtered at IF filter110into filtered IF signal111and forwarded to analog-to-digital (A/D) converter112for conversion to a digital signal113. Digital signal113is subjected to post processing stage114, where post processing activity is performed in order to provide a synchronous data signal115capable of being processed by the DSP118.

In accordance with the present invention, filtered RF signal105is sent to first detector system122for detecting the presence of all-channel signals passing through preselector filter104that meet or exceed one or more of the thresholds set by the “n” detectors. In accordance with the present invention, filtered IF signal111is sent to second detector system124for signal detection. Second detector system124is said to be the on-channel detector given that signal111has been filtered to a single channel by the IF filter110. The first and second detector systems122,124are set with predetermined thresholds for each desired metric. For the all-channel signals that exceed at least one predetermined threshold set by first detector system122, a detector output123is provided to trigger DSP118. For the on-channel signals meeting the predetermined thresholds set by second detector system124, a detector output125is also provided to trigger DSP118.

In response to being triggered, and in accordance with the present invention, DSP118indicates to host120that adjustments are needed to optimize the received signal. These adjustments may include scaling the thresholds set by detector systems122,124; adjusting an integration period within the detector systems122,124so as to fix or track the received RF and IF signal power105,111; adjusting front-end hardware; and/or adjusting functions of controller116such as scaling processing speeds and algorithm selection. Both the ADC112and post processor114can also be controlled dynamically based on input signal conditions reported by the detectors122,124. Parameters including, but not limited to, clock rate, current, bit width, and noise shaping, are just some of the adjustments possible in these two blocks.

As an example, in response to being triggered by signals123and/or125, DSP118can scale forward-error-correction (FEC) parameters, such as block and convolution coding vectors, engage “soft-decoding” algorithms vs. hard decoding algorithms, and/or schedule interrupt service requests (ISRs) so as to reduce the consumption of instructions and intrinsic error correction complexity at host microprocessor120.

As a further example and as mentioned above, the channel information provided by detector systems122,124can be used to scale the active stages within the receiver100, such as gain and filter sections, for maximum linearity when required, or to conserve current if environmental conditions warrant. In this case, host microprocessor120generates a serial port interface (SPI) signal126to make adjustments, as appropriate, to one or more of the active stages such as, LNA106, mixer108, filters104,110and/or ADC112. The adjustment to one or more of these receiver front-end devices impacts the metrics of the RF and IF signals105,111being detected by first and second detectors systems,124respectively. The all-channel and on-channel detector systems122,124continue to detect various metrics of the incoming signal and compare detected metrics to thresholds while the controller116, via DSP188and host120, makes adjustments to the SPI signal126for adjusting the receiver front end130. In this manner, a continuous adjustment loop is formed of detector systems122,124, controller116and receiver front-end130prior to the sampled signal115reaching the controller116.

WhileFIG. 1shows a dual conversion receiver with detector system122applied to the RF stage and detector124applied to the IF stage, a Direct Conversion Receiver can also derive improved signal reception by applying at least one detector system to the RF section and/or baseband section with each detector system providing a plurality of detector thresholds.

FIG. 2is a flowchart200summarizing a method for processing a received signal in accordance with the present invention. An RF signal is received at step202and compared to predetermined set of thresholds to detect all-channel signals and on-channel signals at steps204,206prior to the received signal reaching a latency stage of the receiver. As discussed previously, the all-channel signal detection can include both off-channel and on-channel signal detection. The detected signals are evaluated at the DSP at step208, and adjustments, if needed, are made at step210, to the controller section (DSP and/or host) and/or post processor, along with adjustments to the one or more pre-latency analog receiver circuits, scaling of detector thresholds and/or adjustments to the detector integration periods at step212. By utilizing method200of the present invention, it is now possible to determine, several milliseconds before a sample arrives at the DSP that the received signal's RF envelope is varying at a reasonably accurate estimated rate. Thus, fading and dynamic RF environmental effects on the received signal can be mitigated by making adjustments to the front-end circuitry, the ADC, and/or the post processing block114and DSP.

FIG. 3is an example of detector architectures that can be incorporated into receiver100to detect the all-channel signals and on-channel signals in accordance with an embodiment of the invention. The all-channel detector system122includes a plurality of reference detector offsets302, an integrator304, a plurality of summers306and n-level threshold detector308. When the fixed offsets302are summed at summers306with the integrated value303from integrator304, a resulting plurality of thresholds310are generated and used by multiple detectors in the n-level detector block308. The plurality of thresholds310vary in time as a function of the integration period set by integrator304with fixed offset between threshold values defined by reference302. The on-channel detector124ofFIG. 3has a similar architecture to that of detector system122but can have different threshold levels, shown here as k-levels. The on-channel detector124includes a plurality of reference detector offsets312, an integrator314, a plurality of summers316and k-level threshold detector318.

In accordance with the present invention, signal reception in receiver100can be optimized by making adjustments such as: scaling the thresholds set by detector systems122,124; adjusting the integration period of integrators303,314to allow signals303,313to fix or track the received RF signal power105,111; adjusting front-end hardware; and/or adjusting controller functions such as scaling processing speeds and algorithm selection.

The multi-detector systems122,124of the present invention take the real-time received RF signal105and compares it against multiple thresholds set at threshold detector308with reference thresholds tracking an integrated value303of the input receive signal105. Subsequent thresholds are offset via threshold detector offsets302by offset values delta-n for first detector system122. The second detector124takes received filtered IF signal111and compares it against multiple thresholds set at threshold detector318with reference thresholds tracking an integrated value313of the IF signal111. Subsequent thresholds are offset via threshold detector offsets312by offset values delta-k for second detector124.

The output of n-level detector308and k-level detector318is signal123and125respectively. The logic signal for123and125is generated based on the following representation.1) Once the integrated signal303or313exceeds a specific threshold within the plurality of thresholds310or320respectively, the output logic from the detector associated with that specific threshold within detector blocks308or318is held logic high, and continues to be high as long as the integrated signal303of313exceeds said threshold2) Once the integrated signal303or313falls below a specific threshold within the plurality of thresholds310or320respectively, the output logic from the detector associated with that specific threshold within detectors blocks308or318is held logic low, and continues to be low as long as the integrated signal303of313is below said threshold.3) The output logic123and125is a composite representation of all threshold values at any given time for the plurality of detectors within308and318respectively. The logic level of the output of any single detector within308and318is representative of whether the RF level at the input of said detector is above or below the associated threshold for that detector.

The offsets for the all-channel and on-channel detector system122,124do not have to be the same. Both the integration period of integrator304and314, and delta offset302and312, can be independently controlled by the host120via SPI126. Using the SPI126to control the integration period and delta offsets enhances the versatility of the receiver architecture by allowing the multi-detector architecture to generate metrics for the received signal modulation and/or general channel telemetry indicative of channel dynamics. Metrics for the received signal modulation include, but are not limited to, peak-to-average signal ratios, average power and timing rates to name a few. Metrics of general channel telemetry include, but are not limited to, fading, multi-path and presence of blocking signals to name a few.

The integration period set by integrator304,314and separation between thresholds set by reference detector offsets302,312can be adjusted depending on the targeted information. For example, in some receiver systems fading variations can exceed 30 dB with periodicity spanning several 5 to 100's of a mS, while digital modulations can exhibit peak-to-average ratios that approach 6-8 dB constrained to slot lengths of 10 mS to 30 mS or more.

The post analog-to-digital converter (ADC) section presently incorporated is some radio architectures utilizes sample rates of 20 kilo-samples per second (kps), with internal clock and filter structures for the post-ADC processing that introduces a delays approaching 1-2 ms. It is apparent that this latency can be larger or smaller depending on the sample rate, filter type and complexity (e.g. number of taps) and intrinsic clock speeds for the internal digital circuitry; however, digital latencies ranging from 500-800 μs are reasonably expected for many of the digitally centric radio platforms used today. While these delays are reasonably small in absolute time, as a percentage of slot duration in a Time Division Multiple Access (TDM) protocol, 1 mS latency can approach 5-10 percent of a slot length, which is appreciable for many systems. For Frequency Division Multiple Access (FDM) strategies, including analog FM, the latency is not significant but can still be used to advantage in highly dynamic RF environments such as fast fading.

The utilization of multiple on-channel and/or off-channel detectors having known relationships relative to each other allows for a multi-variant and dynamically scalable channel estimator132ofFIG. 1. The subsequent triggering of specific detectors by the RF signal103, taken together with relative timing from previous threshold triggers from different detector sets (at different threshold levels) facilitates the generation of channel envelope predictions to be provided to the DSP118before the actual channel data115arrives to the DSP. The latency associated with the post-processing, decimation and filtering of the on-channel signal allows channel parameters to be provided to the DSP118“ahead of time” prior to processing the actual data. Thus, “future estimations” of the channel condition are generated prior to the arrival of the actual samples, which may then be used to scale appropriate adjustments in DSP filtering, processing gain, error correction and hardware adjustments thereby improving BER and linearity in dynamic RF environments, such as multi-path and fast/slow fading environments. Additionally, even in steady state channel environments, the multi-detector strategy of the present invention can be adapted to determine the approximate linearity of the received signal modulation, which can then be used to scale the receiver hardware to either maximize subsystem linearity (such as by increasing the LNA/mixer/ADC current and/or bias adjust current to name a few) or reduce subsystem linearity for constant envelope so as to increase battery life.

FIG. 4shows a graph400providing an example of a measured fading RF envelope response402having minimal hysterisis along with a multi-level trip profile404across dual fixed thresholds406,408. The fade rate for this measurement was 100 kph and the fixed thresholds406,408were separated by 10 dB (−65 dBm and −75 dBm). The periodicity of trip profiles for dual detectors is a function of the RF envelope rate-of-change and hysterisis setting. Higher hysterisis eliminates fast deep fade detection. It is apparent from graph400that significant changes in the RF signal level occurred within 1 mS windows of certain portions of the fading response as indicated by designator450. Because of the nature of the response being measured, any hysterisis built into the detector system must be very small. For the example ofFIG. 4, hysterisis approached 200 μS. The single fade valley450and its associated trip profile is shown again in detail inFIG. 5.

From the graphs ofFIGS. 5A and 5B, it is apparent that a differentiated detector trigger (sequential triggering of differing detector thresholds) may be viewed as a pulse-width modulator, where the pulse time difference between transmissions from different detectors within detector blocks308or318ofFIG. 3gives both time502and amplitude504information about the received RF envelope. This may be used to predict future RF environments based on a set of simplifying logic. This logic may include:1) Any slope calculated from contiguous triggering of different thresholds (differentiated detectors triggering) will be used until a new differentiated detector trigger is detected (510).2) Inflection for a fade minima (506) or fade peak (508) is located between contiguous detector triggers at the same thresholds with threshold trip logic having the same value for all detectors (all high or all low).3) A localized inflection of an RF envelope within a multiple detector system (three or more detectors) occurs between contiguous triggers at the same threshold with the direction of the inflection determined by legacy triggering of different threshold states (detector states with different threshold previously triggered prior to contiguous triggering of inflection threshold).4) The absolute fade maxima are limited to 6 dB above the integrated reference thresholds value from the last trigger.5) The absolute fade minima are limited 35 dB below the integrated reference threshold value from the last trigger.
Applying the assumptions listed above in a fading environment for multiple detectors within308and318with fixed thresholds produces slope and timing information from which a “predicted” response may be generated.

FIG. 6shows an example of a graph600representing a fading RF envelope602and an associated predicted envelope604using fixed threshold levels606,608in accordance an embodiment of the invention. A fixed threshold response can be achieved in the present embodiment by setting the output of integration blocks304,314ofFIG. 3to a fixed constant. In this example, the first fixed threshold is set at −65 dBm and the second fixed threshold is set at −75 dBm. The variations in RF envelope602is typical of a signal received by a receiver traveling at 8 kilometers-per-hour (kph), or simply a received signal with a fade rate of 8 kph that is sampled by the ADC (block112inFIG. 3) at a sample rate of 20 ksps. The physical dynamics that induce variations in the RF envelope, known as fading or multi-path effects, are well understood by the RF communication system designers, and will not be described in detail here. However, it should be understood that the variations seen in the RF envelope are related to the speed that the receiver is traveling (8 kph, 100 kph, 220 kph), how many RF reflective surfaces are in proximity to the receiver, and how spectrally congested the RF environment is in the vicinity of the RF channel of operation. In addition,FIG. 6Bshows graph610of the detector slope variation612versus time614, whileFIG. 6Cshows graph620of the dB error622versus sample time624for the actual-minus-predicted RF envelope612and the mean +/−3 sigma614,616.

For the fading envelope and fixed threshold profile illustrated by graph600(which includes fading valleys approaching 30 dB), it is apparent that the worse case error associated with the predicted envelope relative to the true RF envelope may approach 10 dB (as indicated by graph620). However, the nominal error is usually much smaller, with error excursions typically being less that 5 dB. The error response can be improved upon by allowing the output of the integrators304,314ofFIG. 3to be variable and proportional to the integrated value of the true RF envelope as will be shown in conjunction withFIG. 7.

FIG. 7Ashows an example of a slow fading envelope702and an associated predicted envelope704using tracking thresholds706,708in accordance with an embodiment of the invention. As the reference thresholds706,708track the integrated RF envelope, the accuracy of the predicted response704increases, with most error excursions being reduced by 3 to 5 dB. The effect of allowing the reference threshold to track the integrated value of the RF envelope (i.e. signal303tracks the integrated output of block304resulting in the real time variation of threshold level310inFIG. 3) causes the threshold to vary as a function of the RF variations. This results in improved tracking of localized inflection points750,752that previously went unrecognized using the fixed threshold strategy. The identification of localized inflection(s)750,752can also be achieved with fixed threshold values; however, this necessitates the use of additional detectors whose thresholds are fixed at level with smaller differences to increase sensitivity to smaller RF variations. Hence, coupling the reference thresholds to the integrated RF envelop mitigates the need for additional detectors to some extent, while adding extra precision in the predicted envelope generation.

FIG. 7Bshows graph710of the detector slope variation712versus time714, whileFIG. 7Cshows graph720of the dB error722versus sample time724for the actual-minus-predicted RF envelope712and the mean +/−3 sigma714,716. Thus, by allowing the output of the integrators304,314ofFIG. 3to be variable and proportional to the integrated value of the true RF envelope, the error response was improved over that of the fixed threshold levels used in the exampleFIG. 6.

Accordingly, there has been provided a method and apparatus for improving signal reception in a receiver of a portable or mobile communication device by performing all-channel and on-channel estimations of a received signal so as to predict future RF environments. While shown in terms of a dual conversion receiver, the apparatus and method of the present invention applies equally as well to Direct Conversion Receivers (DCR). While shown and described with two detectors systems, the receiver can be implemented with one or more detector systems, each system containing a plurality of detectors. Each detector system can also be implemented without summers or integrators in applications where fixed SPI selectable thresholds are used. The receiver can be integrated into a single chip in which a simple control bus replaces the serial port interface.