Patent Publication Number: US-2004059547-A1

Title: Method of signal quality estimation

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to a method for estimating signal quality in cellular communications systems in general, and more particularly to methods and apparatus for estimating transmission signal quality in TETRA systems.  
       BACKGROUND OF THE INVENTION  
       [0002] When effecting handover of a call in a cellular radio system, such as Terrestrial Trunked Radio (TETRA), a mobile station (MS) generally measures the quality of the signal received from its serving base transmission station (BTS) and its neighboring BTSs and picks the BTS with the best signal quality. The TETRA standard defines this quality measurement to be received signal strength (RSSI). However, in an interference-limited system, such as TETRA, GSM, or any other cellular-type system where the density of BTSs is great enough such that the predominant cause of signal degradation is interference from other BTSs rather than the noise generated in the receiver, RSSI is not a good measurement of quality, and alternative signal quality estimates (SQEs) which measure the signal-to-interference ratio are required.  
       [0003] Two basic characteristics of a good measurement of signal quality are large dynamic range and high accuracy (i.e., low variance in the measurement). These characteristics allow the quality of radio signals from various base sites to be accurately compared to determine the site with the best signal quality. The desirability of these good measurement characteristics may be illustrated with reference to FIG. 1A in which contour lines  10 ,  12 , and  14  represent signal qualities of radio transmissions by BTSs A and B and at various distances from the BTSs. For the purpose of the illustration, signal quality values 1, 2, and 3 are used to represent the relative signal qualities at each of contour lines  10 ,  12 , and  14 . An MS travelling along a line from serving BTS A to BTS B will experience the best possible service if it changes its serving BTS to BTS B when the quality offered by each BTS is equal, i.e., at a point C.  
       [0004] Two possible SQEs are graphically illustrated in FIGS. 1B and 1C, to which reference is now additionally made, with FIG. 1B representing an SQE having a relatively good dynamic range and FIG. 1C representing an SQE having a relatively poor dynamic range. An MS employing an SQE with the range of FIG. 1B would be able to determine almost immediately that it passed the optimum handover point C in FIG. 1A in the direction of BTS B since the signal from BTS B would be measurably better than that from BTS A, and the MS would thus initiate handover almost immediately after passing point C in the direction of BTS B, allowing for normal systemic delay.  
       [0005] In contrast, an MS employing an SQE with the range of FIG. 1C would not be able to distinguish between the quality of the two signals until the quality from BTS A had dropped below level 1, since the dynamic range of the SQE is limited. Thus, handover from BTS A to BTS B would not occur until MS reaches point D in FIG. 1A, with MS needlessly suffering from low signal quality between points C and D.  
       [0006] A similar scenario would apply for SQEs of greater and lesser sensitivity to variances in signal quality, with a more sensitive SQE resulting in an earlier handover and, therefore, better average signal quality from the MS vantage.  
       [0007] It is also important to have an SQE with a large dynamic range to allow for differentiation between the quality of signals from two BTSs even when the difference in quality experienced by the user (such as the speech quality) is not perceptible, as this ensures not only that the current quality is maintained, but also that the probability of continued good quality of service is maximized  
       [0008] In addition to dynamic range and accuracy, it is also desirable for the SQE to have a short measurement time, thus minimizing the delay in changing BTSs, and to be of low incremental complexity to the receiver.  
       [0009] Having established that a good signal quality measurement should have a large dynamic range and high accuracy, just what defines good signal quality remains to be determined. Preferably, the SQE should be related to the quality perceived by the user since, in voice-dominated digital radio systems, the voice quality is perhaps the most important aspect of the call to the user. As voice quality is difficult to measure objectively, the signal bit error rate (BER) may be used as a measurement of user-perceived quality, to which the SQE should be related. Furthermore, the SQE should also relate to the probability that the BER will remain at an acceptable level. In many radio systems, the signal-to-noise-plus interference ratio (C/(I+N)) is used as an SQE, as it is related to error rate for a given channel condition, and the margin in C/(I+N) over the threshold for acceptable BER is a reasonable indication of the likelihood that the signal quality will remain good.  
       [0010] SQE measurements are typically taken of both the serving BTS and neighboring BTSs in determining when to effect handover. The ability of the MS to take measurements of neighboring BTSs depends in large part on which transmissions are captured from the neighboring BTSs. In a TETRA system, the SQE measurements should preferably be taken of a continuous downlink transmission, should support both synchronized and unsynchronized BTSs, should take into account oscillator switching time, and should support both half-duplex and full-duplex calls. SQE measurements taken under such circumstances should attempt to capture at least 65% of a slot&#39;s transmissions in order to ensure that at least one training sequence is captured, although its position in the captured data stream is unknown, and, thus, SYNC information would not be recovered and data blocks other than the Access Assignment Channel (AACH) would not be recovered.  
       [0011] Thus an SQE that has good dynamic range, high accuracy, low complexity, fast convergence, is related to C/(I+N), and meets system-specific constraints such as those discussed above for TETRA systems is required.  
       [0012] There are several known technologies for estimating signal quality, including those based on signal strength, Euclidean distance measurement, bit error rate estimates, and signal envelope fluctuations. These are now discussed.  
       [0013] Signal strength SQEs provide an indication of the best BTS to serve an MS in systems that are range-limited, but are inadequate in interference-limited systems as the measured signal strength can not discriminate between wanted and unwanted signals.  
       [0014] Euclidean distance measurements calculate the mean squared distance of the received data point from the position it should be at in the modulation constellation. This measurement requires a good estimate of the channel impulse response and of the data values to form a local estimate of the correct data point. Hence, both frame and data symbol timing must be recovered to support this measurement, which requires that at least one training sequence be captured for processing. The burst structure of such systems is specifically designed to give a good single-tap estimate of the channel impulse response by employing strategically placed training sequences and pilot symbols. The estimate of the channel is required for reliable and coherent detection. TETRA, which is designed for differential detection, does not provide for pilot symbols, and channel estimation and adaptive prediction is therefore not as accurate.  
       [0015] Bit error rate estimates may also be used as a basis for estimating signal quality. GSM cellular telephone systems make signal quality estimates based on comparing re-encoded data recovered from a slot with hard decision detected data from the received slot. Differences between the hard detected data and the re-encoded data provide an estimate of the raw channel bit error rate (i.e., uncoded error rate). In TETRA systems, the only data sequence captured whole that could be used is the AACH sequence, which contains 14 bits of data and 16 bits of CRC, as training sequences cannot be used due to simulcast problems. With only 14 bits of data per slot, the BER at 25 dB would have to be measured over 250 slots of data for an accurate estimation.  
       [0016] The measurement of signal envelope fluctuations is described in “Co-Channel Interference Measurements Method for Mobile Communications”, S. Kozono, IEEE Transactions on Vehicular Technology, vol. VT-36, no. 1, February 1987, pp. 7-13, which describes how the envelope of the sum of two interfering signals contains components at the different frequencies of the two signals&#39; carriers. Carrier-to-interference is measured by isolating the beat frequency components from the more slowly varying Rayleigh fading. In “On Co-Channel Interference Measurements”, J. Chen, Conference of the Proceedings of PIMRC 1997, pp. 292-296, the performance of the method is extended in noisy conditions and to multiple interferers. However, as the TETRA modulation does not have a constant envelope, the signal envelope varies at multiples of the symbol rate. Simulations have shown that the dynamic range of this method is poor when used for TETRA, and that the averaging time is prohibitively long.  
       [0017] Another signal quality estimation method known as the orthogonal data method is described in “In-service Signal Quality Estimation for TDMA Cellular Systems”, M. Austin &amp; G. Stueber, Wireless Personal Communications 2, 1995, pp. 245-254. In this method the radio channel is modeled as an M-tap transversal filter with taps represented by the vector f=[f−1, . . . , f m ] T , where the T denotes transpose. The received vector is y=[y 1 , . . . , y L ] T , which is written as: 
         y=Af+w   (EQ. 1) 
       [0018] where w is the vector consisting of samples of interference plus noise, and A is an L×M Toeplitz matrix form by the transmitted complex symbols:  
             A   =     (           a   M           a     M   -   1           ⋯         a   1               a     M   +   1             a   M         ⋯         a   2             ⋮       ⋮       ⋯       ⋮             a     M   +   L   -   1             a     M   +   L   -   2           ⋯         a   L           )             (     EQ   .              2     )                       
 
       [0019] If L&gt;M, there exists a vector c=[c 1 , . . . ,c L ] T  such that c T A=0, so that: 
         c   T   y= 0 +c   T   w   (EQ. 3) 
       [0020] The estimate of the noise plus interference power for a given data series is then:  
               σ     I   +   N     2     =         y   T          c   *          c   T        y            c        2               (     EQ   .              4     )                       
 
       [0021] where * denotes conjugate, and the signal plus noise plus interference power is  
               σ     S   +   I   +   N     2     =       1   L          E   [       y   H        y     ]               (     EQ   .              5     )                       
 
       [0022] where H denotes the Hermitian transpose. The estimates of S+I+N and I+N are averaged over several slots. The estimate of C/(I+N) is then:  
               C     I   +   N       ≈         σ     S   +   I   +   N     2       σ     I   +   N     2       -   1             (     EQ   .              6     )                       
 
       [0023] In short, in the Austin-Steuber method a complex vector, c, is found that is orthogonal to as many shifted versions of the complex data sequence as taps in the channel impulse response, and the dot product of the orthogonal vector with the received vector is formed. This removes the data related components in the received vector, thus isolating the interference plus noise.  
       [0024] Unfortunately, the Austin-Steuber method is relatively complex, is difficult to implement, and has several shortcomings that precludes its use with systems such as TETRA. For example, the Austin-Steuber method uses known sequences within the transmitted data, so that the orthogonal data vector is pre-calculated. However, because of the simulcast effect, the training sequences in TETRA cannot be used. Thus, orthogonal data vectors may not be pre-calculated. Furthermore, Austin-Steuber does not address carrier breakthrough which can destroy orthogonal data signal quality measurements. Austin-Steuber is also ineffective in both fading conditions and in the presence of a carrier offset, as EQ. 3 only holds true if the channel is invariant over the duration of the L data samples. Finally, the Austin-Steuber method does not work well in dispersive channels due to errors in estimating the data sequence prior to finding the orthogonal vector. In non-dispersive channels, where the error rate in estimating the data symbols falls continuously with increasing C/I, the error in the SQE measurement due to detection errors is much smaller than the measured C/I. However, in dispersive channels where an error floor dominates, the errors in the SQE caused by mis-detection are much larger than the C/I being measured. This limits the dynamic range of the algorithm.  
       [0025] A signal quality estimation method that overcomes disadvantages of known signal quality estimation methods is therefore required.  
       SUMMARY OF THE INVENTION  
       [0026] The present invention seeks to provide methods and apparatus for estimating transmission signal quality in TETRA systems using orthogonal data sequences that overcome disadvantages of the prior art discussed hereinabove. A method is provided whereby complex data samples are captured on a slot-by-slot basis, and any carrier offset is corrected. The capture data samples are then partitioned into sets, and the signal is reconstructed by differentially detecting the data and re-encoding it. A vector orthogonal to the reconstructed (i.e., re-encoded) data sample set is then found. The S+I+N and I+N values are then calculated for each set.  
       [0027] The S+I+N values are averaged over several slots, with each slot contributing as many S+I+N values as partitioned data sample sets. The I+N values from several slots are gathered, with each slot contributing as many I+N values as partitioned data sets. The average I+N value is calculated after discarding the largest of the SQE values to correct for measurement errors.  
       [0028] The present invention provides for finding an orthogonal vector “on the fly” from detected data symbols. Dummy data sequences are used to calculate the orthogonal vector, as well as to combat carrier breakthrough. One slot carrier offset correctors and short data sequences are also employed to combat time varying channels in measuring SQE. Data conditioning is also used to reject data not conforming to the expected probability distribution.  
       [0029] There is thus provided in accordance with a preferred embodiment of the present invention a method of estimating signal quality, the method including a) taking a plurality of complex samples of a decoded signal at its symbol rate for a plurality of slots, for each of the slots b) partitioning the samples into a plurality of partitioned sample sets, for each of the partitioned sample sets c) deriving data symbols from the samples, d) re-encoding the data symbols into a re-encoded symbol set, e) forming a vector of length L that is orthogonal to a set of vectors formed from L consecutive samples of the re-encoded symbol set, where L is an integer less than or equal to the number of samples in the partitioned set, and f) forming an estimate of the signal quality from the plurality of complex samples for the plurality of slots using at least one of the orthogonal vectors.  
       [0030] Further in accordance with a preferred embodiment of the present invention the forming step e) includes forming the vector of length L that is orthogonal to at least one dummy vector of length L.  
       [0031] Still further in accordance with a preferred embodiment of the present invention the number of samples in each of the partitioned sample sets is K+2, where K is an integer sufficient to allow the formation of the orthogonal vector.  
       [0032] Additionally in accordance with a preferred embodiment of the present invention K is greater than 2M, where M is an integer greater than or equal to 1 and less than or equal to the number of significant components in the impulse response of the channel.  
       [0033] Moreover in accordance with a preferred embodiment of the present invention K is equal to 2M+1.  
       [0034] Further in accordance with a preferred embodiment of the present invention the length of the orthogonal vector L is K−M+1  
       [0035] Still further in accordance with a preferred embodiment of the present invention the set of vectors formed from L consecutive samples of the re-encoded symbol set are a set of M vectors of length L and form the columns of an L*M matrix.  
       [0036] Additionally in accordance with a preferred embodiment of the present invention the set of vectors to which the orthogonal vector is formed include all vectors of length L that can be formed from consecutive samples of the set of re-encoded symbols.  
       [0037] Moreover in accordance with a preferred embodiment of the present invention the set of vectors to which the orthogonal vector is formed include all vectors of length L that can be formed from consecutive samples of the set of re-encoded symbols excluding the first and last samples from the set of re-encoded symbols.  
       [0038] Further in accordance with a preferred embodiment of the present invention the method further includes correcting the samples for received carrier frequency offset.  
       [0039] Still further in accordance with a preferred embodiment of the present invention the dummy vector consists of homogenous components.  
       [0040] Additionally in accordance with a preferred embodiment of the present invention the orthogonal vector is formed using a second dummy vector which is any vector other than a scalar multiple of any of the vectors from the set of vectors of length L formed from the re-encoded symbol set and the first mentioned dummy vector.  
       [0041] Moreover in accordance with a preferred embodiment of the present invention the second dummy vector is linearly independent from the of any of the vectors from the set of vectors of length L formed from the re-encoded symbol set and any the first mentioned dummy vector.  
       [0042] Further in accordance with a preferred embodiment of the present invention the forming step e) includes forming using Gram-Schmit orthogonalisation.  
       [0043] Still further in accordance with a preferred embodiment of the present invention the forming step f) includes rejecting at least one of the interference-plus-noise power estimates.  
       [0044] Additionally in accordance with a preferred embodiment of the present invention the forming step f) includes rejecting approximately 10% of the interference-plus-noise power estimates where the remaining estimates are lower than the rejected estimates.  
       [0045] Moreover in accordance with a preferred embodiment of the present invention A 1 , . . . , A M  are the M vectors formed from the columns of the matrix, where Y 1 , . . . ,Y M+1  are M+1 orthogonal vectors to be calculated from the columns of the matrix, and where the forming step e) includes forming a vector Y M  that is orthogonal to all vectors A k  except A M  according to the equation  
           Y   M     =       A   M     -       ∑     k   =   1       k   =     M   -   1                    Y   k     ·     A   M           Y   k     ·     Y   k              Y   k             ,                 
 
       [0046] providing the dummy vector as A M+1 , and forming a vector Y M+1  according to the equation  
         Y     M   +   1       =       A     M   +   1       -       ∑     k   =   1       k   =   M                  Y   k     ·     A     M   +   1             Y   k     ·     Y   k                Y   k     .                         
 
       [0047] Further in accordance with a preferred embodiment of the present invention the dummy vector is any vector other than a scalar multiple of any of the columns of the matrix.  
       [0048] Still further in accordance with a preferred embodiment of the present invention the dummy vector is any vector other than any linear combination of the M columns of the matrix.  
       [0049] Additionally in accordance with a preferred embodiment of the present invention the method further includes providing a second dummy vector A M+2 , and forming a vector Y M+2  that is orthogonal to the dummy vector A M+1 .  
       [0050] Moreover in accordance with a preferred embodiment of the present invention A M+1  consists of homogenous components.  
       [0051] Further in accordance with a preferred embodiment of the present invention A M+1  is the complex number 1+j0, where j is the square root of −1.  
       [0052] Still further in accordance with a preferred embodiment of the present invention A M+2  includes alternating components (1+j0) and (−1+j0).  
       [0053] Additionally in accordance with a preferred embodiment of the present invention the forming an estimate step f) includes calculating a signal-to-interference-plus-noise ratio.  
       [0054] Moreover in accordance with a preferred embodiment of the present invention the forming an estimate step f) includes g) forming, for each of the sample sets in a single slot, an estimate of the interference-plus-noise power from the orthogonal vectors of the sample sets, h) forming, for a plurality of the slots, an average estimate of the interference-plus-noise power from the slot interference-plus-noise power estimates, i) forming, for each of the slots, an estimate of the signal-plus-interference-plus-noise power from the plurality of complex samples, j) forming, for a plurality of the slots, an average estimate of the signal-plus-interference-plus-noise power from the slot signal-plus-interference-plus-noise power estimates, and k) forming an estimate of the signal-to-interference-plus-noise power from the average estimate of the interference-plus-noise power and the average estimate of the signal-plus-interference-plus-noise power, thereby estimating signal quality 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0055] The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the appended drawings in which:  
     [0056]FIG. 1A is a simplified pictorial illustration representing signal quality at various distances from BTSs;  
     [0057]FIG. 1B is a simplified graphical illustration of a signal quality estimate having a relatively good dynamic range;  
     [0058]FIG. 1C is a simplified graphical illustration of a signal quality estimate having a relatively poor dynamic range;  
     [0059]FIG. 2 is a simplified flowchart illustration of a method of estimating signal quality, operative in accordance with a preferred embodiment of the present invention;  
     [0060]FIG. 3 is a simplified flowchart illustration of a method of carrying out step  150  of the method of FIG. 2, operative in accordance with a preferred embodiment of the present invention; and  
     [0061]FIG. 4 is a simplified graphical illustration depicting the performance of a simulation of the methods of FIGS. 2 and 3. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0062] Reference is now made to FIG. 2 which is a simplified flowchart illustration of a method of estimating signal quality, operative in accordance with a preferred embodiment of the present invention. In a typical receiver which measures signal quality, a signal is captured by the receiver. In time division multiple access (TDMA) systems, the signal captured would typically be a part of the slot of information sent by a transmitter. In a digital receiver, the received signal captured by the receiver is typically sampled by an analog-to-digital converter. “Samples” as used herein refers to the set of complex baseband samples taken, stored, and processed by the receiver, or a subset of these samples. In the method of FIG. 2 one or more windows of complex samples are captured from a signal decoded at its symbol rate using conventional techniques (step  100 ). Preferably, each sample window is sufficiently large to capture most of a TDMA slot where the signal is a TDMA signal. A carrier offset correction is then applied to the data set using any suitable carrier offset correction technique (step  110 ). The offset corrector may be estimated by measuring the phase shift after differential decoding of the signal. The offset corrector is preferably determined and applied separately for each slot.  
     [0063] In order to ensure that the channel is invariant over the data set, the data set is made as short as possible. The minimum length of data set is dependent on the length of the impulse response of the channel. If the impulse response is of length M, then the length of each data vector (columns in matrix A) is set to at least M+2 as 
       L&gt;=M+ 2  (EQ. 7) 
     [0064] Typically, M is an integer greater than or equal to 1, and preferably corresponds to the number of significant components in the impulse response of the channel, with the number being selected using conventional techniques. Further, the M data sets (columns in matrix A) are formed from a data sequence whose length is 
       K=L+M− 1.  (EQ. 8) 
     [0065] Combining these two conditions implies that K, the minimum captured data sequence that forms the matrix A, is constrained such that: 
       K &gt;2 M   (EQ. 9) 
     [0066] Thus, the data are partitioned into sets of consecutive samples having a length M+2 where K&gt;2M (step  120 ). As the minimum length of L is defined by M, then it is clear that a value of L does not always exist that can cope with both a channel of impulse response of length M and a fast changing channel. Theoretically, the smaller M is, the faster the channel fading that the algorithm can cope with.  
     [0067] The data symbols are then derived for each partitioned data set, by detecting and differentially decoding the samples of the received signal, and differentially re-encoded (step  130 ). The first and last re-encoded symbols are then preferably discarded as they were arbitrarily determined (step  140 ). A vector is then found that is orthogonal to the transmitted sequence of each set of re-encoded symbols (step  150 ).  
     [0068] A preferred method of carrying out step  150  of the method of FIG. 2 may be seen with additional reference to the simplified flowchart illustration of FIG. 3. In the method of FIG. 3 a novel, modified version of Gram-Schmidt orthogonalization is used. In Gram-Schmidt orthogonalization a set of N arbitrary N dimension vectors which span a space of dimension N is used to derive a set of N orthogonal basis vectors which define the space. One of the vectors is arbitrarily chosen as the first vector of the basis set. A vector orthogonal to the first is then derived by using a second vector of the arbitrary set. A third basis orthogonal to the first two basis vectors is then derived by using a third vector from the arbitrary set, and so on. While Gram-Schmidt orthogonalization is well known, it may not be used as is to find orthognal vectors when the set is not complete, nor is its use known for calculating SQEs.  
     [0069] In the method of FIG. 3 a vector orthogonal to the M vectors formed from the columns of matrix A is determined as follows. Let A 1 , . . . , A M  be the M vectors formed from the columns of matrix A, and Y 1 , . . . , Y M+1  be the M+1 orthogonal vectors that will be calculated from the columns of A. Then:  
               Y   1     =     A   1             (     EQ   .              10     )                 Y   2     =       A   2     -           Y   1     ·     A   2           Y   1     ·     Y   1              Y   1                 (     EQ   .              11     )               etc   .                             Y   M     =       A   M     -       ∑     k   =   1       k   =     M   -   1                    Y   k     ·     A   M           Y   k                     Y   k              Y   k                   (     EQ   .              12     )                       
 
     [0070] The vector Y M  is now orthogonal to all the A k  except A M  (step  1510 ). In order to find a vector orthogonal to all M columns of A, a dummy vector A M+1  is introduced that is not a scalar multiple of any of the columns of A (step  1520 ). Preferably, the dummy vector should not be any linear combination of the M columns of A. Y M+1  is then calculated as (step  1530 ):  
               Y     M   +   1       =       A     M   +   1       -       ∑     k   =   1       k   =   M                  Y   k     ·     A     M   +   1             Y   k     ·     Y   k              Y   k                   (     EQ   .              13     )                       
 
     [0071] Y M+1  is thus orthogonal to all of the columns of A and may be used as the vector c in EQ. 3 above.  
     [0072] To combat carrier breakthrough, which is a DC offset in the π/4 DQPSK constellation at baseband, the orthogonalisation procedure may be extended so that two dummy data vectors, A M+1  and A M+2  are used. The vector A M+1  is preferably a vector whose components are all identical, typically being the complex number 1+j0, where j is the square root of −1. The orthogonal vector basis set then is formed up to Y M+2  using the second dummy data vector A M+2 , typically having alternating components (1+j0) and (−1+j0) (step  1540 ). The final orthogonal data vector, Y M+2 , is thus orthogonal to the first dummy data vector, A M+1 , and is also, therefore, orthogonal to any vector whose components are all identical.  
     [0073] When carrier breakthrough is present in the vector y in EQ. 1 above, the received data vector, y′, is added to a vector, b, whose components are identical and equal to the DC offset caused by breakthrough after passing through the channel, as follows: (step  1550 ). 
       y′=y+b=Af+w+b   (EQ. 14) 
     [0074] The dot product of y′ with c: 
       c   T   y′=c   T   y+c   T   b= 0 +c   T   w +0  (EQ. 15) 
     [0075] which takes on the value of Y M+2 , thus not only eliminates the effect of the data, but also the effect of the carrier breakthrough.  
     [0076] Returning to FIG. 2, an estimate of I+N may be formed from the results of step  150  across several data sets (from several slots in a TDMA system) using EQ. 4 above (step  160 ). To correct for dispersive channel errors, several estimates of I+N are gathered (step  170 ), and a number of the largest estimates are rejected (step  180 ). This approach to correcting for dispersive channel errors may be understood from the distribution of SQE measurements. The probability density function (PDF) of SQE values in a non-dispersive channel is exponential or χ 2  in nature. The average of this distribution yields the correct C/I. The distribution in a dispersive channel, however, is the expected exponential plus a long tail due to SQEs formed from erroneous data. The solution is to remove the outliers in the tail and leave the values conforming to the χ 2  distribution. Experimentation has shown that removing approximately 10% of the intermediate SQE values used to form the ensemble average with the largest values provides optimal dispersive channel error correction. This proportion removed the majority of outliers in a dispersive channel, without seriously biasing the measurement in a non-dispersive channel. The remaining I+N estimates are then averaged (step  190 ).  
     [0077] An estimate of S+I+N may be formed from the results of step  150  across several data sets (from several slots in a TDMA system) using EQ. 5 above (step  200 ). An average S+I+N is then calculated for several S+I+N estimates (step  210 ). Finally, C/(I+N) may be calculated from the average I+N and the average S+I+N (step  220 ) using EQ. 6 above.  
     [0078] It is appreciated that the steps of the methods described herein may be performed in a different order than that which is described hereinabove and/or that the methods may be performed without one or more of its steps and still result in a useful SQE.  
     [0079] Reference is now made to FIG. 4 which is a simplified graphical illustration depicting the performance of a simulation of the method of FIG. 2, for a TETRA system operating in a TU50 channel (i.e., a radio channel considered typical for a receiver operating in a typical urban situation and travelling at 50 km/h). The dynamic range of the SQE algorithm is the C/(I+N) up to which the measured SQE can easily differentiate between signals with differing C/(I+N) ratios (i.e., the upper value of C/(I+N) before the curve begins to flatten out). The simulated dynamic range of the method of FIG. 2 is shown to be greater than 35 dB for TU50 channels. Experimentation has shown that the method of FIG. 2 is also useful for more stringent channel conditions, such as those of HT200 channels, (i.e. a radio channel considered typical for a receiver operating in hilly terrain at 200 Km/hr), in that it reasonably differentiates between signals below 30 dB.  
     [0080] It is appreciated that one or more of the steps of any of the methods described herein may be omitted or carried out in a different order than that shown, without departing from the true spirit and scope of the invention.  
     [0081] While the methods and apparatus disclosed herein may or may not have been described with reference to specific hardware or software, the methods and apparatus have been described in a manner sufficient to enable persons of ordinary skill in the art to readily adapt commercially available hardware and software as may be needed to reduce any of the embodiments of the present invention to practice without undue experimentation and using conventional techniques.  
     [0082] While the present invention has been described with reference to a few specific embodiments, the description is intended to be illustrative of the invention as a whole and is not to be construed as limiting the invention to the embodiments shown. It is appreciated that various modifications may occur to those skilled in the art that, while not specifically shown herein, are nevertheless within the true spirit and scope of the invention.