Multi-threshold detection for 0.3-GMSK

Abstract: A variable multi-threshold detection circuit for a sampled baseband signal in the receiver of a communication system using GMSK modulation is disclosed. The circuit comprises an integration/dump circuit 600, 610 for accumulating a predetermined number of samples of the baseband signal to generate a sum for a present bit, a level selector 650 for selecting a threshold from a plurality of groups of multi-thresholds based on an estimated phase offset 605 and the binary value 641, 642 of two previous bits, a compare circuit 620 for comparing the sum from the integrate/dump circuit 600, 610 with the threshold selected from the level selector to generate a binary value of either one of "0" or "1" for the present bit, and a delay circuit 630, 640 for delaying the present bit to be used by the level selector 650 to determine a next threshold for a next bit.

Description: RELATED FIELD The present invention relates to the field of telecommunication and more specifically to receiving, sampling and detecting signals through a wireless communication system. ART BACKGROUND It has been common in the field to use a so-called "0.3-GMSK" modulation scheme for wireless communication, where pre-modulated binary data is mapped to +1/-1 and held for a bit duration (T) and filtered with a Gaussian filter of normalized bandwidth (BT) equal to 0.3, where B is the actual bandwidth of the filter. This filtered signal is then FM modulated with modulation index of 0.5, which is the minimum for an FSK system and hence the term "Minimum Shift Keying (MSK)." Figure 1 illustrates a representative 0.3-GMSK system block diagram. On the transmitter side, a bi-polar signal is applied to a Gaussian filter 100, which is connected to a Minimum-Shift-Keying (MSK) 110 before the signal is transmitted to a receiver through an intermediate frequency (IF) to radio frequency (RF) converter 114 and an antenna 115. On the receiver side, the received signal from an antenna 120 goes through an RF-to-IF converter 121 to a bandpass filter 125 connected to a limiter 130, which limits the input signal amplitude. The signal from the limiter 130 is applied to an FM demodulator 140 before the signal is passed through a low-pass filter 150. The resulting analog signal from the low-pass filter 150 is sampled by a sampler 155, through a sampling clock 156 and converted to digital form by an analog/digital (A/D) converter 160 for digital signal processing. The output of the A/D converter is applied to the detection device 170. However, due to intersymbol interference (ISI) intrinsically generated when the signal is filtered by the Gaussian filter 100 in the transmitter and typically introduced by the propagation channel, as well as the filters in the receiver, errors are introduced. Therefore, it is desirable to be able to detect the baseband signal while minimizing the ISI impact at the output of the low-pass filter 150. The FM-demodulated signal can be detected using fixed multi-threshold detection. However, with fixed multi-threshold detection, the accuracy of the decoded data tends to be sensitive to the corresponding phase offset. Therefore, additional mechanisms are needed to periodically adjust the sample time to ensure the correct phase for the detection scheme. These may include adjusting the sampler clock or using interpolation filters to re-generate the sampling phase. Note that the phase offset is determined by a digital processor, shown in block 230 and 330 of Figures 2 and 3, which estimates a sequence of previous samples. Figures 2 and 3 illustrate fixed threshold detection circuits using an interpolation filter and using clock adjustment, respectively. However, as those skilled in the art will appreciate, both the interpolation filter and clock-adjustment would require a more complicated circuit. Therefore, it would be desirable to have a threshold detection circuit without the need to adjust the phases of input signals. Referring to Figure 2 , the analog signal from the low-pass filter 150 in Figure 1 is sampled by the sampler 210 according to a sampling clock 240 and quantized by an A/D converter 215. The resulted signal is applied to an interpolation filter 220 with the phase offset information produced by the digital processor 230. The output from the interpolation filter is applied to the detection circuit 250. Referring to Figure 3 , where the method of clock adjustment is used, the analog signal is sampled by the sampler 310 according to the clock 340 and quantized by an A/D converter 315. Without the interpolation filter, the phase offset is applied to the sampling clock 340 from the digital processor 330. Attention is drawn to document EP 0 417 390, which discloses a GMSK modem. The modem comprises a baseband modulator section and demodulator section coupled to an FM transmitter and receiver, respectively. The modulator section includes a digital waveform generator for generating a GMSK baseband signal from a binary digital data source. The output of the waveform generator approximates the baseband signal output of a premodulation filter with a Gaussian impulse response having a normalized noise bandwidth of between 0.25 and 0.45 in series with a gain controlled amplifier. The gain is adjusted such that the modulation index of an RF signal modulated with the baseband signal is between 0.5 and 0.7. The demodulator section receives the analog output of a discriminator-based FM receiver with a phase equalized Butterworth IF filter. The demodulator section comprises a binary quantized loop bit timing recovery subsystem and one or the other of two data detectors. The two data detectors comprise a two-bit integrate-and-dump type filter and a two-bit multithreshold integrate-and-dump type filter. The binary quantized loop is also connected to a signal quality detector for altering the mode of operation of the loop according to the amount of noise in the baseband signal. In accordance with the present invention a variable multi-threshold detection, circuit, as set forth in claim 1, and a method of detecting bits of a sampled baseband signal, as set forth in claim 5, are provided. Preferred embodiments of the invention are claimed in the dependent claims. SUMMARY OF THE PRESENT INVENTION In order to reduce hardware complexity required by the detection circuitry, a variable multi-threshold detection circuit is disclosed. Based on the estimation of bit timing phase, the corresponding thresholds can be selected. Neither the interpolation filter nor timing phase adjustment is required. What is required is to estimate a timing phase in order to select a suitable threshold. Based on simulation results, performance can be maintained with a given phase offset by varying the thresholds to optimize the circuitry performance. A variable multi-threshold detection circuit for a sampled baseband signal in the receiver of a communication system is disclosed. The circuit comprises an integrate/dump filtering circuit which accumulates a predetermined number of samples of the sampled signal to generate a sum for present bit detection, a level selector for selecting a threshold from a plurality of groups of multi-thresholds based on an estimated phase offset and the binary value of two previous bits, a compare circuit for comparing the sum from the integrate/dump circuit with the threshold selected from the level selector to generate a binary value of either one of "0" or "1" for the present bit, and a delay circuit for delaying the present bit to be used by the level selector to determine a next threshold for a next bit. BRIEF DESCRIPTION OF THE DRAWINGS Additional objects, features and advantages of the present invention will be apparent from the following description, wherein: Figure 1 illustrates a representative 0.3-GMSK system block diagram. Figure 2 illustrates a detection circuit with fixed threshold using an interpolation filter. Figure 3 illustrates a detection circuit with fixed threshold and adjustable sampling clock. Figure 4(a) illustrates a four-level detector as used in Figures 2 and 3. Figure 4(b) is a table of fixed multi-threshold. Figure 5 illustrates a detection circuit in accordance with the present invention. Figure 6 illustrates a variable multi-threshold detection circuit as used in Figure 5. Figure 7 is a representative table of variable multi-threshold to be implemented with the present invention. Figure 8 is a simulated bit error rate chart illustrating the performance of the variable multi-threshold detection in accordance with the present invention DETAILED DESCRIPTION OF THE DRAWINGS A variable multi-threshold detection circuit is disclosed. The detailed description that follows is presented largely in terms of algorithms and symbolic representations of operations within an electronics system. They are the means used by those skilled in the telecommunication art to most effectively convey the substance of their work to others skilled in the art. Figure 4(a) illustrates a four-level detector as used in Figures 2 and 3, 250 and 350. Referring to Figure 4(a) , a sampled signal 405, from the output of the interpretation filter 220 of Figure 2, or from the output of the A/D converter 315 of Figure 3, is alternately applied to integrate/dump circuits 410 and 415 for even and odd bits integration of 2-bit duration, respectively. To determine the binary value of the current bit, the integrated value is compared with a threshold selected from the table of fixed four thresholds in Figure 4(b) , the selection of which is only based on the binary values of the previous two bits. It should be noted that the group of four thresholds, i .e. -a 1 , -a 0 , a 0 and a 1 , correspond to a constant phase offset. Alternatively, another group of four thresholds may correspond to another constant phase offset. Figure 5 illustrates a detection scheme with variable multi-thresholds. The baseband signal 505 is sampled by the sampler 510 according to the sampling clock 520 and quantized by an A/D converter 515. The resulting signal I 1 can then be detected by the detection circuit 530, using the phase offset information I 2 obtained from the digital processor 540. Figure 6 illustrates a variable multi-thresholds detection circuit 530 as used in Figure 5 . The signal I 1 is applied to integrate/dump units 600 and 610, which are alternately connected to the compare unit 620 through a switch 615. The switching rate of the switch 615 equals to data rate. The period of integration 600 and 610 is two-bit duration. The phase offset information I2 is used to select one group of four thresholds for the compare unit 620, based on the previous two bits, 641 and 642, which are generated by the delay units 630 and 640. The groups of four thresholds which can be selected are listed in the table shown in Figure 7 (to be described below). Figure 7 illustrates a representative of eight groups of four thresholds as determined by 2 previous bits, b-1 and b-2, , and an estimated phase offset. The middle two thresholds, -a 0 and +a 0 , for bits "01" and "10" can be fixed, since they generally have little effect from different phase offsets. Therefore, with an estimated phase of, say, 25 %, a 4 can be used when the previous 2 bits are "11", while -a 4 can be used for "00". Therefore, for eight groups of four-thresholds, only half of the thresholds need to be adjusted, based on eight different phase offsets. Those skilled in the art may find different numbers of groups suitable for their systems. However, the system with eight groups is generally efficient enough for most applications. It should be noted that currently two bits are used to select a threshold for the reason that these two previous bits have ISI impact on the present bit. However, using only one previous bit with two-thresholds may reduce complexity at expense of performance. Figure 8 is a simulated bit-error-rate (BER) chart for the variable four-threshold detection in accordance with the present invention. Note that the BER performance shown by the dotted line is similar to that using fixed multi-threshold detection at 25% phase offset. However, as those skilled in the arts will appreciate, the present invention can be implemented with circuits with much less complexity. Therefore, the present invention provides an extremely economically feasible alternative to the conventional system. While the present invention has been described in connection with a communication system using 0.3-GMSK modulation, it should be apparent to those skilled in the art that the present invention can readily be applied to GMSK modulation with BT value between 0.2 and 0.5. Additionally, other filtering schemes such as "raised-cosine" can also be adapted to take advantage of the present invention.

Claims: A variable multi-threshold detection circuit for a sampled baseband signal for a receiver of a communication system using GMSK modulation in the transmitter, comprising: integrate/dump means (600, 610) for accumulating a predetermined number of samples of said sampled signal to generate a sum for a present bit; digital processing means (540) for generating an estimated phase offset (605) of said sampled base-band signal; level selector means (650) for selecting a threshold from a plurality of groups of multi-thresholds based on said estimated phase offset (605) and a binary value (641, 642) of a predetermined number of previous bits; comparing means (620) for comparing the sum from said integrate/dump means (600, 610) with said threshold selected by said level selector means (650) to generate a binary value of either one of "0" or "1" representative of said present bit; and delay means(630, 640) for delaying said present bit as determined from said compare means (620) to be used by said level selector means (650) to determine a next threshold for a next bit. A circuit as defined in Claim 1, wherein: said integrate/dump means (600, 610) comprises first (600) and second (610) integrate/dump circuits for alternately accumulating the samples of two-bit duration (615) from even and odd bits and outputting the sum, respectively, each bit having a predetermined number of samples. A circuit as defined in Claim 2, wherein: said level selector means (650) comprises a plurality of groups of first positive threshold (a i ), second positive threshold (a 0 ), second negative threshold (-a 0 ), and first negative threshold (-a i ), where "a i " is greater than "a 0 ", each "a 0 " and "-a 0 " being of predetermined values for all groups, the selection of one of "a i ", "a 0 ", "-a 0 ", and "-a i " thresholds being based on the binary values of two previous bits and said estimated phase offset (605). A circuit as defined in Claim 3, wherein each group of thresholds corresponds to a phase offset (605) of 6.25% from previous group and the "a i ", "a 0 ", "-a 0 " and "-a i " thresholds within each group correspond to the binary values of "11", "10", "01" and "00", respectively. A method of detecting bits of a sampled baseband signal using variable multi-thresholds in a receiver of a communication system using GMSK modulation in its transmitter, comprising the steps of: sampling (510) said baseband signal (505) to generate said sampled baseband signal with a predetermined number of samples per bit; integrating (600, 610) a plurality of samples of said sampled baseband signal bit to generate a sum for a present bit; estimating (540) a phase offset (605) of said sampled baseband signal; selecting (650) a threshold from a plurality of groups of multi-thresholds based on said phase offset (605) and a binary value of a predetermined number of previous bits; comparing (620) said sum with said selected threshold to determine a binary value representative of said present bit; delaying (630, 640) said binary value representative of said present bit after comparing (620) for selecting the next threshold for the next bit. A method as defined in Claim 5, wherein the step of selecting (650) said threshold selects said threshold from a plurality of four-threshold groups, each group being selectable by a predetermined phase offset (605), each of said four-thresholds being selectable by a predetermined binary value (641, 642) of two previous bits. A circuit as defined in claim 1, wherein said GMSK modulation has a normalized bandwidth between 0.2 and 0.5 in the transmitter. A circuit as defined in Claim 7, wherein: said integrate/dump means (600, 610) comprises first (600) and second (610) integrate/dump circuits for alternately accumulating the samples of two-bit duration (615) from even and odd bits and outputting the sum, respectively, each bit having a predetermined number of samples. A circuit as defined in Claim 8, wherein: said level selector means (650) comprises a plurality of groups of first positive threshold (a i ), second positive threshold (a 0 ), second negative threshold (-a 0 ), and first negative threshold (-a i ), where "a i " is greater than "a 0 ", each "a 0 " and "-a 0 " being of predetermined values for all groups, the selection of one of "a i ", "a 0 ", "-a 0 " and "-a i " thresholds being based on the binary values (641, 642) of two previous bits and said estimated phase offset (605).

Inventors: WANG JIANGZHOU, ZHANG KER, WANG, JIANGZHOU, ZHANG, KER
Applicants: CONEXANT SYSTEMS INC, CONEXANT SYSTEMS, INC.
Application year: 1995
Main classification: H04L25
IPC codes: H04L25, H04L27