Abstract:
A receiver for receiving an analog signal that is digitally modulated using differential binary phase shift keying (DBPSK) and demodulated by a quadrature demodulator into two signals. The receiver converts the two signals into two digital signals using 2-bit analog-to-digital converters. Samples of these digital signals are then encoded according to a maximum likelihood criteria algorithm and decisions are made based on the signal constellation, which identifies the original data transmitted.

Description:
FIELD OF THE INVENTION 
     The present invention relates to data communications, and more particularly, to methods and arrangements for recovering digitally modulated data in a differential phase shift keyed signal. 
     BACKGROUND OF THE INVENTION 
     Differential phase shift keying (DPSK) is a digital modulation technique that is used to encode information in a signal prior to transmitting the signal to one or more receivers. The information in a DPSK modulated signal is carried by the transition between distinct digital states rather than the absolute digital state, hence the term differential. There are several DPSK modulation techniques, including, for example, differential binary phase shift keying (DBPSK) and differential quadrature phase shift keying (DQPSK). 
     In DBPSK modulation, the phase of a constant amplitude carrier signal shifts between two positions, for example, 0 degrees and 180 degrees. As such, two distinct digital states are provided, for example, a binary “0” at 0 degrees and a binary “1” state at 180 degrees. 
     In DQPSK modulation, the phase of a constant amplitude carrier signal shifts between four positions, typically in increments of 90 degrees (e.g., 45, 135, −135 and −45 degrees). As such, DQPSK signals typically provide four distinct digital states, for example, a binary “00” at 45 degrees, a binary “01” at 135 degrees, a binary “10” at 225 degrees (or −135 degrees), and a binary “11” at 315 degrees (or −45 degrees). 
     In both DBPSK and DQPSK, information is differentially encoded in the transition from one state to the next, and/or a lack of thereof. As such, there is no need to recover or track the carrier signal within the receiver, as there would be in a receiver that is configured to demodulate conventional phase shift keyed signals. Instead, the receiver only needs to identify the relative phase shift between two adjacent bits to identify the information contained in a differentially encoded signal. This tends to simplify the design of the receiver. For example, a single channel receiver can be used to receive and demodulate a DBPSK signal. 
     There are, however, problems with single channel receivers. For example, a single channel receiver needs to provide an accurate delay time between bits in order to properly detect the phase shift and demodulate the signal. Further, small differences or jitters in the delay time tend to increase the resulting bit error rate (BER). 
     To avoid such problems quadrature demodulation of DBPSK signals can be employed. By applying quadrature demodulation techniques to a DBPSK signal, phase vectors are produced, which form a constellation when mapped on to a complex plane. The location of the resulting data points in the constellation can then be used to identify the “true” value of the originally encoded data. 
     Identifying the “true” value of the originally encoded data is further complicated by changes that occur to the DBPSK signal during transmission. For example, the medium through which the signal is transmitted typically alters the signal by “twisting” or otherwise changing the phase if either the sine and/or cosine signals within the transmitted signal. As such, the received phase of one or more of the components in the received signal may be different from the transmitted phase. 
     To correct for unwanted phase changes, the sender and receiver will typically attempt to characterize the affects of the medium during an initial transmission sequence. For example, the sender may send a series of differentially encoded binary 1&#39;s to the receiver for a specified duration. The receiver will then attempt to recognize the series and possibly identify the characteristics of the medium. A conventional receiver accomplishes this by sampling the received signal and applying certain decisions, for example, using logic circuitry, to increase the probability of correctly determining the original data sent based on the constellation. Such decisions can be represented in a decision or constellation table. 
     Since there are many dynamic and static factors involved in a transmission, different constellation tables (and logic) have been created in an effort to provide reduced BERs. For example, in certain receivers advanced logic is combined with highly accurate analog-to-digital converters to sample the received analog sine and/or cosine signals and determine the vector intercepts in the constellation. While such complex receivers tend to have lower BERs, they can be very costly. 
     Therefore, as can be appreciated, there is a continuing need for improved methods and apparatus for use in receiving DPSK signals. Preferably, the methods and apparatus will be simple and cost effective to implement, while also providing acceptably low BERs. 
     SUMMARY OF THE INVENTION 
     In accordance with certain aspects of the present invention, methods and apparatus are provided that allow differential encoded phase shift keyed signals to be received, demodulated and decoded using simple, cost effective sampling, decoding and decision techniques that result in a reasonably low bit error rate (BER). 
     For example, in accordance with one aspect of the present invention, an analog signal that is digitally modulated using differential binary phase shift keying (DBPSK) and transmitted over a communication medium, is demodulated by a quadrature demodulator into two signals that are then converted into two digital signals using 2-bit analog-to-digital converters. Samples of these digital signals are then encoded according to a maximum likelihood criteria algorithm and decisions are made based on the signal constellation, which identifies the original data transmitted. 
     In accordance with further aspects of the present invention, a method is provided for accurately identifying the original data within a differential phase shift keyed (DPSK) signal. The method includes the steps of taking a first digital sample of at least a portion of the DPSK signal at a first time, and taking a second digital sample of the at least a portion of the DPSK signal at a subsequent second time. Then the method includes producing a coded digital sample from the first and second digital samples. In accordance with certain further embodiments of the present invention, the first and second digital samples are 2-bit digital samples resulting from an analog-to-digital conversion of portions of the DPSK signal at different, closely-spaced, times, such as, for example, 100 nanoseconds apart. In accordance with still other embodiments of the present invention, the coded digital sample is produced by combining the 2-bit first digital sample with the 2-bit second digital sample to produce a 4-bit coded digital sample. In accordance with still further embodiments of the present invention, the apparatus also includes a receiver that is configured to receive a transmitted DPSK signal and output at least a portion of the transmitted DPSK signal to the sampler as the DPSK signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention may be had by reference to the following Detailed Description and appended claims when taken in conjunction with the accompanying Drawings wherein: 
     FIG. 1 is constellation chart depicting the intersection in the complex plane for a phase shift keyed (PSK) signal shifting between two distinct digital data values, namely “0” and “1”; 
     FIG. 2 is a block diagram depicting a receiving system configured to receive a differential phase shift keyed (DPSK) signal and reproduce the distinct digital data values contained therein; and 
     FIG. 3 is a table containing decision information that can be implemented to identify the distinct digital data values contained within a DPSK signal. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, there is illustrated a constellation chart  10  depicting the intersection in the complex plane for a differential binary phase shift keyed (DBPSK) signal shifting between two distinct digital data values, namely “0” and “1”, in accordance with certain embodiments of the present invention. 
     It has been found that the best demodulator for DBPSK signals, such as that Represented in FIG. 1, is a coherent demodulator. The coherent demodulator can be implemented by using a quadrature demodulator. For a DBPSK signal, the same symbol with the previous encoder output is symbolized as (Acos θ, and Asin θ), and the different symbol with the previous encoder output is (−Acos θ, and −Asin θ). In each case, “A” is the amplitude and θ is an unknown phase. The DBPSK signal is plotted in the signal space as shown in FIG.  1 . The receiver measures the coordinates (S I0 , and S Q0 ) at time t=T b  and subsequent coordinates (S I1 , and S Q1 ) at time t=2T b . For DBPSK, due to its differential nature, the task of detection is to decide if these two vectors are roughly in the same direction or not. If they are in the same direction, then a “1” is detected. Otherwise, if there not in the same direction, then a “0” is detected. Using inner product of the vector S 0  and S 1 , this detection method is mathematically expressed as follows: 
     
       
         If  S   0   T   S   1 &gt;0, then a “1” is detected;  (equation #1) 
       
     
     
       
         If  S   0   T   S   1 &lt;0, then a “0” is detected;  (equation #2) 
       
     
     wherein superscript “T” denotes the transpose of the vector. 
     Thus, equations 1 and 2 can be expressed equivalently as: 
     
       
         If [( S   I0   +S   I1 ) 2 +( S   Q0   +S   Q1 ) 2 −( S   I0   −S   I1 ) 2 −( S   Q0   −S   Q1 ) 2 ]&gt;0, then a “1” is detected;  (equation #3) 
       
     
     
       
         If [( S   I0   +S   I1 ) 2 +( S   Q0   +S   Q1 ) 2 −( S   I0   −S   I1 ) 2 −( S   Q0   −S   Q1 ) 2 ]&lt;0, then a “0” is detected.  (equation #4) 
       
     
     Referring now to FIG. 2, there is depicted a block diagram of an exemplary DBPSK receiver  100  that takes advantage of equations 1 through 4 in receiving and processing a DBPSK signal, in accordance with the principles of tile present invention. DBPSK receiver  100  reproduces the distinct digital data values originally encoded within the DBPSK signal. As illustrated the DBPSK receiver  100  is separated into in-phase components  104   I - 110   I  and quadrature-phase components  104   Q - 110   Q . Many of these components in DBPSK receiver  100  are clocked circuits/functions, as depicted by the clock input&#39;s open arrows. The clock is preferably recovered from the received composite signal using a conventional clock recovery circuit (not shown). 
     With this in mind, DBPSK receiver  100  includes an analog-to-digital (A/D) converter  102  that receives an analog in-phase (I) signal and an analog quadrature-phase signal (Q), for example, from a quadrature-phase demodulator (not shown). A/D converter  102  converts the I and Q analog signals into digital bits or digital signals. In accordance with certain preferred embodiments of the present invention, A/D converter outputs samples of each of the signals (I and Q) at approximately the same time, for example, once every 100 nanoseconds, and outputs a 2-bit digital sample for each signal. 
     The resulting I digital signal from A/D converter  102  is then supplied to delay circuit  104   I  and a latch  106   I . Similarly, the resulting Q digital signal from A/D converter  102  is supplied to delay circuit  104   Q  and a latch  106   Q . Delay circuits  108   I  and  108   Q  are each configured to act as a 1-bit storage devices to store S I0  and S Q0 , respectively. The outputs, following a delay time, from delay circuits  104   I  and  104   Q  are then supplied to latches  108   I  and  108   Q , respectively. 
     Latches  106   I ,  106   Q ,  108   I  and  108   Q  are used for synchronization purposes, such that the appropriate data, in accordance with equations 3 and 4, is provided to respective multipliers  110   I  and  110   Q . The resulting outputs from multipliers  110   I  and  110   Q  are provided to summation device  112 . The output of summation device  112  represents the originally encoded data. It is noted that elements  104   I - 110   I  make up coding circuitry  114   I , and similarly elements  104   Q - 110   Q  make up coding circuitry  114   Q . 
     Using receiver  100 , at each rising edge of the recovered clock, the data from the I and Q signals provides a data point in a two dimensional plane (see FIG.  1 ). The goal of differential decoding is to retrieve the original data that is encoded in the distance between two adjacent data points. Receiver  100  achieves this in two basic steps, namely a multiply step and a summation step. 
     For each channel (i.e., the I and Q signals), the product of the two adjacent outputs is calculated by multipliers  110   I  and  110   Q . In this manner, the result is coded into a format that the decision making of the data distance in the two-dimensional plane can be accomplished by simply adding the results of the two channels, using summation device  112 , rather than calculating the length of the vector. 
     FIG. 3 depicts a table  120  containing decision information that can be implemented in receiver  100 , to identify the distinct digital data values contained within the DBPSK signal, in accordance with the present invention. As shown in table  120 , the output of each channel is digitized at the clock rate into a 2-bit digital format. Thus, the possible outputs are 00, 01, 10, and 11. To preserve linearity of the data distance, the product of the adjacent data samples in each channel is coded according table  120 . The coding occurs in multipliers  110 I and  110 Q, or other similarly configured logic. When the coded results for each of the channels are added together using summation device  112 , the carrier to the upper level of the adder or adder function will yield the original data as encoded in the DBPSK signal. 
     Although certain embodiments in accordance with the present invention, are depicted in the accompanying Drawings and described in the foregoing text, it should be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.