Abstract:
A reverse link modulator modulates the incoming data sequence for the I and Q channels of a wireless device such that the resulting spreading sequence never undergoes a transition through the origin. Consequently, the modulator has a reduced peak-to-average ratio and provides an improved battery life.

Description:
RELATED APPLICATION 
     Provisional patent application No. 60/115,629 was filed with the United States Patent and Trademark Office on Jan. 12, 1999 and is incorporated herein by reference in its entirety. 
    
    
     FIELD OF INVENTION 
     The present invention relates to generation of pseudo random sequences and, more particularly, to generation of pseudo random sequences in a reverse link modulator. 
     BACKGROUND OF THE INVENTION 
     Many electronic systems, such as wireless communication systems, use the Quaternary Phase Shift Keying (QPSK) technique to modulate and demodulate incoming binary input data stream. 
     FIG. 1 illustrates the four phases I, Q, {overscore (I)} and {overscore (Q)}, forming the I-Q plane of a data channel. The QPSK technique uses these four phases to convert the random binary incoming data into a noise-like waveform by modulating the incoming data using a pseudo random sequence of bits. The modulation is carried out at the clock period of the pseudo random sequence generator. 
     As shown in FIG. 1, the four phases I, Q, {overscore (I)} and {overscore (Q)} have respective phase differences of 0°, 90°, 180°, and 270° with respect to the I-axis. Data coming through the I and Q channels of a reverse link modulator are identified by their positions along the I-Q plane. Furthermore, as shown in FIG. 1, such data may only assume positions at points A, B, C and D, which are respectively located along 45°, 135°, 225° and 315° of the I-axis of the I-Q plane. 
     Points A, B, C and D, have respective coordinates of (1, 1), (1, −1), (−1, 1) and (−1, −1), which are the only coordinates in the I-Q plane in which incoming data may reside. 
     Currently known systems do not impose restrictions on data transitions along the I-Q plane. Hence, a transition from, for example, point A to point C, in which both the I and Q coordinates of the data change, is allowed. Such a transition requires passing through the center of the I-Q plane and thus results in modulation of the carrier signal amplitude through a zero value, also referred to as zero crossing. 
     The greater the number of zero crossings, the higher is the degree of modulation of the carrier signal amplitude and the larger is the variation in the peak-to-average signal ratio. The large variation in the peak-to-average ratio, results in the relatively smaller battery life of the existing wireless devices (e.g. cellular telephones). 
     Therefore, a need continues to exist for a QPSK technique that has a reduced average-to-peak ratio and accordingly provides improved battery life. 
     SUMMARY OF THE INVENTION 
     A reverse link modulator, in accordance with one embodiment of the present invention, includes, among other components, a random sequence generator for generating a sequence of random pair of bits to modulate the incoming data bits. With each period of the pseudo random sequence generator clock, one bit of the pair of random bits changes. 
     The reverse link modulator also includes a long code mask, a long code generator, a complex multiplier and a pair of pulse shaping low-pass filters. 
     In some embodiments, the random sequence generator is a 2-bit state machine having four states ( 00 ), ( 01 ), ( 10 ), ( 11 ). The output signals generated by the state machine are the same as its states. Only transitions changing one bit of the 2-bit states are allowed. 
     The reverse link modulator, in accordance with the present invention, inhibits zero transitions of the spreading sequence. Therefore, the reverse link modulator has a suppressed carrier signal amplitude modulation and a reduced peak-to-average signal ratio. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the four phases I, Q, {overscore (I)} and {overscore (Q)} which form the I-Q plane of a data channel, as known in the prior art. 
     FIG. 2 is a block diagram of a reverse link modulator having a reduced peak-to-average ratio, in accordance with one embodiment of the present invention. 
     FIG. 3 illustrates the state transition diagram of the constellation sequence generator of the reverse link modulator of FIG.  2 . 
     FIGS. 4A and 4B respectively illustrate the auto-correlation and the Fourier transform of a sequence of a random reference signal. 
     FIGS. 5A and 5B respectively illustrate the auto-correlation and the Fourier transform of a sequence of random bits PNI(n) generated by the constellation sequence generator, in accordance with the present invention. 
     FIGS. 6A and 6B respectively illustrate the auto-correlation and the Fourier transform of a sequence of random bits PNQ(n) generated by the constellation sequence generator, in accordance with the present invention. 
     FIGS. 7A and 7B respectively illustrate the cross-correlation and the Fourier transform of a sequence of random bits PNI(n) and PNQ(n) generated by the constellation sequence generator, in accordance with the present invention. 
     FIG. 8 illustrates for the same percent of samples, the peak-to-average power ratios of both the reverse link modulator of the present invention and of the reverse link modulator of the IS-2000 standard, as known in the prior art. 
     FIG. 9 illustrates one embodiment of a state machine of the constellation sequence generator whose state transition diagram is shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION 
     A reverse link modulator  100  having a reduced peak-to-average ratio, in accordance with one embodiment of the present invention, is shown in FIG.  2 . Reverse link modulator  100  includes long code mask  102 , long code generator  104 , constellation sequence generator  106 , complex multiplier  108  and pulse shaping low-pass filters  110  and  112 . 
     Long code mask  102  is known in the art and contains a 42-bit wide data. Each reverse link modulator  100  contains a unique 42-bit data in its long code mask  102 . Accordingly, the data in long code mask  102  serves as the identifier of the reverse link modulator  100 . Because the reverse link modulator  100  is embedded in a wireless hand-held device, such as a wireless telephone, the 42-bit data in the long code mask is a tag, differentiating one wireless device from all the others in use. 
     Long code generator  104  is also known in the art (see “ Introduction to Spread Spectrum Communication”  by R. L. Peterson, R. E. Ziemer and D. E. Borth, 1995, Prentice Hall). Long code generator  104  receives the 42-bit wide data supplied by long code mask  102  at its input terminal I and generates random bits LC(n) at its output terminal O. 
     Constellation sequence generator  106 , in accordance with the present invention, receives random bits LC(n) (supplied by long code generator  104 ) as well as signal CLOCK and, in response, generates a pair of random and uncorrelated sequence of bits PIN(n) and PNQ(n) at its output terminals O 1  and O 2 , as described below. 
     Complex multiplier  108 , known in the art, receives incoming binary data DI(n) and DQ(n) (respectively from I and Q data channels) at its respective terminals I 1  and Q 1 . Data input bits DI(n) and DQ(n) are the real and imaginary parts of a complex data. Similarly, PNI(n) and PNQ(n) are the real and imaginary parts of the complex random signal. Accordingly, complex multiplier  108  multiplies the complex data with complex random signal to generate real and imaginary parts of the modulated data XI(n) and XQ(n), respectively. 
     Modulated data bits XI(n) and XQ(n) are supplied to known pulse shaping low-pass filters  110  and  112 , which filter out the low frequency signals to generate signals XI′(n) and XQ′(n). The operation of constellation sequence generator  106  is described next. 
     Concurrent references are made below to FIGS. 2,  3  and  4 A and  4 B. Constellation sequence generator  106  is a state machine whose state transition diagram  200  is illustrated in FIG.  3 . An embodiment  900  of the state machine of the constellation sequence generator  106  is shown in FIG. 9 of the drawings. Random bits  0  and  1  carried by signal LC(n) of long code generator  104  are the only bits (signals) supplied to state machine  900 . The state machine  900  has four two-bit states: ( 00 ), ( 01 ), ( 10 ) and ( 11 ). The states of state machine  900  are also its output signals. In other words, the states of state machine  900  are bits PNI(n) and PNQ(n) supplied to output terminals  01  and  02  of constellation sequence generator  106 , as shown in FIG.  2 . 
     As seen in FIG. 3, starting from state ( 00 ), if the input signal bit to the state machine is a binary 0, a transition to state ( 01 ) takes place and, accordingly, signal bits PNI(n) and PNQ(n) are respectively set to 0 and 1. If, on the other hand, the state machine receives a binary 1 while in state ( 00 ), a transition to state ( 10 ) takes place, thereby setting signal bits PNP(n) and PNI(n) to 0 and 1, respectively. 
     If while in state ( 01 ), the state machine receives a 0, it transitions to state ( 11 ) and sets both signal bits PNQ(n) and PNI(n) to 0. If while in state ( 01 ), the state machine receives a 0, it transitions to state ( 00 ) and sets both signal bits PNI(n) and PNQ(n) to 0. 
     If while is state ( 10 ), the state machine receives a 0, it transitions to state ( 00 ) and sets both signal bits PNI(n) and PNQ(n) to 0. If while in state ( 10 ), the state machine receives a 1, it transitions to state ( 11 ) and sets signal bits PNI(n) and PNQ(n) to 1 and 1, respectively. 
     If while in state ( 11 ), the state machine receives a 0, it transitions to state ( 10 ) and sets signal bits PNI(n) and PNQ(n) to 1 and 0, respectively. If while in state ( 11 ), the state machine receives a 1, it transitions to state ( 01 ) and sets signal bits PNI(n) and PNQ(n) to 0 and 1, respectively. 
     Advantageously, in accordance with the present invention, output signal bits PNI(n) and PNQ(n) are random and uncorrelated. Also, as seen from FIG. 3, state transitions are restricted to those involving only one bit of the two-bit states. For example, only transitions to states ( 01 ) and ( 10 ) are allowed from state ( 00 ). Therefore, a transition from e.g. state ( 00 ) to state ( 11 ) and from state ( 01 ) to state ( 10 ) are prohibited. The restriction ensures that in moving from one state to another state, one of the random signals PNI(n) and PNQ(n) remains unchanged. 
     Because constellation sequence generator  106  eliminates all zero crossings in the sequence of pseudo random bits (i.e., the spreading sequence) by inhibiting state transitions which require zero crossings, constellation sequence generator  106 , in accordance with one embodiment of the present invention, limits the modulation of the carrier signal amplitude and, as such, reduces the peak-to-average ratio of the modulated signals XI′(n) and XQ′(n). 
     FIGS. 4A,  4 B,  5 A,  5 B,  6 A and  6 B respectively illustrate the auto-correlation function and the Fourier transform of a sequence of random reference bits, as well as a sequence of random bits PNI(n) and PNQ(n) generated by the constellation sequence generator  106 . As seen from these Figures, the auto-correlation of sequence PNI(n) and PNQ(n) generated by the sequence code generator  106  is similar to that of the reference sequence, indicating that bits PNI(n) and PNQ(n) are random. 
     FIGS. 7A and 7B respectively illustrate the cross-correlation and the Fourier transform of a sequence of random bits PNI(n) and PNQ(n) generated by the constellation sequence generator  106 . As seen from FIGS. 7A and 7B, bits PNI(n) and PNQ(n) are random and not correlated with each other. 
     FIG. 8 shows the ratio of the envelope of the peak-to-average power in Decibels (dB) of a sequence  810  of random bits PNI(n) and PNQ(n), generated by constellation sequence generator  106 , after the sequence is modulated by Walsh code and filtered by the IS-95A pulse shaping filters (see “ Mobile Station - Base Station Compatibility Standard for Dual - Mode Wideband Spread Spectrum Cellular Systems, TR 45,  TIA/EIA- 95, Nov. 18, 1997). Also shown in FIG. 8 are the ratios of envelope of the peak-to-average power in dB of a sequence  820  obtained using IS-95C standard (also known as IS-2000, see  Physical Layer Standard for CDMA  2000  Spread Spectrum Systems, TR 45,  PN -4429, To be published as  IS- 2000.2, July 1999) as well as of a sequence  830  obtained using random sampling. 
     As seen from FIG. 8, at 0.1 percent of samples (i.e., the y-axis is at 0.1), which is defined as the percent of the sample points which cross the envelope, the ratio of the peak-to-average power of sequence  810  is higher than that of sequence  820  by 0.2 dB and is smaller than that of sequence  830  by 0.8 dB. Therefore, the reverse link modulator  100 , in accordance with the present invention, achieves substantially the same peak-to-average power as those of the IS-2000 standard. 
     FIG. 9 shows a logic block diagram of state machine  900  whose state transition diagram is shown in FIG.  3 . State machine  900  represents one embodiment of constellation sequence generator  106 , shown in FIG.  2 . State machine  900  includes flip-flops  902 ,  904 , exclusive-OR gates  906 ,  908  and clock generator circuit  910 . 
     As seen from FIG. 9, state machine  900  receives input signals LC(n) and signal CLOCK and, in response, generates signals PNI(n) and PNQ(n). Signal LC(n) is supplied by long code generator  104 , as discussed above. The signal to the clock input terminal, CLK, of flip-flops  902  and  904  is supplied by clock generator circuit  910 . Clock generator circuit  910  is an AND gate, which transfers signal LC(n) from its input terminal to its output terminal whenever signal CLOCK is at a high level. 
     Output signals PNI(n) and PNQ(n) are respectively generated at the output terminals OUT of flip-flops  902  and  904 . Exclusive-OR gate  906  has two input terminals I 0  and I 1 . Input terminals I 1  and I 0  of exclusive-OR gate  906  respectively receive signals LC(n) and the output signal of flip-flop  902 . The output signal generated by exclusive-OR gate  906  is applied to the input terminal IN of flip-flop  904 . Exclusive-OR gate  908  also has two input terminals I 0  and I 1 . Input terminal I 1  of exclusive-OR gate  908  receives and inverts signal LC(n). Input terminal I 0  of exclusive-OR gate  908  receives the output signal generated by flip-flop  904 . The output signal generated by exclusive-OR gate  908  is applied to the input terminal IN of flip-flop  902 . 
     With each occurrence of the CLOCK signal, if signal LC(n) is at a high level, a new low-to-high transition occurs at the clock input terminals CLK of flip-flops  902  and  904 . When a low-to-high transition occurs on the CLK input terminals of flip-flops  902  and  904 , the data present at the input terminals IN of these two flip-flops is transferred to their respective output terminals OUT, changing both PNI(n) and PNQ(n) bits. 
     The changes in the PNI(n) and PNQ(n) bits affect the output signals of exclusive-OR gates  906  and  908  which thereby change the input signals to flip-flops  902  and  904  respectively appear at PNI(n) and PNQ(n) when another low-to-high transition on signal CLOCK occurs. The state transition diagram of state machine  900  is shown in FIG.  3 . 
     Clock generator  910  receives clock signal CLOCK at its input terminal CLK and long-code signal LC(n) at its input terminal EN and generates output signal O. Output signal O changes when there is a change in signal LC(n). 
     The exemplary embodiments of the above invention are illustrative and not limitative. The invention is not limited by the state transition diagram of the random sequence generator; nor is the invention limited by the particular state machine that generates the random sequence. Further modifications are obvious to those skilled in the art and are intended to fall within the scope of the appended Claims.