Turning our attention to spread spectrum systems, specifically DS-CDMA systems, we notice that
IS-95 uses OQPSK on the UL for the above-mentioned reasons. Many researchers have shown the
benefits of OQPSK over /4-DQPSK, which is the main reason for its use. Since we are transmitting
chips in the CDMA systems and we typically operate with negative chip SNR values, it doesn??™t make
much sense to be able to have an option to differentially detect the chips themselves. This will be discussed
in more detail in Chap. 7.
114 CHAPTER TWO
Now CDMA2000 and WCDMA have chosen to use what is called Hybrid-PSK (HPSK). This
modulation scheme reduces the number of times the phase trajectories travel close to the origin.
Hence, instead of completely avoiding the origin, they have chosen to dramatically limit the number
of such occurrences. It accomplishes this task by limiting the phase transitions every other chip time
interval. Note this can be extended further.
REFERENCES
[1] R. E. Crochiere and L. R. Rabiner, ???Interpolation and Decimation of Digital Signals??”A Tutorial Review,???
Proceedings of the IEEE, Vol. 69, No. 3, Mar. 1981, pp. 300??“331.
[2] J. G. Proakis, Digital Communications, McGraw-Hill, 1989, New York.
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