Differential PSK Signalling in CDMA Networks

1. Field of the Invention

The present invention relates to the field of telecommunications. More particularly, the present invention relates to a method and a system for Code Division Multiple Access (CDMA) communications.

2. Description of the Related Art

In current Code Division Multiple Access (CDMA) systems, both coherent and non-coherent demodulation techniques are used. Coherent demodulation techniques require an accurate carrier phase estimation of a received signal, which in turn necessitates a pilot signal or a phase-locked loop. Both requirements are impractical, especially for an uplink to a satellite or a base station. Non-coherent orthogonal demodulation techniques require extensive correlation and comparison computations for symbol detection, resulting in a high implementation complexity. Additionally, non-coherent demodulation techniques provide poor performance compared to coherent approaches.

When DS-CDMA (Direct-Sequence Code Division Multiple Access) signals are coherently demodulated, accurate phase tracking of individual phases is required. When DS-CDMA signals are detected via non-coherent orthogonal demodulation, a performance penalty is paid, while also involving complex implementation. What is needed is a simple, yet powerful signalling scheme for CDMA-based communications that reduces implementation complexity while improving system performance.

The present invention provides a method and a system for demodulating multipoint-to-point and point-to-multipoint signals transmitted in a DS-CDMA network without requiring an estimate of the carrier phase. Further, the present invention provides a simple detection technique for processing demodulated signals and has a better performance than conventional non-coherent techniques.

The advantages of the present invention are provided by a method and a system in which (coded) user message bits are grouped into successive groups. A differential phase signal is generated for each message bit group by mapping each message bit group on to a predetermined PSK constellation. An absolute phase signal is generated for each message bit group by adding the differential phase for the current message bit group to the absolute phase signal for the preceding message bit group. PSK modulation is invoked with the absolute phase for each message bit group to form an RF signal. The RF signal is spread using two code sequences and then transmitted.

At the receiver, the RF signal is non-coherently quadrature demodulated and despread using the code sequences. Successive blocks of the demodulated, despread RF signal are phase compared for extracting the differential phase carrying the (coded) user message. Lastly, the user message is recovered.

The present invention is illustrated by way of example and not limitation in the accompanying figures in which like reference numerals indicate similar elements and in which:

FIG. 1 is a block diagram showing the basic functional elements of a transmitter using differential PSK signalling in a DS-CDMA network according to the present invention; and

FIG. 2 is a block diagram showing the basic functional elements of a receiver using differential PSK signalling in a DS-CDMA network according to the present invention.

The present invention simplifies RF demodulation and subsequent detection processes for CDMA communication systems and, consequently, reduces implementation complexity and power requirements that place critical constraints on some applications. For example, in a wireless communications system, the present invention greatly facilitates frequency conversion and baseband processing at a base station in a cellular or PCS network, or aboard a satellite in a space system, thereby requiring less power since the present invention does not require a conventional prediction of the carrier phase for each user signal received on an uplink. Further, collective down-conversion of a received RF signal to a baseband signal for all user signals transmitted on the same carrier is possible. Corresponding hardware and power reductions are also achieved at a user terminal for a downlink, which is significant, especially for handsets or portable communication devices. Moreover, the present invention provides an efficient mechanism for separating different uplink signals for on-board satellite processing and/or downlink switching.

FIG. 1 is a block diagram showing the basic functional elements of a transmitter using differential PSK signalling in a DS-CDMA network, for example, as part of a terrestrial wireless network, such as a cellular and a PCS, or a satellite communications network. Groups of information bits that form a user message from an application are encoded, such as trellis encoded, before being mapped into signal points in a chosen PSK constellation in a well-known manner. A bit/symbol interleaver may be employed either before or after symbol mapping for hard- or soft-decoding. The coded information bits are mapped by MPSK mapper 11 into a differential phase signal .DELTA..phi..sup.(i) (n). The resulting phase from each group of coded information bits is treated as a differential phase. An absolute phase .phi..sup.(i) (n) is derived for each signalling interval by adding the differential phase .DELTA..phi..sup.(i) (n) for the current signalling interval to the absolute phase value .phi..sup.(i) (n-1) for the preceding signalling interval by a differential phase encoder 12 in a well-known manner.

The absolute phase .phi..sup.(i) (n) is impressed on a carrier having a desired carrier frequency by using a standard PSK modulator 13. After carrier phase modulation, the modulated signal is spread in a well-known manner at 14 using a Walsh function assigned to the user by the system. Then, a group code sequence associated with, for example, a terrestrial base station or a satellite beam, is used for further spreading the modulated signal at 15 in a well-known manner before RF transmission at 16. RF amplification may be applied for generating a desired transmitted power prior to feeding the signal to a transmit antenna.

FIG. 2 is a block diagram showing the basic functional elements of a receiver using differential PSK signalling in a DS-CDMA network, for example, as part of a terrestrial wireless network, such as a cellular and a PCS network, or a satellite communications network. At the receiver, a received RF signal is noncoherently down-converted to a baseband signal, and then despread by code sequences corresponding to the code sequences used at the transmitter for spreading and, thereby, extracting a desired user message signal from other user signals sharing the same CDMA channel. A phase comparator is used that calculates the phase difference between two consecutive despreader outputs for detecting the user message signal, followed by a channel decoder.

In FIG. 2, after appropriate RF filtering and amplification, a received RF signal is down-converted to a baseband signal by non-coherent quadrature demodulation at 21a and 21b, and lowpass filtering at 22a and 22b. A message signal intended for a particular user is extracted from the quadrature phase components of the message signal by first despreading in a well-known manner using the group code for the user at 23, and then despreading using the user Walsh function at 24 and an integrate-and-dump process at 25a and 25b. For uplink reception, the down-conversion and group code despreading may be done collectively for all received user signals affiliated with a group. The outputs from integrator 25a and 25b are then differentially phase compared in a well-known manner at 27 using a delay of T at 26 for yielding a differential phase that carries the desired information. When bit/symbol interleaving is used at the transmitter, bit/symbol deinterleaver is employed in a well-known manner in accordance with the interleaver position. A channel decoder 28, such as a Viterbi decoder providing soft- or hard-decision, recovers the user message signal.

While the present invention has been described in connection with the illustrated embodiments, it will be appreciated and understood that modifications may be made without departing from the true spirit and scope of the invention.