AU769743B2

AU769743B2 - Diversity reception by combining vectors weighted by coefficients which vary inversely to offset angles

Info

Publication number: AU769743B2
Application number: AU47429/99A
Authority: AU
Inventors: Hideaki Takahashi
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1998-09-09
Filing date: 1999-09-08
Publication date: 2004-02-05
Anticipated expiration: 2019-09-08
Also published as: CN1126313C; JP2000091971A; CN1248112A; AU4742999A; JP3543632B2

Description

S F Ref: 478141

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

O e@ Name and Address of Applicant: Actual Inventor(s): Address for Service: NEC Corporation 7-1, Shiba Minato-ku Tokyo

JAPAN

Hideaki Takahashi Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Nales, 2000, Australia Diversity Reception by Combining Vectors Weighted by Coefficients Vary-I. Inversely aS Offset Angles A A Invention Title: The following statement is a full description of this invention, including the best method of performing it known to me/us:- 5845 -1- DIVERSITY RECEPTION BY COMBINING VECTORS WEIGHTED BY COEFFICIENTS WHICH VARY INVERSELY TO OFFSET ANGLES BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates generally to diversity reception of uncorrelated signals from an array of antennas, and more specifically to diversity combining of complex vectors of diversity branches.

S o DESCRIPTION OF THE RELATED ART :Conventional diversity receivers are of several types. One of these types is the maximal ratio combining that is known for its excellent signal-to-noise performance.

!"-.-Usually, digital signal processors are used to implement this type of diversity receiver. In order to avoid expensive DSP circuitry, Japanese Laid-Open Patent Specification Hei-7- Is 307724 discloses a diversity receiver which can be implemented with low-cost digital circuits.

S4. If field strength variations occur in the transmission paths and hence irregularities among diversity branches are significant, a strong, out-of-phase signal may i be produced by one antenna while other antennas are producing weak, in-phase signals.

The shortcoming of the prior art diversity receiver is that if such a strong, out-of-phase *signal becomes a dominant factor, it will pull the diversity combined signal away from the signal phase point of the system's phasor diagram. This results in a lowering of the receiver's sensitivity to incoming signals.

A need thus exists to provide a diversity receiving technique that can be implemented with low-cost digital circuits and is tolerant of field strength variations.

SUMMARY

According to a first aspect, the present invention provides a diversity receiver comprising a plurality of antennas for detecting a signal modulated with a digital signal and producing therefrom a plurality of mutually uncorrelated antenna signals, and a plurality of diversity branch circuits associated respectively with the antennas. Each of the diversity branch circuits normalizes the amplitude of the signal supplied from the associated antennas, determines from the normalized signal an offset angle relative to a reference angle, produces a pair of in-phase and quadrature component vectors and an SR:\LIBQ] 1613.doc:gm

I

-2angle weight coefficient which varies inversely to the absolute value of the offset angle, and provides weighting of the vectors by the angle weight coefficient. A diversity combiner combines the weighted vectors of all of the diversity branch circuits and determines therefrom a combined angle. A decision circuit receives the combined angle s and produces a replica of the digital signal.

According to a second aspect, the present invention provides a method of diversity combining mutually uncorrelated antenna signals modulated with a digital signal, wherein the antenna signals are associated respectively with diversity branches.

The diversity combining method comprises the steps of normalizing the amplitude of 0to each of the antenna signals, determining, from the normalized signal of each antenna signal an offset angle relative to a reference angle, producing, for each of the diversity branches, a pair of in-phase and quadrature component vectors and an angle weight coefficient which varies inversely to the absolute value of the offset angle, weighting the vectors of each of the diversity branches by the angle weight coefficient, combining the weighted vectors of all of the diversity branches and determining therefrom a combined angle, and producing a replica of the digital signal from the combined angle.

The vectors are weighted independently of the strength of the signals associated with the diversity branch circuits.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be described in further detail with reference to the .i accompanying drawings, in which: o.:e* eeoc.

.*oeeo oie eoee .eee IR:.LIBQ1613.doc:gmm NE-960 -3- I Fig. 1 is a block diagram of a diversity receiver according to 2 a first embodiment of the present invention; and 3 Fig. 2 is a graphic representation of relationships between 4 offset phase angles and weight angles.

DETAILED DESCRIPTION 6 Referring now to Fig. 1, there is shown a diversity receiver 7 according to one embodiment of the present invention. The 8 diversity receiver comprises an array of antennas 1-1 through 1-k 9 for receiving a digitally modulated RF (radio frequency) signal 0 io such as 7/4-shift QPSK signal. A plurality of diversity branch 11 circuits 2-1 through 2-k of identical circuitry are respectively o: 12 connected to the antennas 1. Antennas 1 are spaced at such 13 intervals that their output signals are uncorrelated with each 1 4 other. Each diversity branch circuit 2 produces a pair of sin and cosine components of the branch signal. A diversity combiner (or S 16 vector combiner) 3 is connected to the diversity branch circuits 2 S" 17 to combine the outputs of diversity branch circuits 2 to supply a 18 combined phase angle to a decision circuit 4.

19 More specifically, each diversity branch circuit is comprised by a downconverter 10 where the RF signal is converted to a 21 complex values of IF (intermediate frequency) signal. The 22 amplitude of the in-phase and quadrature components of 23 the IF signal is limited, or normalized by a limiter 11. The 24 complex values of the signal normalized by the limiter 1 1 are used by a phase detector 12 to detect the offset angle 0 from the 26 reference signal point on a 3 6 0-degree circle on the phasor 27 diagram of the PSK signal.

28 Consider a polar coordinate system, or IQZ space in which 29 the vector is represented by magnitude R and angles 0 and 4, where 4 is the angle of the vector to the Z-axis that is normal to 31 the IQ plane. Since the polar coordinate system must be 32 transformed to the orthogonal coordinate system before the -4diversity branch signals are combined by the diversity combiner 3, the vector is projected on the IQ plane, yielding the following Equations: In Rn sin 4n cos On (1) Qn Rn sin 4n sin On (2) Zn Rn cos n (3) where n is 1, 2, k. Therefore, the combined vector that results from the combining of the diversity branch vectors is given by: (In.ZQnZZ.) S= Rsin. cosOn, Rn sin4. sin0, R cos) n (4) 10 Since the final decision of phase angle is determined on the IQ plane, the resultant vector is given by the following Equation: (Z ZQn) n IR,, sinn cosO,,~ R sin( sinO.) Therefore, the angle 0 sum of the resultant vector is represented by: Is sum= tan' (R n sin4, cosOn E R n sin, sinOn (6) Each diversity branch circuit 2 includes a read-only memory 13 for converting the offset phase angle 0 to a pair of cos 0 and sin 0, which are supplied to multipliers 14 and 15 respectively. Further provided is a field intensity detector 16 which detects from the IF signal the field intensity of the signal received by the associated antenna and produces a received signal strength indication (RSSI) signal which represents the magnitude R.

The signal R is used as a amplitude weight coefficient in the multipliers 14 and for weighting the cos-O and sin-O vector output signals of the ROM 13.

Read-only memory 13 further produces sin 4 which varies inversely to offset phase angle 0. This sin-4 signal is used as a phase weight coefficient in the multipliers 14 and 15 for weighting the cos-O and sin-O vector signals. Therefore, the output signals of multipliers 14 and 15 are respectively represented by Equations and Diversity combiner 3 includes an adder 17 for summing the in-phase components of all diversity branch circuits to give an output signal I In and an adder 18 for summing the quadrature components of all diversity branch circuits to give an output signal Q (R:\LIBQ1613.doc:gmm A division circuit 19 divides the output of adder 17 by the output of adder 18 to produce the phase angel Osum.

Decision circuit 4 checks the phase angle Osum against the reference phase angles of the phasor diagram and recovers a replica of the original digital signal.

Since the 7r/4-shift QPSK signal has eight signal phase points located at degree intervals on the 360-degree phasor diagram, decision thresholds for a given signal point includes the range between a plus 45-degree point on one side of the given phase point and a minus 45-degree point on the other side. The decision thresholds of the given signal point are established by equally dividing this 900 decision range at 11.250.

As shown in Fig. 2, phase offset angles 0 of 0, 11.250, 22.50, 33.750 and 45.00 are mapped to weight angles of 900, 67.50, 45.00, 22.50 and 00, respectively.

Since the offset phase point of 0° is the signal point, the value of at this point is set equal to the maximum weight angle of 900. Whereas, the offset phase points of 45.00 are located on the borderlines of adjacent decision ranges. To ensure clear-cut discrimination between the adjacent decision ranges, the values of at these points are set equal to the minimum weight angle of zero. Intermediate values of weight angles 4 are assigned to the remainder offset phase angles 0 such that varies inversely as the offset angle increases.

Since there is a total of 32 weight angles each weight angle is represented by a 20 five-bit code. Therefore, the phase detector 12 produces a five-bit digital code to represent the offset [R:\LIBQJI613.docgmm NE-960 -6- 1 phase angle 0. In the ROM 13, the five-bit offset angles 0 of 00, 2 11.250, 22.50, 33.750 and 45.00 are respectively converted 3 to five-bit codes of sin 900, sin 67.50, sin 45.00, sin 22.50 and sin 4 00, respectively.

It is seen therefore that by the weight control of the present 6 invention the diversity combined signal no longer deviates from 7 its decision angle due to the arrival of an out-of-phase strong 8 signal at one antenna while other antennas are producing in-phase 9 weak signals.

10 The diversity receiver of this invention is thus tolerant of oO 11 variations and irregularities that can occur in the signal strengths 12 of the diversity branches, and hence the decision error rate of the 13 diversity receiver can be constantly maintained at an acceptable 14 level.

In addition, the diversity receiver can be implemented with °.o16 simple modifications of conventional diversity receivers both in 17 respect of hardware and software.

S"18 In a modified embodiment of the present invention, 19 diversity combining is performed by using only offset phase information. This is achieved by setting R n 1 in Equation 21 Thus, Equation is rewritten as: 22 Osum tan (I sin On cos On/- sin On sin On) (7) 23 Since Equation implies that no errors can result from power 24 level variations of antenna signals, the modified diversity receiver may have a good reception sensitivity in comparison with the 26 previous embodiment. However, in the modified embodiment, 27 the phase information must be quantized with a greater number 28 of bits than the previous embodiment. It is seen that the field 29 intensity detector of Fig 1 is not required in the modified diversity receiver.

Claims

2. A diversity receiver as claimed in claim 1, wherein said diversity combiner 20 includes: :a first adder for summing the in-phase component vectors of all of said diversity branch circuits; ooa second adder for summing the quadrature component vectors of all of said diversity branch circuits; and a division circuit for dividing the output of the first adder by the output of the second adder to produce said combined angle.
3. A method of diversity combining mutually uncorrelated antenna signals modulated with a digital signal, said antenna signals being associated respectively with diversity branches, the method comprising the steps of: a) normalizing the amplitude of each of the antenna signals; b) determining, from the normalized signal of each antenna signal, an offset angle relative to a reference angle; [R:LIBQJ 1613 doc:gmm -8- c) producing, for each of said diversity branches, a pair of in-phase and quadrature component vectors and an angle weight coefficient which varies inversely to the absolute value of said offset angle; d) weighting the vectors of each of said diversity branches by the angle weight coefficient and independently of the strength of the signals associated with said diversity branches; e) combining the weighted vectors of all of said diversity branches and determining therefrom a combined angle; and f) producing a replica of said digital signal from the combined angle. Io
4. A method as claimed in claim 3, wherein the step comprises the steps of: summing the in-phase component vectors of all of said diversity branches to produce an in-phase combined vector; summing the quadrature component vectors of all of said diversity branches to Is produce a quadrature combined vector; and dividing the in-phase combined vector by the quadrature combined vector to produce said combined angle. A diversity receiver substantially as herein described with reference to the accompanying drawings. S".i 6. A method of diversity combining, said method substantially as herein described •.with reference to the accompanying drawings. 25 DATED this twenty-first Day of November, 2003 NEC Corporation Patent Attorneys for the Applicant SPRUSON FERGUSON °oiO° o•*e. oooe *•ooo [RLIBQj 1613.doc:gmm