CA2095717C - Phase combining method and apparatus for use in a diversity receiver - Google Patents

Abstract

A phase combining method and apparatus for use in a diversity reception radiotel ephone is described. The phase combining method and apparatus is a hybrid diversity technique which combines elements of maximal ratio combining (MRC) and level comparison selection diversity, which results in a simple and effective implemen tation of a diversity receiver with superior performance. The diversity receiver demodulates the received signals (115, 117), extracts the phase, forming two phase signals (209, 211). Then, the two phase signals (209, 211) are combined to form a third phase signal (213). One of the three phase signals is selected to be used for interpretation of a symbol in the Quadrature Phase Shift Keying (QPSK) constellation. The selection process is based on the received signal strength of the received signals.

Description

PHASE COMBINING METE[OD AND APPARATUS FOR
USE IN A DIVERSITY RECEIVER

Field of the Invention Generally, this invention relates to diversity receivers in radiotelephones and more specifically, to a method and apparatus for combining the phase of at least two received l 0 signals resulting in superior per~ormance in a digital or analog radiotelephone.

Background of the Invention l 5 A post-detection selection diversity receiver is one which uses at least two ~ntenn~?~ ~or i.LI~.~,ved reception of radio frequency (RF) signals. Each ~ntenn~ and corresponding rece*er together generate a received signal. From the at least two réceived signals available, one of the signals is selected.
2 0 Generally, there are two types of post-detection selection diversity receivers available today. In the first type, the selection of one the received signals is dependent upon the received signal strength (RSO of the signals. This type of diversity receiver is known as the level comparison diversity 2 5 technique. The selection in the second type is dependent upon phase error es~ tsc This second type of post-detection selection diversity receiver is known as the phase 1ikelihood comparison technique.
In the level comparison divelsi~y technique, the RSS of 3 0 each rece;ved signal is detected and compared to the others.
The received signal having the highest RSS value is selected for use by the radiotelephone. This simple system is relatively ef~ective in a Rayleigh fading environment and supe~ior to a SVBSTITIJTE SHEET

wo 93/06668 PCr/USg2/07193 209~71~ '' system having a single ~ntenn~ and receiver. However, the level comparison dive~ily technique does not offer any advantage over a single antenna system in a static environment.
The phase likPl;hood comparison technique utili~es a phase error estimate from each of the received signals, selecting the signal with the smallest estimated phase error.
This embodiment is the subject of a Japanese patent application number HEI2-253727, entitled "Diversity Reception l O Circuit", assignee Nippon Telegram and Telephone Corporation, filed March 28, 1989. First, the phase of each of the received signals is detected. Ne~t, the phase is plotted onto an ap~lop~;ate signal constellation. The constellation contains symbol decision points. A phase error vector is 1 5 created between the plotted phase vector and the nearest symbol rleri.~ion point. The phase error vector is created for each received signal. The rece*ed signal having the smallest phase error vector is used by the radiotelephone for ~urther proces~ing. The other received signals are ignored. Note that 2 0 the phase likPlih~od comparison technique does not utilize the RSS information.
An enh~ncemQnt to the phase likelihood comparison technique is discussed in the paper, All Digital Adaptive Carrier Trackin~ Coherent I)emodul~tor. Shigeki Saito, Harui 2 5 Yamamoto, Yasushi Yamao. NTT Radio Communication System Laboritories, February, 1991. The enh~ncem~nt c~mh;n~s the use of RSS information and phase error infom ~t;on for a two branch diversity receiver. The RSS of the received signals are compared to each other. If the difference 3 0 between the RSS values exceeds a predetermined threshold, then the received signal with the larger RSS value is used. If the difference between the RSS values is less than the predetermined threshold, then the phase error information is SiJESTiTlJT~ ET

WO93/06668 2û9~ 717 PCI/llS92/07t9~

used in the aforementioned m~nnPr to make the selection.
This enh~nceln~nt offers illlyl~)ved performance over straight phase likelihood comparison technique; however, further improvement in performance is desirable. Furthermore, the requirement to generate a phase error estimate restricts the phase likçlihood comparison technique to digitally phase rnodulated ci~
The market today is forcing m~nllf~cturers of radiotelephones to reduce the size, weight and power 1 0 consumption of the radiotelephone while increasing the perfo~nance criteria. Therefore, a need e~ists for a simple diversity receiver circuit which has improved received signal performance when compared to the conventions available today.

Summary of the Invention The present in~ention enCor~r~C~es an apparatus for cr.mhin;nE the phase of a first and a second received signal.
2 0 Each rece*ed signal has a phase and a signal quality indicator. The pparatus demodulates the rece*ed signals thereby extracting the phase from each received signal, and creating two phase signals. The apparatus combines the two phase signals to form a third phase signal. Then, the 2 ~ apparatus selects the most Llpl)lopl;ate one of the three phase s;gn~l~ The selected phase signal is used to recover information.

Brief Des~iption of the Drawings FIG. 1 is a block diagram of a radiotelephone system employing the present invention.

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WO 93/06668 ~ ~ 9 ~ 7 ~ l PCI/US92/07193 .. .

FIG. 2 is a block diagram of a divel~ily phase comhiner as employed in the present invention.
FIG. 3 is a phase comhinPr as employed in the block diagram of FIG. 2.
FIG. 4 is a phase selection circuit as employed in the block diagram of FIG. 2.
FIG. 5 is a state table for use in the circuit in FIG. 4.
FIG. 6 is a simulated performance chart of bit error probability versus signal-to-noise ratio in a static environment 1 0 for the present invention, the phase likAl;hood comparison diversity and the level comparison div~l~ity.
FIG. 7 is a sim~ ied performance chart of bit error probability versus signal-to-noise ratio in a flat Rayleigh fading - environment for the present invention, the phase likelihood comparison diversity, the level cornparison diversity and a single branch detector.
FIG.8 is a QPSK con~tell~tion having a symbol decision point in each quadrant.

Detailed Description of a Preferred Embodiment The present invention encompasses a phase combiner circuit for use in a ~live~ily receiver in a radiotelephone. A
2 5 diversity receiver cont~in~ at least two ~ntenn~ for receiving elctrom~gnetic waves. Each antenna has a corresponding receiver and attempts to receive a signal, resulting in two received signals, each received signal having a phase and signal quality indicator. The phase con~h;nPr is used to 3 0 comhine, the phase from two received signals creating a third phase signal. The signal qualil~ indicators include RSS, zero crossing jitter, phase error estimates or any other equally sufficient indicator. A selected signal quality indicator or a SU~3STE L aJ T ~ IEET

WO 93/06668 2V9~ 71 7 PCI/US9~/07193 com~in~t;on thereof is used to determine which of the $hree phase signals is to be used in decod-ing the data cr.nts.;ned within the rece*ed .sign~lR. In the preferred embodiment, the RSS is the chosen signal quality in~ic~tor.
S Although the present invention can be employed in communication systems using either analog or digital phase modulation srheme2~ the ~efe-led embodiment described herein operates in a communication system employ~ng a digital radiotelephone. A way in to measure performance in a system employ~ng a digital phase modulation scheme is to measure the bit error rate (BER) of the received signals. This embodiment offers an improvement in the BER of recovered data when comparing it to the aforementioned phase lik~lihood comparison diversity technique and the level l 5 comparison technique.
The performance i~ L~,ve~ent is achieved by implem~nting a hybrid ~l;vd~ily tech~igue which comhines çlernPnts of m~im~l ratio comhining (MRC) and level cnmr~qri.con selection livelsily. In fact, phase combining 2 0 livelsily is an effective a~lox;~ t;on of MRC. MRC is theoretically the opli~ liv~l~il,y technique; however, a two branch MRC becomes very complex involnng multiplication of the received signal and the comple~ ch~nn~l gain resulting in the equation stated below:
RMRC(t)= [ al2(t)s(t) + ~l(t)n'l(t)] + [a22(t)s(t) ~ cc2(t)n'2(t)~

where:
aj(t) is the fh~nnel gain, 3 0 nj(t) is complex Gaussian noise, s(t) is the signal component at the comhiner input, and n'1(t) and n'2(t) are phase shifted versions of nl(t) and n2(t) with identical statistical properties.

S~ B~ S-~i~LET

WO 93/06668 2 0 9 5 717 P(~/US92/07193 In the case where the two received signal levels difEer greatly, al >> a2, or ,a2 ~> al, then the MRC output can be ap~o~ ated by a level comparison selection diversity, namely:

RMRC(t) ~ al2(t)s(t) + al(t)n'l(t); if al ~ a2, or RMRC(t) ~ a22(t)s(t) + a2(t)n'2(t); if a2 ~> al If the two received signals are nearly equal, al ~ a2 ~ a, 1 0 then the M:RC output phase can be appro~imated by the modulo - 2Il average of the two phase signals as shown below.

RM~C(t) z [a2(t)s(t) + a(tjn'l(t) ] + [a2(t)s(t) + a(t)n'2(t) ]; if al ~ a2 ~ (X
l 5 ~ Al(t) ej~l(t~ + A2(t) e j~2(t) where:
Ai(t) is the individual branch amplitude, and ~l(t)iS the individual branch phase 2 0 In order to remove the branch amplitude dep~n~lçnries, the app~ ation Al(t) - A2(t) ~ A(t) is used. The identity 1 + eix -= 2cos(~12)ei (x~2) is used leaving ~MRC (t ) ~ ~ ~1 (t ) + [ ~32 (t ) ~ Ç~l (t ) ] mod2~ / 2]mod2~;; if al ~ ~ ~ a~2~
Thus, the phase comhining technique implemented herein is an approximation of an ideal m~m~l ratio combiner output.
FIG. 1 is a block diagram of a radiotelephone system 3 0 which employs the present invention. In the radiotelephone system, the fi~ed site transceiver 103 sends and receives electrom~enetic waves to mobile and portable radio telephones con~;ned within a geographic area. The radiotelephone 101 is STiTU','rr Slb~EET

. : . . . ' .

wo 93/06668 2 ~ 9 ~ 717 P~l/US92/07193 one such radiotelephone contained within the geographic area served by the fixed site transceiver 103.
The radiotelephone 101 contai~s two antennas 105, 107, which couple electrom~Fnetic waves transmitted from the S fi~ed-site transceiver 103, CO11Y~ ,lg them into electrical RF
signals. The electrical RF signals are received by the receivers 111, 113 for use within the radiotelephone 101. The rece*ed signals contain phase and RSS. The RSS indication is transmitted via signals 127, 129, and the received signals are 1 0 transmitted na signals 115, 117, from each of the two receivers 111, 113, to the diversity phase comhiner 119.
The diversity phase comhiner 119 demodulates the received signals 116, 117, thereby recover~ng the phase from each of the received signals 115, 117 and creating two phase signals. The l ~ two phase signals are combined to create a third phase signal.
One of the three phase signals is selected to be used to recover the information cont~inpd within the rece*ed signals 116, 117.
The s~le~;on of the phase signals is based on the RSS of the two received signals 116, 117. In the preferred ~mho~imPnt 2 0 the selected phase signal is decoded into symbols which are output to the processor 121, such as a MC68000 available from Motorola, Inc. and some accompanying memory, via the symbol signal 123. Other embo~;ments implemented in an analog system may use equally sufficient demodulation 2 ~ schemes.
The processor 121 is used to format the data between the user interface 12~, the tr~n~mitt~r 109 and the receivers 116, 117. The user interface 125 cont~in.~ a microphone, a speaker and a keypad. Upon reception of the decoded data, the 3 0 processor 121 uses the data symbols to recreate the voice or the data for use by the user interface 125.
Upon tr~n~mi.~inn, the data or voice signals are input from the user interface 125 to the processor 121. The signals ~IJE~TlTlJTr ~EET

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2~95717 WO 93/06668 - PC~/US92/07193 are digitized, encoded and timed im the processor 121 and sent to the transmitter 109 which converts the data into electrical RF signals. The electrical RF signals are converted into electrom~Fnetic waves and output by ~ntennp~ lO~i. The electrom~Fnetic waves are rece*ed by the fixed site transceiver 103. In the ~e~e~led embo~imPnt two antennas 105, 107 are used; however, more than two ~ntenn~.~ may be used on a given radiotelephone.
FIG. 2 is a block diagram of the diversity phase combiner l 0 119 of FIG.1. The received signals 115, 117, are input into individual phase demodulators 201, 203. The phase demodulators 201, 203 create a digital signal corresponding to the phase contained in the rece*ed signals 11~, 117. Tha phase signals 211, 209 are input into the phase combiner 207.
1 5 The phase comhinPr 207 creates a third phase signal 213 which is a modulo 2-lI average of the two phase signals 211, 209 input into the phase comhiner 207.
it;nn~lly, the phase signals 211, 209 are input into the phase sPlec~;on circuit 21~ which is used to select one phase 2 0 signal from the three available phase signals 211, 213, 209.
The selection of the phase signals is dependent upon the outcome of the level comparison circuit 205.
The level comparison circ~ut 205 compares the RSS of the first received signal 127 to the RSS of the second received signal 2 5 129. If the difference between the RSS of the first received signal and the RSS of the second received signal is greater than a predetermined threshold, then the received signal having the larger RSS is selected with the phase selection circuit 215. If the di~l e~lce between the RSS of the first 3 0 received signal and the second received signal is less than the predetermined threshold, then the comhine~ phase signal 213 is selected. In the ~e~led embo~iment, the predetermined threshold is set to 3 dB; however, it may be adjusted for any X;UE3ST~TlJTE SHEE~T

920~717 particular applir~tion The selected phase output signal 217 is input into the symbol slicer 219.
In the preferred embodiment, the symbol slicer 219 is used to determ~ne the information content of the selected signal and 5 is dependent upon the mo~ tion technique of the communication system. A quadrature phase shift keying (QPSK) modulation scheme is ~ e~ Other embodiments may employ e~ually sllffi~ient in~ormation recovery techniques and modulation techniques such as Gaussian minimum shift 1 0 keying (GMSK). The determination is made by plotting the phase of the selected signal within the QPSK constellation as illustrated in FIG. 8. The QPSK constellation has four quadrants formed by the I a~is 809 and the Q a~is 811. The phase is measured starting ~rom the I axis and rotating l S counterclockwise from 0 to 360 degrees around the ~our quadrants. Cont~ine~l in each quadrant is a symbol decision point 801, 803, 805, 807 located 45 degrees ba~ween the two a~es. The plotted selecte~l phase signal 217 falls within one of the quadrants and it is interpreted as one of the four possible 2 0 symbols by choosing the symbol ~leci~ion point which is closest to its plotted phase vector. For e~ample, a ~electe~ phase signal 217 having a phase of 48 degrees would fall in the first quadrant, thus~ it would it would be interpreted as symbol 801.
If the selected phase signal 217 had a phase of 91 degrees, then 2 5 it would be interpreted as symbol 803 from the second quadrant. The symbol decisions are output on signal 221 to the processor 121 for inteIpretation and ~ecollvel~ion into voice or data.
FIG. 3 is a block diagram of the pnase u~mhin~r 207 3 0 illustrated in FIG. 2. The phase comh;ner 20~ generates a third phase output signal 213 which is a modulo 2Il average of the two phase signals 209, 21i. The ;m~l~rn~ntation of the average consists of modulo ari~hmetic plus a divide-by-two. It SUB~ E SffEET

WO g3/06668 PCI/US~2/07193 209~7~7 lo requires only one adder, one subtracter, and a divide-by-two.
The phase coTnhinRr 207 may be im7, l~Tn~nte~l in either a digital signal processor or s~nple logic hardware.
FIG. 4 is an illustration of the hardware implementation 5 of the phase selection circuit 213 illustrated in FIG. 2. The phase selection ~;h. u~l ~ is a three-way multiple~er act*ated by the result in the level comparison circn~t 205. The level comparison circuit 205 inputs a three state, two-bit signal illustrated in the state diagram of FIG. 5. If the phase signal I 0 211 from the first rece*ed signal 115 is desired, a 0 is output on signals both D1 and D0. If the phase signal 209 from the second received signal 217 is desired, a 1 is output on signals D1 and D0. If the comhined phase signal 213 is desired, then a 0 is output on the signal D1 and a 1 is output on the signal D0, 1 5 resulting in the proper signal output at 217.
The two branch Lvel~ily reception described herein is used to il~lprOve perfoT~7~nce in a radiotelephone system. The phase comhining terhniq-ue offers performance il,lp~ovements over the two embor7.im~nt,s rli~c~ ser7 in the bach~ound, 2 0 namely, the level comr~T7~on diYelsily and the phase likelihood comparison dive~ijily. There are two en-vironments typically used $o evaluate performance of diversity reception.
First, a static environTnpnt is used to simulate a stationary radiotelephone and is characte7ized by con~t~nt, equal level 2 S signals at both ~7ntPnn 7~. Second, a theoretical Rayleigh fading e~viro~m~nt is used to simulate the motion of the radiotelephone in a moving car and is characterized by received signals which are modulated with independent Rayleigh distributed random process. The performance of a 3 0 diversity rece*er as measured by these two criteria illustrate the fi7nr7,~7m~nt~1 performance characteristics of a radiotelephone. Figures 6 and 7 are graphical representations of simulated results of single branch detection, le~lel-selection ~lll3~IITllTE S'rlEET

WO 93/06668 ~CI/US~2/07193 209~717 ~livel~ily, phase lik~lihood divf3~di~y and the phase comhiner Lvt:l~il,y in static and Rayleigh ~ading envirnnm~nt,s respectively. The results of FIGS. 6 and 7 assume II/4 QPSK
modulation with ideal delay detection.
FI~. 6 is a gr~phic~l repres~nt~t;on of probability of bit error versus signal-to-noise ratio of a received signal in the three aforementioned llive~ y receivers in a static environm~nt The graph illustrates that the probability of a bit error when using the phase combining diversity receiver is 1 0 lower than the other diversity receivers aforementioned. The phase comhining diversity is shown by the graph 606. The graph 603 illustrates the performance of the phase likelihood companson dive~ y as described in the background, and the graph 601 is the simple level comparison diversity performance also discussed in the background of this disclosure. As illustrated in FIG.6, level comparison selection ~livelsi~y o~ers no BER i~l~v~lllent in a static ellvil~l..m~nt relative to a single branch detector. Phase likPlihoo~
;Ron live-siiy outperforms level comparison diversity 2 0 in a static ellvho.n-.Pnt but, it is inferior to phase comh;ning diveL~i~y The graph FIG.7 illustrates the probability of a bit error versus signal-to-noise ratio in a Ralyeigh fading envlronment.
The results bf the level con~palison diversity, the phase 2 5 likelihood comparison divelsi~,y and the phase combining .live~sily are represented by graphs 703, 70~ and 707 respectively. All three Lvef~ily receivers show a great illl~l~ ve~ ! .e~t over a single branch detector 701 in this ~llvilo-~mPn~ however, the phase comhininE~ diversity receiver 3 0 mz~int~in.~ a slight performance advantage over the other div~l~ily receivers.
Disclosed is a phase combining diversity technique which offers superior static and Rayleigh faded performance over the SU~3STITUTE S~EET

WO g3/06~68 PCI-/US92/07193 2095~17 ~-' a~oreme~ioned selection diYes~ily methods. Furthermore, phass combiIung dive~ y can be easily im~lçmçnted in either a digital signal processor or a di~ital hardware detector.
Unli3~e phase l;k~lih~od comparison diversity, phase ~i con~hinin~ Lverii~y caIl be employed m systerns ut.il;7ing either analog or digital phase modulation tec~iques. Finally, ~e present invention can be e~ended to receivers wit~ more than two di~ersity branches.

10 What is claimed is:

SUE~ lJ I E ~ET

Claims (10)

CLAIMS:

1. An apparatus for combining at least a first and a second received signal having a phase and a signal quality indicator said apparatus comprising:

means for demodulating the first signal, thereby extracting the phase and creating a first phase signal;

means for demodulating said second signal, thereby extracting the phase and creating a second phase signal;

means for combining said first phase signal and said second phase signal, forming a third phase signal;

means for selecting a phase signal as one of said first, said second and said third phase signal; and means for recovering information from said selected phase signal.

2. An apparatus in accordance with claim 1 wherein said means for recovering information further comprises means for selecting a symbol representing said selected phase signal.

3. An apparatus in accordance with claim 1 wherein said means for combining the phase from said first and said second phase signal further comprises a modulo-2II average of said first phase signal and said second phase signal.

4. An apparatus in accordance with claim 1 wherein said means for combining said first and said second phase signal further comprises an effective maximal ratio combination (MRC).

5. An apparatus in accordance with claim 1 wherein said means for selecting a phase signal further comprises:

means for comparing the signal quality indicator of the first received signal and the signal quality indicator of the second received signal, including determining a difference between the signal quality indicator of the first signal and the signal quality indicator of the second signal and determining the higher signal quality indicator;

means for selecting, upon determining said difference is less than a predetermined threshold, said third phase signal;
and means for selecting, upon determining said difference is greater than said predetermined threshold, the received signal having the higher signal quality indicator.

6. An apparatus in accordance with claim 5 wherein said predetermined threshold is equal to 3 decibels (dB).

7. An apparatus in accordance with claim 5 wherein said signal quality indicator is the received signal strength.

8. A radiotelephone including at least a first and a second antenna, at least a first and a second radio frequency receiver and a diversity phase combiner, the first and the second radio frequency receiver coupled to the first and the second antenna respectively, the first and the second radio frequency receiver outputting a first and a second received signal, each received signal having a received signal strength (RSS) and a phase, the radiotelephone comprising:

means for demodulating said first received signal, thereby extracting the phase and creating a first phase signal;

means for demodulating said second received signal, thereby extracting the phase and creating a second phase signal;

means for combining said first and said second phase signal, forming a third phase signal;

means for selecting a phase signal as one of said first, said second and said third phase signal;

means for selecting a symbol representing said selected phase signal;

means for formatting said selected symbols into a sound pattern; and means for communicating said sound pattern to a user.

9. An radiotelephone in accordance with claim 8 wherein said means for combining said first and said second phase signal further comprises an effective maximal ratio combination (MRC).

10 A radiotelephone in accordance with claim 8 wherein said means for selecting a phase signal further comprising means for comparing the RSS of the first received signal and the RSS of the second received signal, including determining a difference between the RSS of the first signal and the RSS of the second signal and determining the larger RSS;

means for selecting, upon determining said difference is less than a predetermined threshold, said third phase signal;
and means for selecting upon determining said difference is greater than said predetermined threshold, the received signal having the larger RSS.