CA2163952A1

CA2163952A1 - Mobile telephone receiver with adaptively inserted if filters and an if filter inserting method

Info

Publication number: CA2163952A1
Application number: CA002163952A
Authority: CA
Inventors: Taisuke Nakamura
Original assignee: Nec Corporation; Taisuke Nakamura
Current assignee: NEC Corp
Priority date: 1994-11-28
Filing date: 1995-11-28
Publication date: 1996-05-29
Also published as: EP0715417A2; US5758296A; AU3911295A; EP0715417A3; AU695017B2; JPH08154062A

Abstract

A mobile telephone receiver, utilized in a cellular communications system, is provided with a group of IF
bandpass filters (20a, 20b) which are arranged in parallel and which respectively have different passbands with each other. The IF filter group allows an incoming IF signal to be filtered out using one of the IF bandpass filters. An IF filter selector (30) is operatively coupled to the IF filter group and determines a degree of signal contamination by examining a signal which has passed through the IF bandpass filter selected for filtering the incoming IF signal. The selector changes the selection of the IF bandpass filters according to the degree of signal contamination.

Description

TI~LB OF THE lNv~ ON
A mobile telephone receiver with adapti~ely in~erted IF ~ilters and an IF filter inserting method RACR~ROUND OF THE ~Nv~r.
1. ~ield o$ the Invention The present invention relates generally to a mo~ile ~elephone receiver and mo~e specificall~ to such a recei~er having a plurality of IF ~intermedia~e frequency) bandpass filters one of which is adaptively in~erted Ln a signal path in accordance with recelve signal ~uality. The present invention is applicable to either of analog or digital type mobile radio co~munications systems.

2. ~scription of the Related Art It is known in the art to apply frequency reuse in a mobil~ radio communications system in order to satisfy various ob~ectives s~ch as large sub~cri~e~s capacity, effictent spectrum U~e~ adapta~ility to traffic density, etc. The frequency reuse refers to the use of channel~
on ~he ~ame carrier frequency to cover different are~s which are sepa~ated fr~m ~ne another by su~ficient distance6 so that cochannel in~erference is no~
ob jectionable. A ~ystem utilizing ~u~h concept is known a~ a cellular mobile radio system.
2S The most serious e~ect o~ the mo~ile en~ironment o~
cellular sy~tem performance is the ~reation of fuzzy cell boundaries. The ~ignal contour is actuall~ only a con~enient ideaLization of what turns out to be a very messy reality. The fuzziness of cell boundaries i~
caused by topographical ~actor~ which ~ary wi~h different cell~ ~n~ also by constantly chan~in~ ~ran~mis~ion charactcristlc~.
Adjacent channel interference occu~s when ~ignal energy ~rom one channel spills ove~ into an ad~ac~nt ch~nnel or when a filter on the recei~er is too "loose"

- 2 - ~E-683 and captures energy ~rom a broader band ~han it really nee~s to. Adjacent channel interference can be eliminated if receiver filte~ing is considerably tighter.
~owever, th~ tighter th~ ~ilter requirements, the more S e~pensive the implementation, and as such, i~ is a common pr~ctice that adjacent channels are not used within the same cell because o~ loose receiver filtering.
However, as mentione~ a~ove, it is practically impo~ible to exactly define or estimate the actual cell boundaries. Therefo~e, it i~ a current practice to narrow or t~gh~en the pass~and of an IF h~nAr~ss, p~ovide~ in a ~eceiver, so as to obviate adja~ent channel interference. Thig prior approach, however, is objectionable in ~erms of high fidelity of Le~Lo~ced lS vo~ce. Further, even if a traffic density is lower~d to such an extent that the a~iacent channel interference rarely occurs during mi~night (for example), the narrow I~ ban~pa~ filte~ r~r~ utilized under such a condition.
What is ~esired is there~ore to adaptively ~elect one of a plurality of IF ban~paqs filters ~epen~in~ on actual mobile environm~ts.
STJMMA~Y OF THE INVENTION
~t is an ob~ect o~ ~he pre5ent invention ~o provi~e a mobile radio telephone receiver which is equipped with a plur~lity of I~ filter~ one of which is adaptively in~e~ted into a si~nal path depending on mea~ured re~eive signal quality.
Anothe~ object of the present invention i~ to 3~ pro~ided a method of adaptivel~ inser~ing one o~ a plurality of IF fLlters in a ~ignal path depending on measured receive signal quality.
~hese objects are fulfilled by a mobile telephone receiver, utilized in ~ cellular communlcations system, is provided with a group of IF bandpass fllters which are _ 3 _ NE-683 arranged in para~lel and ~hich respecti~ely have different pas~ands with each other. The IF fil~r group allow~ an inco~ing IF signal to be ~iltered out usin~ one of the I~ bandpass filters. An IF filter selector i5 operatively coupled to the IF filter group and det~ ;nP-s a degree of signal contaminati~n by examining a signal which has pa~sed through the IF bandpass filter selec~ed for fllter~ng the incomlng IF signal. The ~elector changes the selection of the IF ~n~paæs filter~
~0 according to the degree of signal contA in~tion.
More spe~i~ically, a first aspect of the present invention resid~s in a mobile ~elephone receiver used in a cellular ~ommuni~ations system, comprisi~g; a group of IF (int~r~7~iate frequency) b~p~ss filters which are arranged in parallel an~ which have respectively different passbands with each other, said group allowing an incoming IF signal to ~e filtered out ucing one of ~aid I~ bRn~pa~3 filter~; and an I~ fil~er selec~or for det~r~; n; ng a degree of signal contamination ~y ~ ng a signal which has pa~sed through said one of said IF
~ndpass filters, said selec~or ~hanging selection o~
said IF bandpass filter~ ac~ording to said degree of signal cont~rinætion.
A seccnd aspect of the present invention resides in a method of adaptively controlling IF signal filt~ring at a mobile telephone receiver used in a cellular C~ -- n; cations system, ~aid method comprising the steps of: ~a) allowing an incoming IP signal ~o b~ filter~d out using one of a plurality of IF ~ s filter~ which are arr~nged in parallel and which hav~ respecti~ely different passband~ with each o~her; (b) deter~ ng a degree of ~ignal cont~m;n~tion by ~m~n~ng a signal which ha6 pa6sed th~ough said one of sald ~F h~nAr~ 3S
filter~; and (c) changing selection of said IF bandpass filters according to said degree of ~ignal contamination.

- 4 - N~-683 BRIEF D~.~r,~TPTIoN OF ~HE DRAWINGS
~he features and advantages of the presen~ invention will become mo~e clearly ~pprecia~ed from the following description taken in conjunction with the accompanyins drawings in which like elements are denoted by like reference numerals and in which:
Fig. 1 is a block diagram showing a preferred embo~iment a~ the present invention;
Fig. 2 is a diagram schematically showing a passband of a filter o~ Fig. l;
Fig. 3 is a diagram schematically showing a passband of another ~ilter of Fig. 1 Fig. 4 is a sketch schemati~ally showing a waveform of an I-ch~nnel ( or Q-~hannel) ~ignal appe~ring in ~he arrangement of Fig. l;
Fig. 5 is a pha~e diagram for explaining a signal deviation from a normal signal level according to the embodiment of the presen~ invention; and Figs. ~-8 are sketche~ e~ch for describing the operation of the embodiment of the present in~ention.
DE~ATT~n DES~RIPTIO~ OF
PREFERRED ~MRODIME~TS
A preferred embodiment of ~he present invention is dis~ussed with reference to Figs. 1-8.
The present invention re~ides in provision of a plurality of IF bandpass filtPrs on~ of which is adaptively inserted in a receive signal path in accordance with a degree of measured receive signal contamination.
Re~erring to Fig. 1, there i~ shown part o~ a mobile radio telephene receiver in block diagram ~orm. A~
shown, ~ QPSK (quaternary phase sh~ft keying) modula~ed signal is applied to a front end 10 by w2y o~ an antonna 12 and a duplexer 14. A si~nal to be transmitted is ~pplied to the duplexer }4 from a transmitter (not shown) which is ir~elev~nt to the present invention. An RF
(radio frequency) signal, which is picked up by the front end 10, is amplified and applied to ~ mixer 1~ which con~erts the RF signal to an IF signal. A local oscillator ~oupled ~o (or included in) the mixer 16 is not shown in Fig. 1 in that the operation of the m~xer per se i~ well known in th~ art~ The IF signal is then applied to an IF bandpass filte~ 20a via a switch 18 in the illustrated case.
The pas-~band of f~lte~s 20a ~nd 20b are 6chemati~slly shown in ~ig~. 2 a~d 3, respectively. A~
shown in Fig~. 2 and ~, the IF bandpas~ filter 20a has an e~fec~iv~ pas~band of 40KHz (=20KHz ~ 20~Hz), while the IF filter bAndpass ~Ob has an effective pass~and of 20~z (=lOKHz + lOKHz). It is assumed ~hat a center f~equency Fc of each of the filters 20a and 20b is 80MHz merely by way of example.
The output of the filter 20a is applied, via ~
switch 22, to a demodulatox 24 which takes the fonm of coherent ~PSK demodula~or in this particular case. As is known in the art, according to coheren~ detection which is use~ to demo~ulate a received M-~ry PS~ modulated signal, the received signal iQ mixed with a locally reproduce~ carrier to produce I- and Q-channel qignals (viz., baseband signal). For further ~etai~ of N-ary PSK demodul~tor, reference should be made to U.S. Patent 5,157,604 granted ~o Iwasaki, et al. and assigne~ to the sam~ entity of the present application. The I- and Q-~hAnn~l signals u~dergo analog-to-digital ~A/D) con~er~ion at a ~unctional block 26.
Fig. 4 is a sketch for the sake of a better undPrst~ g of 6ampling and quantization of the I-channel (o~ Q-channel) sign~l applie~ to the ~/~
~onverter 26. The sampled le~els are quantized by 16 3S (viz., 4 bits) levels in the present embodiment. The A/D

conversion is notorious to one skilled in the ar~ and hence a further description ix deemed redundant and accordingly omitted for brevity.
The I and ~-channel signals, which ha~e been digitized at ~he A~ converter 26, are fed ~o a sig~al deviation ralculator 28 which ~orms part of a filter selector 30. The operation of the ~ignal deviation cal~ulator 28 is briefly discussed with reference ~o a phase diagram shown in Fig. 5. Four black circles (depicted by Q1, ~2, I1 and I2) on I and ~ axes indicate normal (re~erence) sign~l point~. If the coor~inate6 of a ~eceived signal point (depicted by "x" in Fig. 5) applied from the A/D converter 26 i~ ~I=10, ~-14), it is r~adily underst~od that a de~iation of the receive signal poi~ x from the normal point Q1 can be calculated. A
plurality of deviation~, each of which is calculdted as mentioned above, are successi~ely added in a suitable me~ory (not ~hown) in a ~eviation accumulator 31 predet~ InR~ time interval (lOms merely by way of example). When the predet~r~ne~ time perio~ elap~es, the accumulator 3~ applies the sum o~ ~he devia~ions to a comparator 32 to which a threshoLd is also applied fro~ a re~erence level generator 34. When the c~ r~tor 32 dete~ts that the sum of the deviation cro~æes the threshold, it applies a filter change signal to the swi~ches 18 and 22. It is to be n~ted that the threshold level may be empiricall~ ~ete~m;ned through field work investigation.
~he operation of ~he present embadiment is furthe~
de~cribed.
When the telephone recei~er of Fi~. 1 is initially power~d, the comparator 32 applies a logic 1 (for example) to the switches 18 and 22 so as to originally select the I~ filter 20a whose passband is broader than that of the IF filter 20b. If an ad~acent channel i~ not ~ 7 - NE-683 utilized in neither of the neighboring cells, no ad~acent channel interference occurs.
On the contrary, it i5 assumed, when the receiver of Fig. 1 is initially powered, tha~ a given receiver located in an a~jacent cell has alread~ used an IF
bandpa~s ~ilter which is a counterpart of t~e filter 20a.
In such a case, ther~ exists the high proba~ility that the adjacent channel interfe~ence is induced a~
understood from Fig. 6. Thus, the deviation accumulator 31 outputs a value which exceeds the threshold level produced from the reference leve~ generator 34. ~he comparator 32 re~ponds to issue a lo~ic O which render~
the switche~ 18 an~ 22 to select the ~ilter ~Ob.
Following thi~, the above mentioned given recei~er within ~he ad~acent cell is also ~orced to use a narrower IF
bandpass filter which corresponds to the filter 20b. It is to be noted that the abo~e ~entioned ordex o~ changinq to ~he narrow bandpass filters at the two ~eceiver~ is reversed ~epending on the operating conditions of the two rece1~ers. If the two recei~e~s located in the respecti~e ad~cent cells begin to use the narrower IF
ban~pass filters, no adjacent channel interference occur~
a~ under~tood from Fig. 7.
on the other han~, it is a~sume~, when the receiver of Fig. 1 is ini~ially powere~, that a given receiver located in an ad~acent cell has already u~ed a n~rrow IF
bandpass filter which is a counterpart of the filter 20b.
In such a case, there also exi~t~ the probabilit~ that the adjacent channel interference occurs as unders~ood from Fig. 8. Thus, the devi~ion accumulator 31 outputs a value which exceeds the threshold le~el produced from the referen~e le~el generator 34. The comparator 32 responds to such a ~ituation and i~sues a logic O which renders the switches 18 an~ 2~ to select the filter 20b.
35 Sin~e ~he receiver of P~g. 1 begins to use the narrow IF

a - NE-68 bandpass fil~er ~Ob, adjacent ~h~nnel inter~erence can effectivel~ ~e avoided.
The outputs of the A~D converter 26 are also appl$ed to a control signal extractor 40 ~rom which a con~rol signal~s) is extracted and applied to a con~roller such as a CPU (central processing unit) fo~ super~i~ing an overall operation of the ~eceiver of Fig. 1. An audio signal from the con~rol signal extractor 40 i~ applied to a speaker ~ia a dri~e~ 42. These blo~k~ are not co~rerned wi~h the present invention.
It will be under~tood tha~ the above disclosure i8 representative o only one possible embodiment of the present invention and that the concspt on whi~h the invention is ~sed is not specifically limi~ed thereto.
For example, the receiver of ~ig. 1 can readil~ be modi~ied such ~s to include more than ~wo IF ~ilters.
With ~his modification, the co~parator 32 has to be modified to i~sue three di~ferent switch control signals and, the reference level generator 30 is modifie~ so as to produce two different threshold~.
Furthe~, the present invention is in no way limited to the QPSK modulated signal but is applicable to m-ary (m=8, 16, 32, ...) PSK system. Still further, the pre~ent invention is able to use various signals 2S modulated using various technique~ ~uch as ~S~ (f~equency shift ke~ing)r ~tc.

Claims

1. A mobile telephone receiver used in a cellular communications system, comprising:
a group of IF (intermediate frequency) bandpass filters (20a, 20b) which are arranged in parallel and which have respectively different passbands with each other, said group allowing an incoming IF signal to be filtered out using one of said IF bandpass filters; and an IF filter selector (30) for determining a degree of signal contamination by examining a signal which has passed through said one of said IF bandpass filters, said selector changing selection of said IF bandpass filters according to sadi degree of signal contamination.

2. A mobile telephone receiver as claimed in claim 1, wherein said IF filter selector includes:
a calculator (28) for receiving a baseband signal outputted from a demodulator and calculating a receive signal deviation from a normal signal point, said calculator successively producing a plurality of receive signal deviations;
an accumulator (31) coupled to receive and accumulate said plurality of receive signal deviations at a predetermined time interval, said accumulator outputting a value accumulated over said predetermined time interval; and a comparator (32) for comparing said value produced from said accumulator with a threshold, said comparator issuing a control signal which is used for said selection of said IF bandpass filters.

3. A method of adaptively controlling IF signal filtering at a mobile telephone receiver used in a cellular communications system, said method comprising the steps of:
(a) allowing an incoming IF signal to be filtered out using one of a plurality of IF bandpass filters (20a, 20b) which are arranged in parallel and which have respectively different passbands with each other, (b) determining a degree of signal contamination by examining a signal which has passed through said one of said IF bandpass filters; and (c) changing selection of said IF bandpass filters according to said degree of signal contamination.

4. A method as claimed in claim 3, wherein said step (b) includes the steps of:
(d) receiving a baseband signal outputted from a demodulator (24) for calculating a receive signal deviation from a normal signal point, and successively producing a plurality of receive signal deviations;
(e) receiving and accumulating said plurality of receive signal deviations at a predetermined time interval, and outputting a value accumulated over said predetermined time interval; and wherein said step (c) includes, comparing said value produced at step (e) with a threshold, and issuing a control signal which is used for said selection of said IF bandpass filters.