Note: Descriptions are shown in the official language in which they were submitted.
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FREQUENCY AND TIME SLOT SYNCBONIZATION
USING ADAPTIVE FILTERING
Fi~l~i Of th~ TnY~nti. n
s
The present i~e,~tion relates generally to the field of
CG~ niC-s-t;Qn~ and particularly to frequency and time slot
synchro~i70t;Qn in a mobile cc ~ ication environment
1 0 R~rl~, lo~ of tl e Inven~ior
Since a mQ~;le ~ ~ioteleph~ne does not have an accurate
~9n?.~gh frequency Lefelehce internally, it musst make fine
frequency a~ t~ent~ to achieve the ,eq~ilc~ frequency
15 s~ucLo .i~9t;Qr~ to a base 9totio' frequency. In a time ~lom~in
multiple access (TDMA) s~Ole~ multiple logical h~nnels
are transmitted on the same frequency, but separated in time,
FIG 2. To ~Q ~ i~te with a base station, the mobile
radiotelephone must also fint the boundaries of these time
2 0 slots, caLled time slot synch~o ~i70~n
The different base stst;Qn~ in a cellular radiotelephone
system msir~toin very accurate frequency refe~e..ces, but
utiLize di~e~eut tr~o-nsmis~iorl frequencies, and possibly
tifferent time slot ,lignm ~nt~ When a mo~ile ratiotelephone
2 5 is handed off from one ceLl to another in a cellular
~ iqtelephone system, the mobile may need some minor
requency atj~a~n ^nt as weLl as complete time slot
s~uc~ ~i7~tiQn to co~nml~nic~te ~nth the new base station
To accomplish this in a digital ceLlular radiotelephone
3 0 system, the radiotelephone first inds a frequency correction
nnsl (FCCH), which is part of the b~oA i~st control
~honnel (BCCH). FIG. 2 illustrates the FCCH slots (201) and
other data control ch-nnol~ that make up the multiframe
TDMA structure of the BCCH This format is described in
3 5 greater detail in the digital cellular st-n~ d specification
GSM Re~a-nmer~ ;on 5 02, Version 3 3.1, October 13, 1989
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The ba~e band signal of the FCCH is a frequency
cor.e~Lion burst (FCB), a pure tone (~ine wave) at 67.5 IcHz,
consisting of 148 s~mple~, sent prio~lic~lly and it always
occurs in time slot zero of the data stream. The offset between
S the carrier freql~en~es of the base ~t~t;on and the mobile
radiotelephone i~ tr~n~l~te~ to the base band as a dev~ation
from 67.5 l~Hz. The bo~m~l~ iea of the FCB ~ ne~te the time
dots of the I DMA structure. From the FCCH tetected, the
mobile r~liote?ep~on e synch~o. izes its local os~ tQr
10 frequency and time ~lot bo ~ ;es with tho~e of the base
st~t;Qn using the frequency co~ lion burst in the FCCH time
slot.
Since the burst i~ relati~ely short, the mobile must find
it in the data 8~ u and ~l-ch,~,lize with it in this short
1 5 period. There is a resulting need for a y.oc~ss that can detect
the l.,e~e..ca and bo~m~l~ries of the FCB verg rapidly, ant
e~tim~e the frequency offset very accurately, even when
are ~ece;~,ad in the presence of noise.
2 0 ~llmm~ry Of t~ ~nVPn~ n
The syncLo~ .6 process of the ~.eoe..t in~ention i~
compriset of the step~ of filtering a ,ece;~,ed signal with
ataptive filtering me~nq~ buae,~g this signal in storage
2 5 means, and deter~ if the frequency cG,.e_1.ion tone is
present, (the detection process). This also cotQhli~hes the
boundaries of the TDMA time slots. When this frequency
correction tone is ~eC-e..t, filtering the b~,~re~d signal and
determining the li~,~nce b¢t..ee.l the frequency of this
3 0 filteret signal and 67.5 kHz, (the c~, el frequency offset
e~;mQ~;on process).
Rrief n~tior~ Of tl~ nr~
3 S FIG. 1 show~ a block diagram of the process of the
present inve~tin~
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FIG. 2 shows the TDMA, multiframe, bro~ S ~t control
ch~nnel format.
FIG. 3 show~ a typical radiotelephone, using the
yl~ces~ of the present inV~nt~ for use in a TDMA type
5 system.
ne~ e~l nes~il~t;nn of tlle I~ .?fel l ~ .1 li.mho~liment
The y~o~v~ of the present i~ lion provites rapid
10 frequency and time slot ~ cLo-~i7~tiQn b~,..~.l a mobile
~iot~vle~hone and the base 9~ n with which it is
co~ nic~S~ting This i~ acco.-~pliQ~l by detecti-g the
presence and bQ~mA~ries of the frequency ~o.l~c~ion burst and
det,er...i.~i-.g the frequency of this base band tone.
1 5 The y.ef,.~d em~oAiment of the proces~ of the present
e~tion i~ illustrated in FIG. 1. The input to this process is
one of the two ba~e band quadrat~ , the I or Q tata
sl,ea~, ~mpl6'l at one sQmple per bit-time, from the l~ceiver
of the ~r lioteleph~ne. This Qign~l, lslhel~ l ~cn iIl FIG. 1, is
2 0 initi~lly filtered by a secon~l orter, infinite impulse response
(I~) bS~nApS~s filter (101). Both the gain ant the pole of this
filter are adapti~e. The gain i~ atjwtet in order to maintain
9lth6t~n~s ny unit~r gain through the filter, that is, the energy
at the output i~ equal to the energy at the input. The pole of the
2 5 Slter is mo~ed _o that the ~Qvsh~ of the filter e--~c--.~qses
the ~ece;~c~ ~gn~l The signal output from the filter is
lshql~ Yn- The filtering is l~elfol.-led as follows:
Yn+l=bn~n+l+anYn+(~ro)Yn_l
3 0 The energy of the input signal ant the energy of the
filte ed aignal are then c9t;m~t~1 in the energy estim~tiQn
block~ (103 ant 104). Ihe es~im~tion for the input energy is
aCco~ isl~e~ as follows:
3 5 E(~C)n+l = ( 1~) ~ (~C)n + ae ~b+ l
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The estimate for the filtered signal energy i8 as follows:
E(y)n~ ae) Eyn I e Yn l l
where a. is the energy allapt~tion coPffirient and is set equal to
S 0.091 for the estim~*on operations.
The input and the output energies, E(s)n~l and E(y)n~l,
are co~ ed in the gain ad&~t~lion block (105) and the gain
of the Slter is r l~pte~l to match the input and filtered signal
energies. This adaptet gain is then fet back to the filter. This
10 col.,p~;son and adaptation is pe,f~l~ed as follows:
gn+l = ~E(S)n+l / E(Y)n+l
bn~l=bn (l+~b(gn+l~
1 5 where b is the gain in the adaytive filter and ab is the gain
ad~ytation coe~;~Pnt and is set to 0.077 for the gain adaptation
operation.
The pole a~1ayt~:or~ block (102) estim~tes the
inst~nt~necus frequency of the filtered signal. The pole of the
2 0 ada~ ,e filter is r l~pted toward this frequency and the new
pole looPt;~r is fed back to the filter (101). This operation is as
follows:
It ( Yn > 0-15 E(y)n+l ) the~
~( ¦2Yn¦ ~ IYn_l+Yn+ll )
en+l = (Yn-l + Yn+l) / (Yn)
an+1 = ap an + (1~p ) r en+
E~lt~
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where en is an instaneous pole P~timote and ap is the pole
adaptation co~fficient and iB set to 0.083 for the pole adaptation
operation. When the aday1 i~e filter tracks a pure tone, such
as in the frequency C~r~ e_lion burst, all the energy in the
5 input signal is in the band of the bon~lp~s filter. Unit gain
can thus be achieved through the filter (101) with the lowest
value of the filter gain. This condition is ch~rlre~ to deterrnine
the in~ont~r~ presence of a tone in the tone detection block
(106). If gn+l is less than a threshold of 1.2 and bn+l is less
l 0 than a threshold of ffan), then the tone i8 present.
The timer block (107) ~ es the length of time for
which the tone p.e~c.,t condition persists. If this tone is
t for at least 100 somple~ in the ~r~ fe.~cd emho~limerlt~
the p,eee.lce of the frequency cG,.e_Lon burst has been
1 5 verified. This integration }~ e.lts the algorithm from falsely
d~tecting a ~ , that, for short periods, may appear like a
na~,u. l.and signal.
The signal, ~cn~ that was input to the filter (101), is also
stored in a shift registered buffer (108). Once it has been
2 0 dete~nined that this stored signal is the frequency COl . e~l ion
burst, the signal from the buffer (108) is input to the band-pass
filter (101) again using opt~ coefficients, a~ ant b~,
determined du~ the det~ct;on process. Since the pocsb~o-ncl
of the filter (101) is now tuned to the frequency of the frequency
2 5 correction burst, after the above ad-pto~on ~ cass, it passes
this signal without ~ttenllotiQr~ and filters out the background
noise, thw i~l.,o~i, g the e~_Li~,e signal-t~noise ratio.
Yn+l s b~ ~Cn~l ~ a~ Yn ~ (~ rO) Yn-l
The filtered sigr~ Yn, is ne~ct proce~se~l using a Least
Squared Error estim~t;l~n ~,oce3s to generate a frequency
est;m~te q~, of the base band tone.
The dlQ`e~ce between q~ and p/2 (67.5 kHz) is the
3 5 frequency offset bel ~ the carrier freqll~n~es of the base
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st~o~ and the mobile radiotele~n~le. This is entered to the
local os~ tor circuit of the r~ otelephone to comren~te for
the carAer frequency offset. The above described process is
~,f,.~et perio~iic~lly to keep the mo~ile radiotelephone
S locked to the ba~e station carrier frequency.
An e~ !e of the .~ce;~e portion of a typical mobile
.~~iote!e~ ne for use in a TDMA sy~tem is illustrated in
FIG. 3. The I and Q ~3eco~3er block cor~t~in~ the
gynchroni7~tjor ~)rOCe~9 of-the present invention tisclosed
10 herein. This type of L - iioPule~l~one is t~ se~l in greater
detail in copen~ling Canadian patent application serial
No. 2,071,866 "Interference Re-lueti~n Using an Adaptive
Receiver Filter, Signal Sll~ ~gtll, and BER Sensing"
l 5 filed Se~ he~ 3, 1991 on behalf of Cahill.
In ~ . y, a novel ~ ocess has been shown that will
synchronize a mobile r~(li¢t~'ephone's local os~ tor frequency
and time slot positioning with those of the received signal from
a base station. This ~yucllro~ iQ~ ocGurs in real time and
with signifi~ntly enh~nce~ accuracy.
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