CA1127781A - Orthogonal spread spectrum time division multiple accessing mobile subscriber access system - Google Patents

Abstract

Abstract of the Disclosure A time division multiple access mobile communications system employ-ing frequency division multiplexing and perfect noise codes to enable utiliza-tion of the system. A central node, or repeater, is employed, with all trans-missions to it being performed at a first frequency, and with the frequency division multiplexing being such that all re-transmissions from the central rode are performed at a second, different frequency. The noise codes employed are of a type termed code mates having correlation functions which upon detec-tion provide an impulse autocorrelation function. The described arrangement enables random access, or direct call-up, to be accomplished with total non-interference between users. Large improvements in signal-to-noise power ratio and in signal-to-jamming power ratio will be seen to result.

Description

1~,778~

Field of the Invention This invention relates to mobile communications systems and, more particul~rly, to a mobile subscriber accessing system utilizing direct call-up with omnidirectional antennas.

Background of the Invention As is well known and understood, one of the problems associated with mohile subscriber accessing systems offering direct call-up is that of inter-ference between users. As is also well known and understood, this results from the utilization in such communications systems of omnidirectional antennas.
Attempts at utilizing such antennas with very narrow pulses to reduce the ~lZ~7~1 probability of self-interference has been shown to be limited. Use of time division multiple accessing, by itself, has not completely solved the inter-ference problem, as signals from moving transmitters often threaten the chan-nel (or time slot) in which another signal is being received. Interference has especially been found prevalent when a call-up is first initiated, before any synchronized acquisition is first established. Although time division multiple accessing communications systems have been developed where directional antennas have been used (see, for example, my issued United States Patent No.
3,908,088), it was not until U.S. Patent No. 4,215,244 of July 29, 1980, that a workable implementation has been proposed for the mobile arena where omni-directional antennas are utilized.
As will become clear hereinafter, the mobile subscriber access system of the present invention represents an improvement over that described in my U.S. Patent No. 4,215,244, by providing total non-interference between users in environments where very large numbers of subscribers are operating. As reference to such Patent will show, the system there described provides a self-adaptive feature which permits switching to vacant channels as interfering users move to an occupied channel. ~hus, the elimination of self-interference will be seen to depend upon how quickly the self-adaptive feature can work, upon how quickly synchronous operation can be acquired, and upon how many sub-scribers begin transmitting at the same time. Analysis has indicated that where very large numbers of users will be accessing the system, the self-adaptive feature might not guaranty, at all times, the presence of a vacant channel to which to switch. As a result~ some self-interference might still be present.
As will become clear hereinafter, however, the mobile subscriber access system of this invention employs frequency division multiplexing for users ,.~

llZ77~31 l ; to transmlt to a central node, or r~peater, at a flrst frequencY, ~ith re- j l` l transmlss~ons therefrom to the lntend~d r~slnlcnt to be at a second freauency.
~hus, indlv;dual tr~nsm1sslons from an adjacent user would not ~enerally tend to prnduce 1nterference In receivers oDeratina at a dlfferent fre~uency hdd~-t10nally, the mob11e subscriber access system ~lll be seen to ~e orthoqonal ln opetatlon, such that perfect (1.e., lobeless) noise codes could be utillzed In the re-transmlss10ns from the central node so that the 1ntended rec1D1ent ~ould respcnd only to co~un1cat~ons slanals addressed to 1t. A loop back synchron-o~s tim;ng scheme ~s employed to maintain a time reference for all of the mob11e users, so as to keep the system orthoqonal and ensure receptlon onlr to~
lntended transmlsslons, w~th total non-~nterference. As will be seen, such an ¦ access1ng system ser~es to proYlde ~am~1ng protect10n and ranae extens10n caDaIl b111t1es, as ~n added attraction. ~hereas the moblle subscrlber access systemI~ of my concurrently flled ~ppllcatlon would operate perfectlY acceptably In 1ts jl tntended env1ronment, the added co~plexlty of ut111~1ng the central node con-¦~ struct~on of thls 1n~ent10n has been found necessary to m~1nt~n the hlqh leYell of non-1nterference where very large numbers of subscrlbers are present, each ¦
I of whom ar~e us1ng omnldirectlonal antennas In the~r accesslng.

I Br1ef Descr1pt10n of the Draw1ngs ~hese and other features of the present 1nvènt10n w111 be more read11y understood from A cons~deratlon of the follow1ng descr1Dt10n taken In connec-t70n w1th the accompanylng drawlngs ln whlch FIGURE 1 1s a slmpl1f1ed lllustratlon of a ground moblle access system emplo~1ng ortho~onal t1me dlvlslon multlple accesslnq accordlnq to the lnven-t1on;
FIGURE 2 ls a funct~onal block d1agram showing the loo~ back synchron- ¦
ous tlmlng control whlch malntains the varlous ch~nnels ln the central node re Il transm~ss10ns orthogonal ll FIGI)RE 3 ls a functtonal block d1anram of the transm1tt1ng Dortlon f !
Il ~

,l -3-¦ ! , ' 112~78~L

l t such a communlcat;ons system for use wlth a mult;plexed code constructed accord-~;ng to the ~nvention; and FIGURE 4 is a functional block dlanram of the receivinn p~rtion of such a co~luniCations system for Use with the ~ultinlexed ct~de employed in FIr,llPE 3.
i' _tailed Description of the Draw;ngs Il The arran~ement of FIGURE l pictorially illustrates a ~oblle subscriber.
i! access system employinq time d;vision multiple accessin~ established at a cen-tral node, or repeater, lO, with a synchronous time reference Tn~ Such refer-o l! encintJ may be accompl1shed ln any appropriate manner, such that each mob;le , user CommUniCates ~lth another durinq an ass~qned channel (or ti~e slot). Thvsl, one user Rl,Tl transmits to a second user R2,T2 alonq the mobile llnks 12, 14 durinq the time slot Tl, a third user ~3,T3 tranSm~tS to a fourth user ~4,~
alontl the mobile links 16, 18 durinq the time slot T2, the uier D.5,T5 transmits to the user R6,T6 during the time slot T3, etc. ~s indicated in FIGI~RE 1, all ~', tranSmissions to the central node 1O from the Various subscrlhers are In a il first freguency band fl whereas those transm~ssions from the central node 10 ¦ to the mobile subscribers are in a different frenuencv band f2. Whether sub-I' scriber Rl,Tl iS connected with subscriber R2,T2 via time slot Tl, subscriberll R3,T3 ~S connected w~th subscriber R4,T4 v;a t~me slot T2, or via different ¦l time slots, it will be appreciated that each subscr;ber ~air has their associ-ated t~me slot (or pair of t1me slots for tlme-multi~lexed du~lex operatlon) I established and ma~ntdined synchronous at the central node lO, or common sta-¦j t~on, 1O. As such, transmission in the frequency band fl would not Produce !¦ ~nterference ~n recelvers tuned to frequenc;es ~n the band f2. This fret~uency div~slon mult;pleY.1nq to elim~nate self-interference is of Darticular imDor-tdnce ~Ihere very large numbers of subscr;bers are aCcessintl the cor~unications' 5ystem.
¦l In order to keep the Various channels existinn in the transmltted out-1~, put from the central mode ln orthnnonal ~ a lonr t~acl: svncl~r~l~rllls 'l~lnq sche~e _A_ ., ~ llZ~7~31 1 lis employed, as lllustrated in FIGURE 2. ]n neneral, a pulse ti~ina sianaldeveloped by a voltaqe controlled osc;llator 20 ls cnunled hv a n~te aenerator 22 to a transmitter 24 for rad;ation, as bv antenna 2h. at a freqlle~cv ~
t~Yards the central station 10. Pecelvinn cuch transmlsslnn ~ith a don~ler ,, shift fd, the central node 10 retransmits such s;~nal to the receivin~ a~tenra ,1 ~
28, relative to the synchronous reference frequency fO. The donPler sh~fted signals intercepted by antenna 28 wlth the frequencies (f + ~fdl and (f~ + fd~) are coupted through a receiver 30 and de-multlplexer 32J and a pair of prf lock Il loop circu~ts 34, 36, for a comparison of the phase or time of the ~rans~itted I¦ signal against the clock standard pulse repetition frequency that is simultane-Il ously received from the central node 10. The output of a ti~e discri~inator ¦ 38 coupled to the prf lock loop circuits 34, 36 contrls, as by a filter 40, the 1', voltage control oscillator 20 to a lock condition which occurs when its fre-Il quency is equal to (fO - fdO). ~Ihen the transmission loop is thusly locked, 1¦ the output of the time discriminator 38 is zero, and f 2fd fo + fd tl) Since the path to and from the central node, or common station~ 1~ is the sare, ~ fd fdO (2 Therefore, f ~ fo fdO (3) Thus, the signal frequency recelved from each mobile subscriber at the central l n~de 10 is l f + fd ~ fo fdO ~ fdO fo ( ) l such that time multiDlexing is Dossible lnto any availahle ti~e slot at the cel-¦l tral station 1~. As ~111 be seen, the fre~uency of the voltaae controlled os-l Il, cillator 20 wi11 be automatically and cont~nuously channed until the looD-around ¦¦ transmission thus becomes synchronous. Since the foreqoina system is ortho~onal ~n operation, ~erfect (loheless~ noise codes wlll additlonally be seen to be .

:

, ~127781 l I ut~l~zable t~ qreat advanta~e.
l ,~s ~s descr1bed ln my 'ssued I!n1ted States Patents ~o. 3,~61,4;1, ; t~o. 3,519,746 and No. 3,634,765, for example, a numher o~ classes of co~es (1.e., pa~rs of code s~cna1s ter~ed code ~ates~ have am~lltudes and ~utocorre-t10n ~unct~on s which provide a peak outrùt at a ~ven ti~e and a zero outDut (or outputs hav1nq the same magnltude but opposlte ~olar1ty) at all other t~ntes. When the code m~tes are detected and the resultant detected outputs are l~nearly added, there ls Provided the impulse output of h~qh amplitude at l,I a ~ven time and a zero out~ut at all other tlmes ll To ut~l ke mult1~1exed codln~ in the tlme divlsion multi~le access communlcatlons system of the 1nventlon, the nate qenerator 22 and transm~tter II 24 of FIGURE 2 ls replaced by the blnary modulator 50, coder-~ultl~lexer 52 j', and power amplif~er 54 of FIGI)PE 3. These un1ts may be located at tbe moh~le '~I subscr~ber, with the modulator 50 and coder-mult~plexer 5Z compr1sln~ clock,.15 Il synchronlz1n~ generator, code generator and mlx1na apparatus to Drovlde propa-¦¦ gat10n by the omn~d~rectlonal antenna 26. As w111 become clear from the dls-cussion that follo~s, the coder-multlplexer 52 is designed such that code ~ate 'pa1rs ar~ transmltted to the central node lO wh1ch compress to a sincle lm~ulse,II lobelessly. I
Il FIGURE 3 symbollcally sho~s A transmltter arranqement for the mult~- ¦
¦¦ p~exed cod1ng system for use ln an lllustrat1ve ranner w1th mate code palrs ¦¦ wh1ch meet the autocorrelat10n requ1re~ents and 1n wh~ch:
¦ Code (a~ = 0001 Code (b) ~ 0100 ~here:
0 Ind1cates a plus (+~
1 Ind~cates a m1nus (-) he arrangement between the blnary modulator sn of FIGURE 3 and the ~ultlplex II unlt 56 w111 be understood to essent1a11y compr1se a match f11ter conf~quratlon.
I' Referr~nq, more part~cularly, to the constructlon Or FIr~JRE 3, lt ~111 , ~

, I

,: I

11277~

1 be seen that a patr of llnear adders 60, 61 are Included, w~th the outputs of each be'ng acpl ied to one 1npl.t of the nrultlplexer 56 for ~r~Y~n(l the po~ r ampl~f~er 54. The input si~nals to the adders 60, 61, on the other hand, are provlded by means of a plurality of time delay clrcu1ts and by means of a p7ur-al~ty of phase control clrcutts. Speclftcally, the c~rc~lts 70, 71, 72, 73, 74, 7~ each delay the detected code s1~na1 by one t~me slot of the synchronous t~m1ng cycle. The phase control ctrcuits ~6, 77, 78, ~9, 80, 81, ~2, 83 are of constructlon to pro~ldè a signal feedthrough e1ther w;th 0 or 1Bn phase shlft, depend1n~ upon the spectftc code mate o~erated ~pon. In FIGUR 3, the c~rcu1ts Ident~fled by the reference numerals 76, 77, 78, 80, 82, and 83 pro- ¦
Ylde ~ero phase reversal for the code s1gnal, whereas the clrcuits 79 and 81 provlde the 180 ph~se reversal requtred. In the descrlptlon that Is shown, 1t wtll be understood that the Incluston of a "0" w1thin these phase controt c~rcu~ts represents a sl~nal feedthrou~h wlth zero phase a1terat~on while the l~ 1ncluston of a ~ nd~cates a phase reversal of 180.
The compresslon of the code 0001 In the top Dortton of the match f11te;
of FI~URE 3 1s Illustrated as follows, ~tth the last llne Indicat~n~ the auto~
. correlat~on functlon fro~ the adder 60.
Tlme Slot l 2 3 4 5 6 7 ~0 Pulse l l l l Comp. 0 0 0 n 1 Fllter 0 0 0 n O O O ~ 1 l' (Sn the foregolng table, 1t wlll be apprec1ated that the exponent Indlcates the 3mpl Itude.~
The co~press10n of the code ~100 In the bottom portlon of the match f~lter of FlGURE 3 Is 111ustrated by the follo~1nn tahle, where the last l~ne ¦ -~nd1cates the autocorrelat~on funct10n fro~ the ~dder ~ ,7781 1 Tlme Slot 1 2 3 4 S fi 7 P~llse n 0 ~ n n Co~np. O 0 1 0 () Fi 1 ter 1 1 0 1 1 ; 0 n 1 ~ n _-- !
'tbC~) F11ter Outpu~ 1 n4 1 o he follow~ng table -- and, spec~f1cally, the last l~ne there~n ~-11 lllustrates the output si~nal of the multiDlexer 56, sh~wlnq that the l~near /matched 1 sum of the orthogonally mult1plexed filter out~uts results In a compress10n ,~ of ehe compos~te code to a s~ngle ~mpulse.
Time Slot 1 2 ~ 4 5 6 70 04 0 i 0 ~ 1 04 1 0 ~ o~
- 15 In apply1ng the ex~mple code pa1rs 1n the t1me dlv1s10n mult~ple acces5-lng system of the present ~nYent~on, 1t w111 be helpful to f1rst constder an uncoded system ~1th three users accessln~ the syst~m ~th the selected channel~
occup~n~ the three ttme slots of one user 1n the f~rst, th1rd and fourth t~mel I¦ slot postt~ons. D1fferent a~p!~tudes and phases ~ill be assumed for the separ~
il ate users ~ccess~ng the system to de~onstrate that ut11izing perfect noise codes enables the system to b~ per~ormed w1th no self-1nterference eYen for rge dtfferences tn recelYed po~er levels. Th~ ~mplitudes and phases of the I three users w111 be assumed to be +2, +4 and ~ 1th the three users occupy-, tng these t~me s10ts, the sum ~d output w111 tppe~r thusly:
T~me 510t 1 2 3 4 User ~1 o2 User ~2 04 i User ~3 ~I ~ o The follow1nq t~ùle 111ustr~tes hoth the rult~Dlexed code siDn~ls th~t ~2~7~31 1 I would be transmitted by the three accessintJ users, as ~ell as the l~near Sum of the coded signals as they would aPpear at the mutli~le ~cc~ss in~uts of the central node ln. The code s;qnal transmittt~d bv user ~ P!~ll bt~ seen to be ~f four timrs the am~litud~ and of the opposite polaritY ~s tht sl~al tr~nsmitted, by user 3, whlle the cotie si~nal transmitted ~y user ~1 ~/ill ~ se~n to he of twice the amplitu~e and of the opposite polarity as the code s~nnal transmitted by user ~3.
Time slot -3 -2 -1 +1 ~2 +3 +~
1 User Yl o2 o2 o2 12 User #2 0 n4 o4 User ~3 1 1 1 0 Il ~R) o2 n2 n6 t1 o3 1~ ~
!i The foreoolnq table representintl the code (a) co~nosite sitlnal, can then be compared w~th the follolint~ tahle, renresentina the code (b~ com~oslte ~j signal.
Time Slot -3 -2 -1 +1 +2 +3 ~4 j l)ser Yl ~2 1~ n2 n2 User ~2 ~a 1~ ~4 n~
I' User ~3 1 n i ~6) o2 12 ~fi 13 n5 n3 The only difference between the foreaoinn two tables beint) that the first rep-¦¦ resents the condition for the code (a) inputs wh~le the secont! re~resents that ¦l for the code (b) inputs.
Il Pulse compressin~1 the compos~te outputs ln their res~ect~ve ~atched ~ f~lters, and llnearly addin~ the results ~n the multiplexer 56 ~roduces the followin~. As for pulse com~ressinq the code (a) comnos~te output:
j Time Slot --3 -2 -1 +1 +2 +3 +~ +5 +k +7 'I Puls~ 1 12 12 16 1 13 ~5 1 ' Corlp O o2 o2 ~ n n~ ~ 5 ~
l' Filter n o2 o2 ok o n3 15 n .

... ., . ~

~llZ77~

o o2 ~ ~fi 0 ~ n __ __ .
~ Fllter OU~ t 12 ~ l2 o~ nfi ol~ 12 1 14 ~ I, Pulse compressin~ the composite output for code (b! resul-ts ln~
T1me Slot -3 -2 -l +l ~2 ~3 ~ ~5 ~ ~7 Pulse 0 o2 l2 o6 13 05 ~3 l ~,om~. ~ o2 12 ~6 13 n5 n3 F11ter l 12 ~2 16 0 ~ l n ~ n2 l2 ~6 13 n5 ~3 _____ ~ ) filter ~utput n2 . o2 o7 l~ n~
The llnear sum of the two matched filter outDuts then yields Time Stot -~ -2 -l- +l ~2 +3 +4 +5 ~ ~7 y (~) 12 . 12 09 ~6 nl5 ~2 1 14 0 ) o2 o2 ~7 16 ol7 16 ~ o4 ~T ~ ) ' ' ' ol 6 . ~3~
1~J The t~o ma~or factors to note from this last table are that the com- I -: pressed 1nformatlon b1ts are tot~lly non-interfer1nn and the siqnal voltaqe rece1ved at the central node lO Is e1ght tlmes ~reater than for an uncoded time d1viston mult1plex access1ng s,Ystem. ~h1s factor of el~ht Is s1mply the t1me-band width product or, equ1valentl,y, the number of noise code b1ts con- 1.2~ , ta1ned in each ~nformat10n h1t, Th1s 1ncrease of eiaht t1mes in the s1gnal , voltage prov1des the advantaqeous result of 1ncreasing s1anal-to-no1se power , ratto by that same factor, as well as the s1anal-to-~jam~1n~ p~er ratio in a ' hostlle enY~ron~ent.
' 1 In usage, a gated automatic gfl1n control 100D Wl11 be employed to ad-ll ~lust the 1nd1v~dual channel s~nals to the same le~el for re-transmiss10n from the central node lO to the 1nd1vidual mobile users. The comDosite ~roup of t~me d1vlsion multlple access svstem channels ~ould then be nolse coded in a stmilar manner, a~atn as a multiplexed no1se code nair (either of the same no1se code structure, or ~n a d1fferent no~se code structure~. Sim1tar de- I3n mutt1plexln~ and matched f~lter detectlon would ~e emploved ~n each receiver ln llZ77~

l ll of the subscriber to result ln the total non-lnterference wlth recept~on of ~11 channel stanals. The ~nd1Yldual subscr1ber would then slmrly qate out th selected tlre slot tntended for him prlor to demodulatlon of th~ ~nfor~at10n, ~ thereby provldina an outDut that is totally non-inter~ered wlth by the other ; subscrlbers tn the system and ~hlch thus slmultaneouslY pro~tdes a protection aga~nst ~ammers equal to the number of accesses, ~1th no required increase 1n band wtdth. In FIGURE 4, th~ receiYer structure ls sho~n for use ~lth the gener-ated noise code palr ln FIGURE 3, wlth three adders lO0, lnl, ln2~ tt~e de-1 lay ctrcutts 103, 104, 105, 106, 107, 108, wlth n slqnal feedthrough phase lo 1! control c1rcutts lO9, llO, lll, 112, 113, 114, and with 18~ Dhase reYersat control c1rcults 115, 116. Analysls stmtlar to that descr1bed w1th the fore-, go~nq tables would show that the tnformatlon h~ts nroduced as an outDut from the adder tO2 would be totallv non-1nterfertnq, and ~lth its recetved sinnal ¦ voltaqe be1nq tncreased by a factor of elnht. The result of uslnq the perfect lS ' no1se codes, therefore, wtll be seen to optiml~e transmlsslon efficlency,Il ~h~le at the same tlme obta1ning a large deqree of anti-~la~m1nn protect~on.
I, Whtle there has been descrtbed what ts cons1dered to be a preferred i embodt~ent of the present 1nvent~on, 7t will he readilv aDnarent to those Il sk~lled ~n the art that modlf1cat~ons may be made hY lmnle~entat10n of differ-~ ent perfect code mates and yet stlll prov1de the impro~ements in sinnall1nq and power characteristlcs wh~ch result. To carry out the teachlnns here1n, all that would be necessary would be to select the Perfect code mate pairs to ¦I be used and to arranqe the phase control clrcu~ts of FIGI!RES 3 and 4 in aDpro-l pr~ate relatlonshlp w~th respect to them. ~lternatlvelv, multl-dlmenslonal 1 codes could be ut~l1zed ~n the proposed system, as mult~plexed codes whlch aretlme coded concurrently wlth qenerat~nq a ~ultlplexed hlnarv code pair to pro-vlde a composlte (l.e., multl-dlmenslonal) code stru-ture whlch retalns ~ero lobes ~n the t~me doma1n wh11e s1multaneously prov1dina verY lo~ side-lobes . throughout the amblqu~ty dlagram Such ~ult1plexed co~es are more part1cularly lldescrlbed ln my tJnlted States Patent tlo. 3,917,q99 , ' , . ;

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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A mobile subscriber system comprising:
a plurality of subscriber stations each of which is assigned a respective distinct time interval within a channel frame period for transmis-sion and reception;
a separate transmitter means at all of said subscriber stations for transmitting communication carrier signals omnidirectionally at a first frequency common to all said transmitter means, said transmitter means inclu-ding a code multiplexer applying the same multiplexed code signals to said transmitter within the respective assigned time interval; a central node station including carrier frequency translation means for retransmitting said code signals at a second carrier frequency relative to a reference frequency without change of relative timing; receiver means at each of said subscriber stations for receiving code signals transmitted by said control node at said second carrier frequency which is common to all of said receiver means; each of said receiver means including means for detecting said retransmitted code signals to produce an output signal occuring only during the respective time interval within the corresponding channel frame period, and means at each subscriber station for synchronizing timing of that station's code signals retransmitted from said central node station with said reference frequency, by controlling that station's transmitter frequency.

2. The mobile subscriber system of claim 1 wherein said transmitter means includes a voltage controlled oscillator; and said synchronizing means includes phase comparison means responding to said reference frequency signal transmitted from said central node and to said code signals retransmitted from said central node to provide a control voltage to said oscillator for locking the oscillator of the respective transmitter means to the frequency of the reference frequency signal at said central node.

3. The mobile subscriber system of claim 2 further including 3 time discriminator receptive of said detected signals for deriving said control voltage for said voltage controlled oscillator.