GB2393358A - Estimated signal-to-interference ratio provides feedback to integrating controller for transmission power control - Google Patents

Abstract

A radio-frequency (RF) transmitter in a telecommunications network, particularly a third generation network, is controlled by an integrating controller 35. An estimated signal-to-interference ratio (SIR) signal SIRest is used as feedback for the integrating controller 35 to prevent the integrating controller 35 from becoming unstable when the maximum transmitter power is used. The integrating controller 35 produces a required SIR signal SIRr from an input error signal. The required SIR signal SIRr may be set to produce a desired bit error rate (BER) at the output of the transmitter.

Description

( - 1 - TE=-OMMU>CAINo NETWORKS The Present i..ver.ton relates co -

te:ecommur_atons etworks, and in patc_lar o thrd generation mobile telephone networks and systems BACKGoU 0 -INTENTION To third gerera'on noble telephony systems, power cor.-ol -s imotar.t to obtain desiaby high 10 capacity and efic ency, Part Cal ably in COMA systems.

The variable that is controlled is called Duality Quality of the communication is controlled with reference to one Of two quality measurements PER (Bit Error ate) or -ER (frame Erasure Rate) Usually an 15 integrating controller is provided to achieve a steady state of performance with zero control error The control scheme used is cascade control, see for example Figure 1 of the accompanying drawings The idea with cascade control is to make an. inner control loop (2) 20 much. faster than an outer control loop (4) For transmission power control (TPC) the inner loop controls s gnat to interference ratio (Sly).r.e oute _oop sets the SIR reference value SIR_ for the inner loop T2.e goal of the cuter loop s to control the SIR 25 refere-.ce to achieve a 3R the" is equal to the BER reference Barr To get a control system that in steady state achieves a BAR that is equal to the sR reference, an integrating con-oller (9) can be used The cascade control er illustrated in figure l 30 comprises an inner control loop (2) and an outer contra' loop (4). 30th control loops have an input of a received signal (yak)) The o'ter control loop serves to estimate the BER (;) fo- comparison w th a B_2 reference signal A Subtracter (7) calculates the 35 differe,.ce between. the reference slgna' and the BAR P13872 - HL77286

-2 est-ae to supply an imp t sigma_ to an integrating cor.tro2ie' (3) The ntesratng cont--l'er 3) produces a SIR reference signal.

The SIP reee-.ce signal is compared with an SIR eat-mate ó'om an FIR estiatio. unit (3) n the inner control Loop (2) The difference between the SIR reverence and t:-e SIR es mate is sp?l_eB to a step cor.tc2le (1') For Bete.=in;ng the 'ar.sm_ssion power of -:se cu-?vt signal u(k) 0 A known. problem with an _rtegrati-.g controller is that it becomes unstable if the control signal saturates This problem is often _eCered to as the windup problem TPC saturation of the control signal corresponds to sltuat ons when the maximum trar.sm-cter power is used The problem with windup in the power control algorithms for third generation mobile telephony systems is well known. Howeve- no solutions to the problem have yet been presented The specific problem 2 of which protection is CODA makes several additions to ani-wr.d'Jo schemes used n othe- areas necessary.

As is well:nowr, iregra_ng co-.t-o" ems have the nice pro?e-y Of being able to achieve zero co-.rol error ire steady state As an. example of an Egret g 25 controller, a continuous time PI-contrcl'er is shown in Figure 2, which illustrates a continuous tome controller Discrete time cor. 'rollers have similar behaviour Fo' example, Karl than Astrom and Tore k:aggiund HID Control'e-s Theory, Design. and Tuning 0 Ir.scramen Socie y of America, Research Triangle Park, NC, second edition, 1995.

A kr.owr problem with integrating controllers is that the ir.tegator part turns unstable when the control signal saturates This instability occurs 35 because feedback from the process is needed to P 13812 - HL77286

-3- stab laze the cor.tro let, which is not open loop stab e. IN the case of trar.sission power cor.-rol, saturation can cccr when maximum rarsssion powe- is used. I.n this situation. the t_ansmisson power can only be decreased, -at ch can be see' as over. loop o?erat on of the _nteg_ator.

As "he con.rol2e_ s not open loop stable the controller state (the integrator, I-part) car. start to DU'2 I '_p a arge sae. This lsua'ly results in that 10 it "ages a lo.g t me for the control loop to s art fu-. cLoni..g again after the saturation state is left.

This problem is usually referred to as tine wr.dup prowl em.

1, STY OF THE PROS- I1W ENTION

An embodiment of the present Invention can provide a method for controlling a radio frequency (RF) transmitter, the method comprising: using an integrating controller to produce a 20 requ'-ed signal-ionterference ratio (SIR) signal from a desired error s anal; product ng an estimated SIR s_gna' to the ir_egrating cor^irol2er as a feedback signal, thereby stab l sing "he required signal-to-i reference Pablo 5 (SIR).

It is emphasized that the term "comprises" or "comprising" is used in this specification to specify

the presence of stated features, integers, steps or copor.ents, but does not preclude the addition of one 90 or more further features, integers, steps o components, or groups thereof.

BRIEF DE5CRroTlON OF THE DRAWINGS Figure 1 illustrates a controller for a third 35 gen.eration mobile system; Pl3872-HL77286

/ Figure 2 2lust-a.es an ir.tegrat'ng controller; and Figures 3 to illustrate respective controllers emcdyirg the present ir.ver.ton s DETAILED DESCRIPTION OF THE PREFERRED -MBODIM=NT

To explain the _nventio.., a s mplified model for the controlled process is derailed, and is illustrated in Figure 3. The STR control loop is modelled as a 10 saturation. (22) and a delay (23) me Process chat raps SR to BED (BLUR) is model2ed as a static function (2) This is not important for the invention and ca.. be modeled by any suitable means A problem is that SIR and BAR (BLER) 15 cannot be diTec-'y measured SIR arc BER (IDLER) car.

Only be estimated This is indicated in Figure 3 with two estimation noises W2 Anew transmission power saturates (i.e. when the maximum transmission power is reduced) SIR will no 20 longer follow S_Rr (SIR reference) In the tracking approach of the present invention tine difference between SIR and cart is calc,'ated and fed back to stab l ze the integrating cor.t-ol2er. If vx ' s small the difference w 11 be smal', except when,.e 25 transmission power is saturated The tracking solution is shown in Figure 4 The PI-cortro'ler with tracking signal input to the integrator is shown in Figure 5.

As Deplore, a continuous time loop is shown, but a discrete time version is simply derived and would have 30 similar behaviour Embodiments of the present invention are applicable to transmission power control systems in both the up-link and the down-link Figure is a schematic illustration of a tracking solu_ior. embodying the present invention. the 3; components of Figure 3 are shown, namely the saturation P13S72 - HL77286

21, de2 ay 23 and the stat c hard ng function 25. A required SIR (IR-)is _r.put to this model to Produce an SIR est:m,ae (SIR es') and a 3 estimate (B-'7. est) The trckir.g so' union is i2_st-a-ed by elements 5 31, 33, 35, 37, 39 and 42 A -equated Sex (B=)is supplied via a Log fu. ^c.io,7. 31 to an adder 3 Also sups red the adder 33 is the HER estimate, v a a log fu^7.ctior. 4', so that the adder 33 prod'_ces an. e--or result equal to -:.e d-=ere.-e etwee.. the requ red PER :5 and he es'ima ed 3=R The Log -u-ctors are r.troduced 'a ensure that the control loop behaves in tinier fashion A contra ler 35 (CHAR) receives as one _r.put the error signal T.e cor.toile also receive a -ackl..g signal IS The cor--oile produces a signal 1- represen'rg the repulsed SIR for suppl y to the SIR control loop model The required SIR signal is also supplied, via a delay element 37, to an adde- 39 which proc'uces the tracking signal by subtotaling the required SIR signal from the estimated SIR signal 20 Figure 5 illustrates the controller 35 in more detail As can be seen, the controller includes a gain element 4 of ga I..< which receives an Output e as supplies an ou_pu" eK to an adde- 4a The erro' signal e is also supplied to a component 45 hav- r.g a transfer So -unction K/-i (where To is the in_egratio time) for supplying to another adder 45 A second i-put of the adder 46 is provided by the output from, a second component 49;av_n.g a transfer function lit: (where T: is the rackir.g time) as supplied with the e-ro_ signal 30 ES The oupu a- the adder 46 is integrated by the integrator 47 (1/s) and supplied to the adder 44 The output of the adder 44 gives the required Sip signal.

It can be seer. that the controller 35 provides the following transfer function as given fir. eq,1aton 1.

PL3872 - HL77286

-6- lieK e SIR' = eK - - -+ s per, alternative imp_emer.ta'ior would be to use the estimated tracking signal eScO do "cor.dit oral s iegra-io." In such an impleme.n_at o. the.ntegra.or -a-- is -tot 1pdated if es is largethan a "nreshold, e _ eS,> ethmshold the integrator is not updated This solution. also prevents the -rtegrator state to build = large Maple in sce-.ar-os o oowe 2C sa urzion.

The best practice implementation the t-ackir.g z--angemer.t includes to do some filtering of es and a dead zone This makes the impact of estimation errors sma'le' in cases of -,o. saturating power IS The Invention s a new application of the tracking approach to the windup problem To the knowledge of the inventor, tracking has no' before been used to tackle the problem of saturation in transmission power control The major improvement compared tc existing 20 approaches are that the saturation is estimated by comparing SIR_ a-d S-Re5 Without this _eat,re it would not be pass b'e to generate a trac'-ing signal 2-.e irven_i Oh s applicable to transmission. cower coerce systems in both the up-link and the down-link Pl3872 IIL71286

Claims (1)

1 A method for controlling a radio reqer.cy (Ray tansmi er, the met:cd comor_s_-.g Vising an '-.tegrating cor.trol_er to o-oc'_ce a -ec,mired sgr,al-to-interference ad_o (SIR, signal from a desired error signal; producing ar. estimaec SIR signal relet ng to a actual SIR of h.e RF transmitter; and lo suplyi.c the estimated SIR sigma to _:ne r.tegra -.5 control = as a eedback sgr.a, thereby sabi2sing the ecuired signal-toincerre-ence ratio (SI fir) 2 A radio freqer.cy transmitter controller 2 comprising an integrating controller operable to produce a des red signal-to--nterference ratio (SIR) signal from an input error signal; and an estimation unit, connected to receive a sampled 20 output signal, and operable to produce an estimated SIR signal relating to a sampled output signal, wherein the estia'ion un- is operable to supply the est mated S_ s gnal to the integrating cor^-roller, and the in_ea-a-ing cor.tro2_er Is operable to practice 25 the des red S signal from the. out error signal and the estimated SIR signal 3 A controller as claimed in claim 2, wherein he integrating controller has the irar.s'er Unction: leK e'\ SIRr= eK±I + s 17 17) n which SIR i s the desired SIR si gral, e is the input errcsignal, X is a constant, e9 is the estimated 35 SIR signal and T and T.: are t He constants e'atlng to P 13 872 - HL77286

( - 8 - the -._egra on arid est.nat on unit respective y.

. A con' roll er as claimed fir. cla rr 2 or 3, wherein the desired srP so gr.ai 1 s set to produce a desk red bit error rate (Err) at the output of the transmitter. Pl3872 - HL71286 TOTAL P. 01