Dynamic Gain Management for On-Channel Repeaters

IEEE TRANSACTIONS ON BROADCASTING, VOL. 59, NO. 4, DECEMBER 2013 685

Dynamic Gain Management for On-Channel RepeatersMichael Mao Wang, Member, IEEE

Abstract—Stability and output signal quality are always twomajor concerns with the deployment of on-channel repeaters.This paper proposes a gain management scheme and evaluatesthe performance of the proposed gain management technique foron-channel repeaters. The gain manager uses a loop gain metricas an indication of stability and output signal quality (SNR) ofthe repeater to continuously and accurately monitor the repeaterstatus, and adjust the repeater gain accordingly. The repeateroutput SNR and stability can thus be tightly controlled. It isdemonstrated that the proposed gain management ensures thesafe and robust operation of the repeater against the dynamicsof the environment.

Index Terms—Amplify-and-forward relays, on-channel (on-frequency) repeaters, single frequency network, stability control.

I. Introduction

THE increasing spread of multimedia broadcasting ser-vices, such as DVB-T/H [1], [2], and FLO-TV [3], and

wireless WAN, such as EVDO and WCDMA, require efficientand economic network technologies for successful delivery[4], [13]. The purpose of an on-channel repeater [7], [6]is to extend the coverage of the network in areas whereservice quality is poor. Repeaters are an essential part of anefficient and effective wireless communications system, pro-viding enhanced coverage to isolated areas. A repeater may beconsidered as an amplify-and-forward relay with no decodingor scheduling ability. The on-channel or on-frequency repeaterreceives the original signal over the air, amplifies it and thenre-transmits it on the same frequency/channel as received. Inthis way, the repeater system improves reception in areas ofpoor coverage, especially in homes and buildings, therebyextending the coverage and achieving the hole filling purposes.From a cost-benefit tradeoff perspective, the low complexityof repeaters makes them an attractive low-cost alternative toother expensive relays.

Sending and receiving concurrently on the same channel, theeffective gain of an on-channel repeater is limited primarilyby the amount of isolation between the transmitter and thereceiver antennas. There is a need for high isolation betweentransmit and receive antennas to minimize coupling and tokeep the system stable. The isolation of the receive (donor)antennas and the transmit (coverage) antennas of the repeateris necessary but usually not sufficient to reduce the impact

Manuscript received April 26, 2013; revised July 27, 2013; accepted August7, 2013. Date of publication October 21, 2013; date of current versionDecember 10, 2013.

The author is with Qualcomm Research, San Diego, CA 92121 USA(e-mail: [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TBC.2013.2284417

of unwanted echoes or the feedback channel. Digital echocancellation techniques can be used to compensate for thelack of antenna isolation and further reduce the unwantedechoes in the feedback path or leakage channel between thetransmit and receive antennas, thereby improving the isolationfigure [7]–[13]. However, the performance of these techniquesdepends heavily on the dynamic multipath characteristics ofthe feedback channels.

The ability to ensure repeater stability has been the criticalissue that is limiting the wide deployment of repeaters.In addition, as the wireless market continues to expand,the use of repeaters that potentially create interference oncarrier networks becomes a growing and pressing issue.Conventional repeater products do not have a robust methodof oscillation prevention and detection methods and must becarefully engineered and installed to avoid oscillations. Dueto the lack of stability control mechanism, the repeater statusin a dynamic wireless environment can hardly be guaranteed,and when isolation/environment changes, a conventionalrepeater can either distort the network signals or evenoscillate. Repeater oscillation can act as a jammer within thenetwork, blocking a channel or a number of channels within anetwork; leading to costly trouble-shooting for the carrier, anddegraded services for subscribers. Therefore, stability controlvia appropriate gain management is a highly desirable featurefor modern repeaters. Unfortunately, there are very fewpublicly available repeater stability and output signal qualitycontrol techniques in the literature [14], [15]. This paperattempts to fill this gap with a gain management techniquethat employs a loop gain metric to continuously monitor therepeater stability and promptly adjust the gain accordinglyto ensure the repeater stability as well as the repeater outputsignal quality. The goal of the design is to ensure that arepeater is unconditionally stable, and to maintain outputsignal quality at a desired level in any dynamic environment.

The remainder of the paper is organized as follows: InSection II, we briefly review the architecture and componentsof modern repeaters; Section III describes in detail the designof the gain management in both the link level and the devicelevel perspectives. In Section IV, we evaluate gain managementperformance. Finally, conclusions are given in Section V.

II. Repeater Overview

Fig. 1 illustrates a typical repeater architecture where arepeater receives the signal over a wireless link using adirectional antenna, i.e., the donor antenna. The received signalis amplified by the repeater, which is then transmitted using acoverage antenna that may be directional or omnidirectional.

0018-9316 c© 2013 IEEE

686 IEEE TRANSACTIONS ON BROADCASTING, VOL. 59, NO. 4, DECEMBER 2013

Fig. 1. A typical repeater block diagram (black) and the proposed gain management module (red).

In between receiving its signal and re-transmitting, the repeaterintroduces a delay of a few microseconds.

Ideally, we want a repeater to have a large gain(e.g., > 70 dB), maintain the input SNR at the output, and haverobust boot-up and operation. However, the repeater gain islimited by the isolation between the amplified signal and itsfeedback path, which can be provided by two different mech-anisms: isolation between the donor and coverage antennas(passive isolation) and isolation by feedback echo cancella-tion (active isolation). The output SNR of a repeater mainlydepends on echo cancellation residual error, and practicallymay also depend on RF noise figure. Repeater may oscillatedue to isolation changes as a result of degraded cancellationperformance or antenna isolation. Special mechanisms there-fore must be in place to ensure the safe operation of a repeater.

A. Antennas

The main problem with on-channel repeaters is the effect ofstrong feedback signals from the coverage (or transmit) anten-nas towards the donor (or receive) antennas. These feedbacksignals can interfere with the weak received signal (i.e., theremote signal) of the repeater causing oscillations and systeminstability. This unwanted feedback signal is also known ascoupling loop interference and arises when the gain of theamplifier is larger than the isolation between the receive andtransmit antennas.

A first solution to the coupling loop interference problem isto increase the isolation of the transmit and receive antennasof the repeater. Depending on the design (physical isolationbetween the transmit and receive antennas of the repeater) andthe operating environment (reflections from the surroundingenvironment), antenna isolation typically ranges from 30 to60 dB. This degree of isolation is necessary but often insuffi-cient for high gain repeaters.

B. Feedback/Echo Cancellation

Digital baseband echo cancellation is a commonly usedtechnique to effectively increase the isolation in modernrepeaters [17], [18]. It typically provides 35 to 45 dB isolationin addition to antenna isolation.

Accurate estimation of the feedback channel is the keyto echo cancellation. One approach to channel estimationis the use of a low power (e.g., more than 15 dB belowtransmitted signal) reference signal, p (cf. Fig. 1), inserted into

the transmitted signal [9]. The reference signal is uncorrelatedwith the transmitted signal. The feedback channel is thenestimated using the reference signal and the received signaly, and used to recreate the feedback signals (echoes) whichare subtracted from the input signals. The drawback of usingan inserted reference signal is that the reference signal acts asan unwanted interference and degrades the output signal SNRof the repeater. Another approach is to use the original signal,i.e., p = q, as the reference signal for channel estimation. Thismethod does not introduce extra interference to the transmittedsignal. In addition, it provides a better channel estimate thanthe inserted reference approach since the original signal ismuch stronger (e.g., 15 dB stronger) than the inserted referencesignal. The drawback of this approach is that the repeater hasto purposely introduce sufficient delay such that the signalcurrently being transmitted is uncorrelated with the remotesignal currently being received. That is, the repeater delay hasto be larger than the remote signal delay spread.

The cancellation performance is directly dependent on thequality and tracking capability of the feedback channel estima-tion. To obtain an accurate channel estimate, channel estimatesare usually averaged over time. The longest allowable averagetime is limited by feedback channel coherence time. The useof cancellation in modern repeaters increases the vulnerabilityof a repeater to channel dynamics, thereby increasing the needfor careful gain management.

C. Stability and Output SNR

As earlier stated, the repeater output SNR and stabilitydepend on the repeater effective isolation (combined passiveand active isolation). As shown in Fig. 2, the repeater outputSNR is upper-bounded by the residual interference strengthrelative to the remote signal, i.e., SNR≤δ. SNR = δ if therepeater RF noise is ignored. It is clear that the inverse ofthe SNR, i.e., −SNR (dB) or −δ (dB) is also related to theloop gain of the feedback system. The repeater stability thusrequires that −δ < 0. However, since the antenna isolationand the cancellation performance are both dependent on thefeedback channel, the effective isolation (both passive andactive) in turn is affected by the environment. Changes in theenvironment result in changes in isolation, and hence changesin output SNR and stability. A mechanism that dynamicallycontrols the output SNR and stability is thus highly desirable.The lack of this control mechanism in most repeater products


Fig. 2. Repeater input/output signal levels. Here the remote signal is thedesired signal to amplify.

is one of the key factors that limit the wide deployment ofon-channel repeaters.

III. Gain Management

The proposed repeater gain management includes 1) linklevel gain control and 2) device level gain management. Theyare the main focus of this section.

A. Link Level Gain Control

The main goals of the proposed repeater gain control includerobust boot-up, oscillation prevention, and accurate outputSNR control. As we’ve already stated that the repeater gain islimited by the repeater isolation whereas isolation is dependenton the repeater operating environment. Ideally, the repeatergain should change in accordance to the repeater isolation toensure the repeater stability as well as the output SNR. Wehence introduce a gain management mechanism, as shown inFig. 1 (red), to dynamically control the repeater gain such thatthe repeater stability and the desired output SNR are ensured.

1) Loop Gain Metric: It is clear that in order to controlthe repeater stability as well as the output SNR, we needan estimate of the repeater loop gain as the metric for gaincontrol. The target value of the loop gain metric is typically inthe range of -20 to -15 dB, depending on the stability marginand the target output SNR. This means that the noise floorof the estimate metric has to be lower than -20 dB, whichis indeed challenging. Moreover, an issue that makes thedesign of the metric even more challenging is the diffusion orspreading effect of the canceller. Let’s consider a simplifiedrepeater diagram as in Fig. 3. The canceller removes thefeedback signals, and as a result spreads the residual energyover the entire cancellation window, which makes the effectivefeedback energy more difficult to detect.

We devise the following estimation scheme. The feed-back energy of the reference signal at the point where the

Fig. 3. Illustration of the effective feedback channel with feedback cancel-lation. The canceller removes the feedback signals, and as a result spreadsthe residual energy over the entire cancellation window.

corresponding reference signal is transmitted can be estimatedby

∑τ∈Nch

∣∣∣∣∣N−1∑n=0

r∗[n]q[n + Ndelay + τ]

∣∣∣∣∣2

(1)

where N is the correlation length in samples, Ndelay is the totalfeedback delay, Nchthe channel estimation length, and

r[n] =p[n]√

N−1∑n=0

|p[n]|2, 0≤n < N (2)

is the normalized reference signal with unit energy. Here p

is the reference signal. SinceN−1∑n=0

|r[n]|2= 1, (1) represents the

amount of the feedback energy from the reference signal p.The ratio between the feedback energy and the transmitted

energy corresponding to the reference signal is then

η =

∑τ∈Nch

∣∣∣∣N−1∑n=0

r∗[n]q[n + Ndelay + τ]

∣∣∣∣2

N−1∑n=0

|p[n]|2

=

∑τ∈Nch

∣∣∣∣N−1∑n=0

p∗[n]q[n + Ndelay + τ]

∣∣∣∣2

∣∣∣∣N−1∑n=0

|p[n]|2∣∣∣∣2 , (3)

which is an estimate of the loop gain. Therefore,

η =

∑τ∈Nch

|R(τ)|2

|S|2 (4)

is used as the loop gain metric, where

R(τ) =

∣∣∣∣∣N−1∑n=0

p∗[n]q[n + Ndelay + τ]

∣∣∣∣∣2

(5)

and

S =

∣∣∣∣∣N−1∑n=0

|p[n]|2∣∣∣∣∣2

(6)

Fig. 4 plots the block diagram of the loop gain metricgeneration. The down sampling by a factor M removes the


Fig. 4. Loop gain metric generation block diagram. The down sampling by a factor of M removes the correlated samples from the transmit filter to reducethe computation complexity. The “Reset” signal clears the contents of Counter N as well as the R and S modules as defined by (5) and (6). When the CounterN expires, it sends out the “Ready” signal indicating that the loop gain metric is available. It also resets the R and S modules. The Median-m filter is usedto remove outlier noise of the loop gain metric that may otherwise give rise to large false adjustment of the repeater gain.

correlated samples from the transmit filter to reduce the com-putation complexity. The “Reset” signal clears the contentsof Counter N as well as the R and S calculation modules.When the Counter N expires, it sends out the “Ready” signalindicating that the loop gain metric is available. It also resetsthe R and S modules. The Median-m module is a non-linearmedian filter with length m. It is used to effectively removeoutlier noise of the loop gain metric that otherwise may resultin a large false adjustment of the repeater gain.

2) Loop Gain Control: With the loop gain metric, theinstantaneous stability status and output SNR of the repeatercan be closely monitored and accurately controlled.

Loop gain control is for controlling the loop gain of therepeater to the desired range such that the stability requirementand the output signal quality of the carrier are met, and at thesame time the output gain is maximized. First, the target rangeof the loop gain should provide sufficient separation from 0 dBto ensure stability. Second, as earlier stated, since the residualfeedback signal not only causes instability but also presentsitself as interference to the original signal, the loop gain metricalso represents the inverse of the output signal SNR and hasto be controlled to meet the repeater output SNR requirement.

As depicted in Fig. 5, loop gain control logic manages theloop gain by monitoring if the loop gain metric η is within acertain desired range, i.e.,

(Tup, Tdown

), corresponding to the

green zone in Fig. 5. Typically, Tdown is set at the value lessthan the target output SNR, i.e., Tdown≤ − SNRtarget.

If the current loop gain metric is smaller than Tup, i.e.,

η < Tup, (7)

which corresponds to the blue zone in Fig. 5, i.e., the loopgain is less than the desired value, indicating there is room forhigher repeater gain, the repeater digital gain, G, is adjustedup by an amount of

δ+ = α∣∣η − Tup

∣∣2(8)

dB, where α is a coefficient that controls the step size of theup adjustment. Whereas if the loop gain metric is greater thanTdown, i.e.,

η > Tdown, (9)

which corresponds to the red zone in Fig. 5 and indicatesthat the current loop gain is higher than the desired value, the

Fig. 5. Illustration of gain adjustment as a function of the loop gain metric.For η < Tup, the gain is increased by δ+ = α

∣∣η − Tup

∣∣2. For η > Tdown,

the gain is reduced by δ− = β |η − Tdown|2. No gain adjustment is necessaryotherwise.

Fig. 6. Gain management flowchart. The gain update procedure is triggeredby any of the three events: loop gain metric update, saturation metric update,and channel isolation update.

repeater digital gain, G, is then lowered by

δ− = β |η − Tdown|2 (10)

dB, where β is a coefficient controlling the down adjustmentstep size.

B. Device Level Gain Management

Device level gain management deals with the device limita-tion to ensure that repeater components, such as ADC, DAC,and power amplifier are not saturated. Saturation may incurdegradation of output SNR and even instability of the repeater.

1) DAC Saturation Control: The output of the transmitfilter q, which contains mostly the gain adjusted remote signalas well as the cancellation residual error, is the signal to be


Fig. 7. Illustration of the effectiveness of repeater gain management under various Doppler fading frequencies in the feedback channel. The maximum repeatergain is set at 70 dB. The top row shows the Doppler frequency of the feedback channel. The switch between Doppler fading frequencies is instantaneous forstress-testing the gain manager’s response to channel dynamics.

sent to the DAC, amplified, and transmitted. We need to firstmake sure that this signal does not saturate the DAC. That is,

(E)dB + G < TDAC, (11)

where

E =1

Nsat

Nsat∑n=1

q(n) (12)

is referred to as the saturation metric and TDAC is a thresholdrelated to DAC range. For example, TDAC = 68 dB for a 14-bitDAC. The DAC “headroom” for the digital gain is

GmaxDAC(E) = TDAC − (E)dB , (13)

which is a function of the saturation metric.2) ADC Saturation Control: The total signal presented to

the input of the ADC is the remote signal plus the feedback(as well as RF noise). Since the feedback interference is muchhotter than the remote signal (cf. Fig. 2), the total energy at theinput of ADC is approximately equal to the feedback energy.We thus require

(E)dB + (GRF + GISO) + G < TADC, (14)

where GRF is the lumped RF gain, GISO is the antennaisolation, and TADC is the threshold determined by the ADCinput dynamic range all in dB. Since (GRF + GISO) is the

effective channel gain from the input of the digital gain tothe input of the ADC, it can be obtained directly from thechannel estimate. The ADC headroom is then

GmaxADC(E, GISO) = TADC − (GRF + GISO) − (E)dB , (15)

which is a function of both the saturation metric and thechannel isolation.

3) Overall Gain Head Room: Repeater gain is also limitedby other factors like the maximum PA and other physicallimits imposed by other RF components. Assuming the cor-responding gain cap is Gcap, the overall gain head room isthus

Gmax(E, GISO) = min{Gmax

DAC(E), GmaxADC(E, GISO), Gcap

},

(16)The gain manager must ensure that this headroom is not to beexceeded at all times, i.e.,

G≤Gmax(E, GISO). (17)

C. Gain Update Procedure

The update procedure of the repeater gain is illustrated inFig. 6. The gain update procedure is triggered by any ofthe update events: loop gain metric update, saturation metricupdate, and channel isolation measure update.


Fig. 8. Stress-test of the repeater gain management’s responsiveness to large-scale changes in isolation. The maximum repeater gain is set at 70 dB. Thecanceller is turned on and off in a random fashion creating a ∼40 dB swing in effective isolation. This instant drop in isolation is responded by the gainmanager with a quick reduction in repeater gain to maintain the repeater stability and output SNR.

Fig. 9. The test of the repeater’s response to large-scale changes in isolation. The maximum repeater gain is set at 70 dB. The canceller is turned on and offin a random fashion creating a ∼40 dB swing in effective isolation. This instant drop in isolation is responded by the gain manager with a quick reductionin repeater gain to maintain the repeater stability and output SNR.


Each time when a loop gain metric update is available orready (cf. Fig. 4), the loop gain controller determines if again adjustment is necessary based on the loop gain metric.If a gain increase of δ+ is suggested, this gain adjustmentwill be on hold until the new saturation metric is availablebefore the adjustment can be finally applied. As soon as thesaturation metric is available, the gain manager updates thegain headroom and checks to see that the headroom is notexceeded. That is,

G.= min

{Gmax(E, GISO), G + δ+

}, (18)

if a gain up adjustment δ+ is pending. Otherwise,

G.= min {�max(E), G} (19)

is asserted.On the other hand, if a gain reduction is ordered by the loop

gain controller, the gain is updated and is applied immediatelywithout further delay,

G.= G − δ−. (20)

IV. Performance

In this section, we investigate the performance of theproposed gain management scheme via a combination of asimulation system and a prototyping system. The system usedWCDMA signals operating on 1850-1990 MHz. The patch-dipole antenna system provides antenna isolation in the rangeof -40 dB to -55 dB. Gain ripple over operation frequencyrange was ±1.5dB. Return loss for each antenna is better than10 dB over operation range. A baseband FFT-based frequencydomain echo canceller gave an additional 35 to 45 dB activeisolation depth. This ensures that the residual error was 15 to25 dB below the remote signal. The resolution of the ADCand DAC was 14 bits, with sampling rate 120 and 240 Msps,respectively. RF gain GRF ≈ 45dB. Gain cap, Gcap, was setto ∼70 dB. Including the coverage antenna gain of ∼16 dB,the maximum repeater gain was ∼86 dB. The repeater targetoutput SNR was 15 dB. The corresponding loop gain metricthresholds were set to Tup = Tdown = −16dB, to allow an1 dB margin for thermal and other imperfection noise. Theconversion of digital gain G in dB to

√G in linear scale was

via a look up table (LUT). The median filter length m was setto be three.

The gain was initialized at 20 dB, resulting in an uncon-ditionally stable total loop gain of -20 dB. There was noecho cancellation during the first 33 μs for channel estimationsettlement. As soon as the channel estimation was stabilized,echo cancellation was activated, thereby lowering the repeaterloop gain, and allowing the gain management to ramp up thegain accordingly.

Fig. 7 through Fig. 9 plot the repeater total gain, outputSNR, and loop gain metric during boot-up and steady op-eration under various isolation conditions recorded from thesimulation system. The maximum repeater total gain is set at70 dB. It is clear that the loop gain metric accurately reflectsthe repeater output SNR (the inverse of the output SNR, cf.Fig. 2).

To show how well the loop gain management scheme canadapt to the changing environment, fading channels with vari-ous Doppler frequencies is injected into the repeater feedbackchannel as shown in Fig. 7 and Fig. 8, where the effectiveisolation changes as the channel changes. In particular, sincethe echo cancellation performance is sensitive to channelvariation speed, the faster the channel variation rate, the poorerthe cancellation performance is as shown by the repeateroutput SNR in Fig. 7.

In Fig. 7, the gain management is disabled. As a result,the repeater output SNR varies with the feedback channelcondition and cancellation depth. The required SNR (≥15dB),as well as the stability, cannot be guaranteed. It is in factcompletely at the mercy of the environment.

In contrast, it is seen in Fig. 8, in which the gain man-agement is active, that the repeater safely boots up and theoutput SNR is maintained above the required level at all times.The gain manager detects the effective isolation via the loopgain metric and adjusts the gain accordingly such that theoutput SNR is maintained above the desired level regardlessof the channel conditions. Note that, although unrealistic, theswitch between different Doppler fading rates is purposely setto be instantaneous for stress-testing the gain manager. Theinstant changes in channel parameters cause degraded chan-nel estimation, and consequently the degraded cancellationperformance, thereby resulting in sudden changes in the ef-fective isolation of the repeater. As a result, the output SNRdips at the time of switch. However, the potential instabilityis detected by the gain manager, as reflected by the soaring-up of the loop gain metric, the repeater gain is thus promptlyreduced and the repeater stability, as well as the output SNR, ismaintained.

To further stress-test the gain management scheme, thecanceller is turned off and on in a random fashion resultingin a ∼40 dB sudden swing in repeater effective isolation.Oscillations are expected without gain management. However,as seen from Fig. 9, with gain management the repeater is ableto quickly adjust the gain and manages to maintain stabilityeven under such harsh disturbances.

V. Conclusion

The feedback of the transmit signal to the input of therepeater in an on-channel repeater not only causes potentialinstability but also acts as interference degrading the repeateroutput signal quality. The potential danger of oscillation anddegradation of output SNR prevent repeaters from beingwidely deployed. Oscillation prevention and output SNR con-trol are thus crucial to the repeater design, and hence is themain focus of this paper. Although there are different typesof repeaters and echo cancellers available in the literature (notthe focuses of this paper), this paper proposes an effectivegain management scheme for on-channel repeaters. In par-ticular, a loop gain metric is introduced to continuously andaccurately detect the stability status of a repeater, allowingthe gain manager to control the output SNR and preventinstability by appropriately adjusting the repeater gain in adynamic environment. The proposed active gain management


scheme is further tested in a simulation and a prototypingsystem. It is demonstrated that the gain management schemeis highly effective and robust in responding to the dynamicsof environments.

Acknowledgement

The author would like to thank the reviewers for theirconstructive comments and suggestions which helped improvethe paper. The author is also grateful to Dr. Gwendolyn Barriacand Dr. Dhananjay Gore for their helpful discussions.

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Dynamic Gain Management for On-Channel Repeaters