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34 www.rfdesign.com September 2006

Software Radio

Mitigating interference to maximize spectralefficiency in 3G/4G networks

With the advent of data applications, interference is a challenge for wirelesscarriers. Consequently, mitigating interference to maximize spectral efficiencyand improve network throughput is on the minds of operators and handsetmakers. This article describes an interference cancellation technologycomprising an ASIC/core hardware and DSP-based software, which cancelsinterference from all traffic channels, and from all interfering sources for 2.5G,3G and 4G networks.

By John Thomas

Canceling intracell interference induced by channel effects andintercell interference introduced by channel effects, as well as

serving and non-serving base stations is a technological challenge.And, it is particularly difficult when the code space is heavily loaded,as in dense voice networks such as fully loaded CDMA2000 1xEV-DOdata networks or mixed-voice and HSDPA networks.

While there may be several receiver architectures in the market toaddress the interference challenge, in reality there are four generalclasses of baseband receivers. And, in terms of increasing complex-ity, these include RAKEs, equalizers, linear minimum mean squareerror (MMSE) receivers, and multi-user detectors (MUDs). Each ofthese receivers is an optimum solution for a different combination ofcommunication protocol, channel condition, and code-space loading.Consequently, when a RAKE is optimum, there is no performance gapto be filled by a more advanced receiver. However, when an equalizeris optimum, the only performance gap to be filled is between a RAKEand an equalizer. Likewise, when a linear MMSE receiver is optimum,there are performance gaps to be filled between a RAKE, an equalizer,and a linear MMSE receiver.

Thus, to fill available performance gaps between a RAKE, anequalizer, a linear MMSE receiver, at low complexity, TensorCommhas developed a novel interference cancellation technology (ICT),comprising an ASIC/core hardware and DSP-based software, thatcancels interference from all traffic channels, and from all interferingsources for 2.5G, 3G and 4G networks. A protean signal-processingoperator, ICT exploits all time-varying source characteristics that code-space profiles and channel conditions leave open for exploitation, in ahardware-efficient architecture.

Mitigating interferenceInterference sets the limit on performance in code-based systems.

By canceling unwanted interference, ICT reduces power requirementsand increases spectrum efficiency so that cells can maintain size andnetwork capacity. The technology requires no modification to existingor evolving air interface standards. Because it is a receive-only tech-nology, it requires no base station modification to effectively reduceinterference on the forward link. It also can be integrated into CDMAand WCDMA chipsets today. The same technology may be appliedto the reverse link and integrated into base stations. The importanteffect is to maintain cell size at high traffic density and to reduce thefrequency of dropped calls in hand-off.

Besides canceling intersymbol and interchannel interference frompilot, paging, synchronization, traffic and high-rate data channels, ICT

is specifically designed to cancel direct and multipath interference

from adjacent base station sectors and interference from multipathwithin the serving sector. Also, this technology is complementary to,and can co-exist with, other technologies, such as transmit/receivediversity technologies, to re-capture power and/or spectral efficiency.Additionally, it is independent of carrier frequency bands, and willapply at 450 MHz, 800 MHz, 1900 MHz and elsewhere.

Theoretical foundations of ICTInterference is the limiting factor in the performance of CDMA

and WCDMA wireless networks. Field conditions such as fading andmultipath defeat all attempts to maintain orthogonality between trafficand control channels in multi-access voice and data systems based onCDMA and WCDMA standards. The lack of orthogonality leads tointerference and a consequent reduction in signal-to-interference andnoise ratio (SINR). Thus, CDMA and WCDMA are interference-limitedrather than noise-limited. As a consequence:After multipath resolution with a RAKE receiver, every resolved

baseband path contains interchannel interference (ICI) and intersymbolinterference (ISI) from every other path.This produces a bit-error-rate (BER) higher than a target energy-

to-noise density ratio (Eb/No) would predict, requiring a) increasedsignal strength and SINR, b) reduced traffic-loading and/or c) reducedbit-rate to maintain network quality of service (QoS). Transmit power increases because neighboring devices ask for

more power to contend with more interference.

S2

S3

S1

S4

Base Station 1 Base Station 2

Figure 1. Two base stations and four paths.

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Overall network capacity should be maximized by havingeach device use the minimum required transmission power so that theinterference caused to other devices in the network is minimized.

There is a body of work for interference cancellation and multi-userdetection suggesting that all interference effects can be managed withsignal processing if the channel can be accurately estimated and the

optimal signal-processing solution can be implemented at the symbolrate for voice and data. Neither of these ideals is achievable. TheTensorComm approach approximates the optimum solution by factor-ing interference cancellation into a sequence of signal-processing stepsthat remove ISI and ICI.

The company has filed more than 75 patents in this area and indi-cates its strength is to blend advanced signal processing with existingtransceiver architectures for CDMA and WCDMA modems.

Illustrated exampleThe example illustrated in Figure 1 describes the application of ICT.

Consider to be the complex baseband signal arriving at a handset(after reception at the antenna and downconversion). This signal canbe resolved into multiple components that represent the different paths

arriving at the antenna, plus thermal noise.For example, we could represent the complex signal in path 1 as:

where and are signals from two different paths from basestation 1, while and are multipath signals from base station 2 insoft hand-off. is the thermal noise in the received signal.

A conventional RAKE receiver assigns these paths to differentfingers, which then recover the message by applying the correct alignedcodes for recovery of the transmitted symbol. While the design andselection of the codes attempts to minimize the cross-correlation of thedesired codes with the codes of other paths, the presence of multipathand hand-off defeats orthogonality and produces non-zero cross-

correlation between signal components.Let the codes for the channels of interest be:

and for the respective paths.

The RAKE receiver then recovers the symbols

and by computing inner products with thecorresponding codes. The estimated symbol in path 1, , is obtainedusing the inner product or correlation

where is the symbol of interest in path 1, is interference andis noise.That is,

Similarly, each path experiences interference from all the otherpaths:

The combiner combines the symbol estimates from each path toarrive at a soft decision, usually using a maximal ratio combiner:

where and are the maximal ratio coefficientsassociated with each path (usually pilot amplitudes).

Because of the correlations between the multipath signals andthe codes , the symbol estimates contain interference from allpaths.

The estimated SINR, also referred to as Ec/Io, is the ratio of thesignal energy (Ec) to the total noise and interference (Io) in the system.Lets call the variance (or power) of interference , and thevariance of the noise. Then ignoring correlation between interferences,a crude but descriptive estimate of SINR is:

In an ICT-enabled handset, the interference in the signal iscanceled, so that the application of the desired code yields a smallerinterference term. Thus,

where is the signal for path 1 with interference canceled.That is,

where the variance (or power) ofis much smaller than the

power of , and is approximately 1. Therefore, the maximal ratiocombined symbol estimate is:

Baseband Chip

Keypad

Ringer

DataPort

External

Memory

RF Rx/Tx

Chips

ICT CORE

Speaker

Microphone

CODE

C Digital

SignalProcessor

(DSP)

Modem

MemoryMicrocontroller

(MCU)

RF

Interface

Analog Domain Digital Domain Analog Domain

AntennaSpeaker

Microphone

Keypad

Receive Chips

Transmit Chips

Battery Management

BasebandChip

SPAM Flash

Figure 2. ICT is an ASIC/core solution that integrates into the modem ofthe baseband chip.

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The new estimated SINR is:

The ICT gain G can be roughly estimated as:

To illustrate, if all are roughly equal to 1

This gain is realized without impacting the diversity that multipathbrings or from the gain due to hand-off. Interference cancellationpreserves all the advantages of the system while increasing SINR. Inhand-off, the receiver resolves multipaths with higher SINRs than itwould have in the absence of ICT.

Architecture and integrationAs shown in Figure 2, ICT is an ASIC/core solution that integrates

into the modem of the baseband chipset. When deployed in the handsetto mitigate forward link interference, no other components and nore-design of the handset RF are required, resulting in no form factorchanges to the handset. Furthermore, no base station modificationsare required.

The sequence of signal-processing steps in ICT occurs after RFprocessing, delivering interference-cancelled signals to the RAKEreceiver. ICT delivers a subspace version of the original path signalsto each finger, relatively free of ISI and ICI, with higher SINR. Thisis illustrated in Figure 3.

ICT has been integrated into platforms and successfully testedin the field on commercial networks. In addition to developing thebasic ICT algorithms, TensorComm has evolved a process for integrat-ing its algorithms and intellectual property into a customers CDMAor WCMDA modem.

Impact on performance of networkIn a CDMA network, as traffic load increases, the total base station

transmit power increases, because a handset requires more transmitpower from the base station to maintain the same performance in dense

Figure 3. ICT sits between I/Q and searcher/finger.

Figure 4. Depiction of network response to ICT A. With interference (b),serving base station broadcasts more power per handset to keep the con-nection, while others nearby have to do the same. ICT is added to cancelinterference (d), and base stations no longer have to transmit higher

power to each handset. Power and spectrum efficiency is restored makingroom for more handsets/subscribers (e).

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interference. The effect on the network is that probability of coveragedecreases and network performance degrades. Furthermore, when thenetwork load exceeds 75%, degradations are more pronounced as cellboundaries collapse, creating coverage holes. The result is that custom-ers experience a greater number of dropped and blocked calls. The sequence of panels in Figure 4 demonstrates in a qualitative

way how ICT reclaims power and spectral efficiency to maintain cellcoverage and traffic density. That is, interference cancellation increasesthe capacity of the network for more users or for higher data rates,while expanding or maintaining network coverage. In essence, it showsthat base stations no longer have to transmit so much power to eachhandset. And can use extra power to increase capacity, coverage, datarates and quality.

A network attempts to maintain uniform quality for users by setting per-

formance targets and adjusting transmit power to meet these targets.At a low level, ICT operates in tandem with fast and slow powercontrol to provide performance improvement. ICT improves SINR oneach signal, which translates to a reduction in demodulation errors andlower frame error rate (FER). Accordingly, the handset compensatesfor this performance improvement by requesting a lower forward-linktransmit power in an attempt to keep base station transmit power ata minimum.

Additionally, the increase in SINR due to interference cancella-tion allows a greater number of base station sectors to remain in theactive and candidate sets. This provides greater signal diversity, whichis invaluable in compensating for signal fading. The technology canprovide large instantaneous gains in fades that would limit handsetperformance. The interference cancellation gains also soften theimpact of fades, since power control movements are minimizedduring fades.

From the base station perspective, each ICT-enabled handset requiresreduced transmit power to maintain the same performance. As a result,the base station is able to increase the number of served users in a cell.From the network, a second-order effect is observed: for a given numberof users, each base station lowers the transmit power for ICT-enabledhandsets, reducing the noise on all mutually interfering sectors. Thisleads to further reduction in network transmit power.

In field environments, there is almost always an opportunity forinterference cancellation. As a result, ICT will be operational formuch of the time. However, the design incorporates a level of intel-ligence, so that it may turn off the cancellers when cancellation is

not beneficial. This no-harm feature guarantees that an enabledhandset always provides performance better than or equal to that ofa non- ICT-enabled handset.

Technology evaluationThe company has proven its technology through comprehensive

technology development, testing, and evaluation, consisting of simula-tions, laboratory testing and field validation. Figures 5-7 demonstratethe performance of ICT by documenting its impact on base stationtransmit power and file transfer speed.

Laboratory tests were conducted to evaluate the prototype undervarious signal conditions, such as fading, multipath and hand-off, over arange of network loads from 0% to 75%. In the laboratory, test scenarioswere specified with a pilot Ec/Io for each base station that depended onproximity to the handset, from the edge of the cell, where pilots wereequal, to close-in, where the difference in pilot strengths was 6 dB.

Figure 5. Forward traffic channel power recorded at base station.

80

70

60

50

40

30

20

10

0

%

CapacityIncrease

% OCNS Loading (distributed over multiple Walsh codes)

ForwardTrafficChannelPowerR

eduction(dB)

0 dB Pilot Ec/lo Separation3 dB Pilot Ec/lo Separation6 dB Pilot Ec/lo Separation

2.5

2.0

1.5

1.0

0.5

0.00 25 50 75

Figure 6. Forward traffic channel power reduction vs. % OCNS loading.

Figure 7. An example of base station logs recorded during commercialdrive test.

Figure 8. Side by side comparison of data throughput for a file transfer toa prototype with ICT enabled and then repeated to a prototype without ICT.Moving from -60 to -40 seconds, the transfer with ICT enabled was com-pleted in less than 15 seconds. Moving from -40 to 0 seconds, the transferwith ICT disabled took more than 35 seconds.

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The forward-link traffic channel power gainranged from 0 db to 4 dB. Figure 6 illustratesthe forward traffic channel power reductions asa function of the cell loading, at three differentpilot strengths. When pilot strength separationis medium to low, ICT provides significant

gains that increase with % OCNS (orthogonalchannel noise simulation).

Drive tests were conducted on a major U.S.operators CDMA2000 commercial network

over an extended period at various times dur-ing the day and night. Tests were conductedsimultaneously with two side-by-side proto-types comparing the network impact betweena prototype with ICT enabled and anotherprototype with ICT disabled. In all cases, the

critical metric recorded was the base stationforward-link traffic channel power.

The commercial test results of Figure 7demonstrate a substantial reduction in forward

link traffic channel power. Gains averaged2.5 dB in a heavily loaded environment withpeak gains of up to 6 dB.

Laboratory tests of data throughput re-vealed a doubling of data rates for thosehandset prototypes enabled with ICT. As

illustrated in Figure 8, a file transferred to anICT-enabled prototype took half the time of afile transfer to a prototype with ICT disabled.This was further validated by the recorded datarate, which was doubled for the ICT prototypeat a reduced data retransmission rate.

Simulation evaluations of WCDMA ver-sions (release 99 and HSDPA) have beencompleted and observed gains are comparableto CDMA2000 gains.

Network modeling of these test resultsshows that the recaptured base station powercan support greater than 40% more subscribersand make all future network capacity expen-

ditures 40% more efficient. This gain can berealized and used in multiple ways: to increasehigher data rates for data applications, supporthigh traffic densities, provide better quality ofservice for voice applications, and reduce basestation transmit power, leading to increasednetwork capacity. This capacity relief allowsthe wireless operator to delay and reducesignificant capital and network operationalexpenditures on expensive infrastructure andspectrum. Network modeling indicates that fora 20 M subscriber system, the realized savingswould be more than one billion dollars in afive-year period.

SummaryBy adopting ICT, a wireless service opera-

tor can increase network capacity, accompa-nied by a substantial savings of capital expen-diture. The ICTs patented approach deliversgains in power and spectral efficiency, therebyimproving cell capacity, increased coverage,improved quality of service and increaseddata rates. Based on commercial networktrials, this technology has proven to increasea CDMA wireless service operators networkcapacity by greater than 40%, thus delayingand reducing the capital and spectrum expen-ditures required to support subscriber growth,increased minutes-of-use, expanded coverage,and increased data services. RFD

ABOUT THE AUTHOR

John Thomas is CEO and co-founder ofTensorComm. Prior to founding Tensor-Comm, Thomas co-founded Data FusionCorp. Before co-founding Data Fusion,he was at NASAs Jet Propulsion Labora-tory. Thomas earned a Ph.D in ElectricalEngineering/Signal Processing from theUniveristy of Colorado at Boulder.

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