896 IEEE COMMUNICATIONS LETTERS, VOL. 13, NO. 12, DECEMBER 2009

Beacon Signaling and its Application to Inter-Frequency Handoff inOFDMA Cellular Communications

Michael Mao Wang, Member, IEEE, Tom Richardson, Ravi Palanki, and Alex Gorokhov

AbstractThis paper introduces a beacon coding and sig-naling technique and its application to handoff in OFDMAcellular communication networks. Beacon signaling provides anefficient alternative to the conventional tune-away approach forinter-carrier/frequency handoff. The proposed handoff techniquetransmits beacon pilots in other carrier frequencies in the net-work allowing for easy and early detection of neighboring cells,and fast and seamless handoffs in a multi-carrier synchronousor asynchronous deployment. The proposed technique therebyminimizes the call drop rate and interference due to delayedhandoff.

Index TermsBeacon, inter-carrier handoff, OFDMA.

I. INTRODUCTION

IN multi-carrier deployments of a cellular communicationsystem, wherein more than one carrier frequency can beused, an access terminal is typically tuned to one carrier.The access terminal may not know the presence of sectorsoperating on other carriers. Traditionally, this problem wassolved by the access terminal tuning-away to other carriersto search for other sectors on those carriers. This approach,however, causes an interruption in data service to that accessterminal and causes delay in handoff. In this paper, weintroduce a beacon signal based inter-carrier handoff techniquefor use in OFDM-based communication networks. OFDMhas become a mainstream modulation technique for nextgeneration wireless communications. For example, the mostrecent proposed next generation wireless wide area network(WWAN) standards, 3GPP2 Ultra Mobile Broadband (UMB)[1], IEEE802.20 Mobile Broadband Wireless Access (MBWA)[2], 3GPP LTE [3], and WiMAX [4] are all OFDMA-based.Mobility management is crucial to the performance of anOFDM-based system since a delayed serving cell switchintroduces interference and increases call-drop rate.

II. BEACON SIGNALING

Conventional inter-carrier/frequency handoff uses tune-away to other carriers to search for other cells on those carri-ers. This approach suffers from the drawback of interruptionin data service since a typical access terminal is equippedwith only one tuner. As a result, tuning-away can not be donevery often thereby resulting in late detection of neighboringcells and delayed handoff. In this paper, a different solution tothis problem is proposed. This solution is based on having asector transmit a periodic sequence of beacons on other carrier

Manuscript received May 11, 2009. The associate editor coordinating thereview of this letter and approving it for publication was H. Wymeersch.

The authors are with the Corporate Research Center, Qualcomm Inc., SanDiego, California USA 92121 (e-mail: wangm@qualcomm.com).

Digital Object Identifier 10.1109/LCOMM.2009.12.091048

Fig. 1. Illustration of beacon transmission scheme for inter-carrier handoff.

frequencies (cf. Fig. 1). The beacons contain information aboutthe sector identifier and the sector carrier frequency. A termi-nal operating on a given carrier frequency can see beaconson the same carrier frequency transmitted by other sectorsoperating on other carrier frequencies. Thus the terminal isable to identify these other sectors without the need to tuneaway.

In an OFDM system, beacon signaling is done by trans-mitting a large fraction of the energy in an OFDM symbol inone subcarrier. The choice of the subcarrier depends on themessage. However, beacon detection can be prone to errors (anevent in which a beacon is detected on a subcarrier where nobeacon was sent by other sectors due to noise or interference)and erasure (an event in which a beacon is sent but not detecteddue to fading). The message , represented by informationsymbols, u = (1, 2, , ), is therefore encoded intoa Reed-Solomon codeword as c = (1, 2, , ), where0 1 is the code symbol whose value correspondsto the subcarrier index on which a beacon tone is transmitted, is the number of tones that can be used for beacon signalingin each of the OFDM symbols. On each of these OFDMsymbols (1 ), a beacon tone is transmitted in oneof the subcarriers (no transmissions on other subcarriers).Since Reed-Solomon codes are maximum-distance separable,it ensures that erasures or half that number of errorscan be corrected. However, the beacon code often needs tobe detected in the presence of timing uncertainty. Note thatthe sector transmits an infinite periodic sequence of beacons , 1, 2, , , 1, 2, , , . An access terminalmay not have information a priori about the boundary of thecode word. Thus, the beacon code should be such that all thecyclic shifts of a code word should be decodable to the samemessage. We therefore represent the message as

=

1=0

( 1)

= 1 ( 1)1 + + 1 ( 1) + 0 (1)1089-7798/09$25.00 c 2009 IEEE

WANG et al.: BEACON SIGNALING AND ITS APPLICATION TO INTER-FREQUENCY HANDOFF IN OFDMA CELLULAR COMMUNICATIONS 897

where =/( 1) mod ( 1). We then define

the information symbols by

u = (1, 2, , ) = (1 , , 1 , 0) (2)where is a primitive element in GF( ). We now Reed-Solomon [6] encode the information symbols u into a code-word c = (1, 2, , ) via the Galois Fourier Transform(GFT)

=

=1

1 =

=1

+1+(1)/ (3)

where = 1, 2, , and denotes summationmodulo . Once the terminal detects code sym-bols , +1, , +1 at OFDM symbols, i.e.,, +1, , +1, we have the following linear system asshown in (4) at the top of the next page. This system does nothave a unique solution if is unknown. But we first solve thelinear system (in GF( )) for 1+

1 = 1, i.e.,

1 + ( 1)/ = log 1 (5)We observe that =

log 1

/( 1)/ and 1 =(

log )mod (( 1)/) if we can assure that

< ( 1)/. (6)The value of can then be used in solving for , =

2, , 1, 0. Therefore, the message can be determinedin the absence of a priori timing information . In addition,the value of provides frame timing information of the adver-tising sector. Hence, the beacon also helps handoff betweenasynchronous cells since beacon signals provides the frameboundary information of the cell from which the beacon issent. According to the size of , we can choose and such that (6) is satisfied. The choice of is restricted bythe total available subcarriers and is restricted by decodingdelay since a minimum of OFDM symbols is needed fordecoding the beacon code.

III. BEACON CODE DESIGN AND DETECTION

A. Beacon Code Design

In this section, is assumed a 12-bit message that containsthe minimum amount of information needed for a sectorswitch, including 9 bits sector identification number (e.g., PNoffset), 1 bit synchronous/asynchronous mode indication, and2 bits of carrier (channel band) identification. It is representedby = 2 information symbols and is encoded into acodeword of = 10 over a set of = 211 subcarriers. Theprimitive element is chosen to be 207. Since < 212,it is always true that 1 < 212

/(211 1) < 15. Hence

1 < ( 1)/ = 21 and thus this beacon code iscyclic shift decodable. Note that the beacon tone may causeinterference to the data transmission on that carrier. However,the beacon tone is only one out of = 211 tones and thereforethe disturbance is minimal.

The 12-bit message is first represented by (1) with = 2, and 1 = / ( 1) and 0 = mod ( 1).Next, we define the information symbols 1 = 1 and2 =

0 . We now apply the GFT to generate the codeword

Fig. 2. Illustration of the transmission of a beacon code ( = 10, = 211)where the code symbol 0 210, = 1, 2, , is defined as thesubcarrier index of a set of = 211 subcarriers. The hollow tones denoteno energy transmitted on these tones.

1, 2, , , 10, where = 1+21 + 0+42, =1, 2, , 10 according to (3). The codeword is then transmit-ted on = 10 OFDM symbols periodically. That is, foreach code symbol , a beacon tone is transmitted on the subcarrier of the 211 subcarriers of OFDM symbol , = 1, 2, , , as illustrated in Fig.2.

The starting position in the transmission frame structureof the = 10 OFDM symbols used to transmit a beaconcode word c = (1, 2, , 10) is a hash function of thesector identification number as is the actual location of the subcarriers within each OFDM symbol of the total bandwidthFFT used for transmission of a beacon code symbol . Theseparation between two consecutive beacon OFDM symbols

= +1 is by design far apart (e.g., = 500 OFDM

symbols) to ensure minimal disturbance to the data traffic.

B. Beacon Detection

At the receiver, a beacon tone is declared to be detectedif the subcarriers energy crosses a given threashold. Since = 2, we need at least two symbols at the terminal to decodethe message . Let us assume that a terminal receives twoerror-free symbols and

+1 at time and +1

= 1+21 + 0+42

+1 = 1+21(+1) + 0+42(+1)

where = 207. Writing the above in matrix form gives[+1

]=

[1 121 42

] [1+21

0+42

].

This system can be solved to obtain 1+21 =+1422142 (in

GF(211)). We then obtain 1 =(log

+1422142

)mod 21

and =log

+1422142

/21. The value of can be used to

solve for 0. With 1 and 0, the message can thereforebe determined according to (1).

In the presence of errors, verification of the decoded mes-sage is necessary. The decoded message is re-encodedand the resulting sequence, + , ++1, , + , iscompared to the beacon tones + ,

++1, , +

898 IEEE COMMUNICATIONS LETTERS, VOL. 13, NO. 12, DECEMBER 2009

+1

...+1

=

1 1 1(1)

2(1) (1)

......

. . ....

(1)(1)

2(1)(1)

(1)(1)

1+(1)

2+2(1)

...

0+(1)

(4)

Fig. 3. Simulated beacon message detection performance (channel model:ITU Pedestrian A and B [8], false alarm probability =106 , i.e., theprobability of decoding beacon signals to a wrong (but valid) message is106. OFDM subcarrier spacing is 9.6kHz. OFDM symbol time offset amongcells is uniformly distributed between 0 to 50% of the symbol duration. Thefrequency accuracy of a sector is 0.1ppm at a carrier frequency of 2GHz (10MHz bandwidth).

at the OFDM symbols of + , +(+1) , + where > . A decoded message can be considered to be validonly if the received beacon sequence matches the re-encodedmessage on a certain number ( ) of OFDM symbols.

With the detected sector identification number and thebeacon symbol position, the timing information (i.e., theOFDM symbol index in a transmission frame structure) of theadvertising sector can also be determined. Hence, the beaconalso helps handoff between asynchronous cells since beaconsignals provides the timing information of the cell from whichthe beacon is sent.

Fig. 3 shows the probability of successful detection of themessage from the example given above with = 4. Aswe can see, this beacon code can be detected at very lowcell geometries (signal to interference ratios (SIRs)). At lowSIR, the Pedestrian B channel has lower detection probabilitythan the Pedestrian A channel since Pedestrian A has muchless channel delay spread resulting in less frequency diversityand more channel variance for the beacon tone. Therefore thedetection probability is higher at low SIR for Pedestrian A.Note that these results do not take into account the low peak-to-average power ratio (PAPR) of the beacon symbols (singletones). Such a power boost would in fact allow the beaconcodeword to be detected at even lower geometry, i.e., higherpenetration to other sectors.

IV. CONCLUSION

Like all other wireless systems, cellular OFDMA sys-tems require the transmission of several control channels.These control channels typically are used to transmit a smallmessage. An effective scheme of transmitting such smallmessages is to use beacon signaling. The proposed beaconsignaling transmits pilots in other carrier frequencies allowingfor fast and seamless handoff in a multi-carrier synchronousor asynchronous deployment, enabling early discovery ofneighbor sectors, thereby minimizing the call drop rate andthe interference due to delayed handoff. Beacon signaling asdescribed in this paper offers several advantages comparedto other forms of signaling. For example, beacons can bedetected with fairly low receiver complexity by just looking forsubcarriers with significantly higher energy. This is in contrastto traditional coherently coded schemes which would requirechannel estimation. In addition, beacons can be detected atvery low SIR due to the high amount of energy placed onthe beacon subcarrier. When the beacon from a sector is tooweak to be detectable, the terminal is usually far away fromthat sector. Theres no urgency to detect that sector sincehandoff to that sector is not yet imminent. Another advantageis that beacon OFDM symbols where all the energy is onone subcarrier have low PAPR. This allows for higher poweramplifier efficiency on these symbols, thereby allowing deepercoverage for beacon signaling. Finally, we note that OFDMsymbol time synchronization among cells helps but is notnecessary in the proposed beacon signaling scheme. That is,beacon signaling can be applied to asynchronous systems aswell since even in the worst case where OFDM symbol timingbetween two base stations are offset by 50%, the receiver willstill pick up 50% of the beacon tone energy together withsome inter-symbol interference and inter-carrier interference.This will result in 3dB degradation with respect to perfectsymbol synchronization.

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