Preamble Design in Ultra Mobile Broadband Communication SystemsMichael Wang, Aamod Khandekar, Alex Gorokhov, Sandip Sarkar, Bhushan Naga, Ravi Palanki,

Avneesh AgrawalQualcomm Corporate R&D, San Diego

E-mail: wangmgqualcomm.com

Abstract The preamble design for a highly flexible wireless Table 1 System bandwidth and the corresponding FFT size.communication system can be challenging. This paper Bandwidth, MHz < 1.25 1.25-2.5 2.5-5 5-10 10-20describes a preamble design for Ultra Mobile Broadband FFT size 128 256 512 1024 2048(UMB) Communication Systems that allows flexibledeployment, such as flexible bandwidth allocation, The subcarrier spacing is fixed at 9.6 kHzsynchronous/asynchronous modes, FDD/TDD, full/half corresponding to an OFDM symbol duration ofduplex, and configurable cyclic prefix duration, etc. The Ts 104pt sec. The length of the cyclic prefix of an OFDMUMB preamble design facilitates the flexibility and yet has slow overhead, low acquisition latency, and low complexity. symbol is variable, T. = NC TS/16 6.51NCp ,usec, to

facilitate various deployment environments, whereKeywords: Ultra Mobile Broadband (UMB), preamble, N 1,2,3,4.system acquisition. cp , .3.

At a UMB transmitter, the transmitted data are

1. Overvieworganized as superframes. For a UMB access network, a

The Ultra Mobile Broadband (UMB) system is designed for superframe consists of a preamble followed by NPHY = 25

rdaccess and is optimized for high PHYframes. Both the preamble and the PHY frames consistrobust mobile broadband ofesNnisotmzdfrhg-spectral efficiency and short latencies using advanced s 8 OFDM symbols. The preamble is used by an

modulation, link adaptation and multi-antenna transmission access terminal for the purpose of system determinationtechniques. In addition, fast handoff, fast power control, and and/or acquisition. The PHY frames are used for data trafficinter-sector interference management are embedded in the transmission. In FDD half duplex mode, each PHY frame isdesign to facilitate communication in highly mobile separated by a guard interval ( = 3Ts/4 =78.13,t sec),environments. whereas there is no separation in full duplex mode (Tg = 0 ).

The UMB system uses OFDM as the main modulation . . .technique. In addition, it incorporates adaptive coding and Thered tissingniiatlysmorex flexibility in UMB

modulation ~ ~~~ ~ ~ . wihsnhoou.AQadtrb.oigwt compared to existing systems. Flexible parameters that canaortlrtrans latencof 5.5 ins.the UMB fwar affect preamble structure are: (1) Bandwidth allocationa short retransmission latency of 5.5 ms. The UMB forward hc orsodao a F ie olink supports MIMO (both single codeword with closed which 2corresponds 2to 8andFth szer ofloop rate and rank adaptation and multi-codeword (layered) NFFT =128/256/512/1024/2048 and the number ofwith per-layer rate adaptation) with closed loop precoding guard tones; (2) FDD/TDD. FDD includes full and halfand space division multiple access (SDMA). The peak rate duplex and TDD includes choice of TDD partitioning; (3)reaches 260 Mbps in a 20 MHz forward link. The reverse Cyclic prefix length (four possible values); (4)link supports quasi-orthogonal transmission: orthogonal Synchronous/ asynchronous modes. The flexibility in UMBtransmission based on OFDMA and non-orthogonal with system configuration requires that the preamble bemultiple receive antennas. The reverse link employs CDMA structured to provide an efficient mechanism for systemfor the control segment that allows statistical multiplexing determination and acquisition for an access terminal.of various control channels and fast access and request. TheCDMA reverse link control segment provides a wideband 2. Preamble Structurereference for power control, subband scheduling and The UMB preamble consists of eight OFDM symbols. Theefficient handoff support. first OFDM symbol is used to transmit the PBCCH

UMB provides interference management through (Primary Broadcast Control Channel) while the next fourfractional frequency reuse for improved coverage and edge OFDM symbols are used to transmit the SBCCHuser performance. The dynamic fractional frequency reuse (Secondary Broadcast Control Channel) and the QPCHalso optimizes bandwidth utilization. UMB optimizes (Quick Paging Channel) in alternate superframes.throughput and fairness through distributed and other The last three OFDM symbols carry acquisition pilotssector/cell interference based power control. TDM Pilots 1/2/3. TDM Pilots 2 and 3 are additionally

UMB has a unified design for full and half duplex modulated by OSICH (Other Sector Interference Channel).FDD and TDD and a scalable bandwidth from <1.25 to 20 The ordering of the preamble OFDM symbols, i.e.,MHz for variable deployment spectrum needs. The system placing PBCCH/SBCCH in front of the TDM Pilots, is tobandwidth and corresponding FFT sizes are listed in Table provide sufficient AGC convergence time for the TDM1. Pilots during initial acquisition.

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The structure of the superframe preamble is depicted Table 2 Relationship between TDM Pilot 1 sequencein Fig. 1 parameters (u, NG, NP) and cyclic prefix (CP) duration

£ £ ~and FFT size.|I C| C| Preamble FFT Size CP (As)

nL n n n co co ( ,3 3 135,6 1,12,10 .5a aa a t ~~0 0

0 0 n C 128 256 512

C) C) C) C) 82H-) 135,5) 1717,20 65

FI X (12,23,23) (22,59,56) (39,127,120) 13.02

OFDM Symbol (14,23,23) (39,59,56) (110,127,120) 19.53

Index 0 1 2 3 4 5 6 7 (22,23,23) (47,59,56) (112,127,120) 26.04Fig. 1 Superframe preamble structure.

2.2 TDM Pilots 2 and 3The UMB preamble is always transmitted on a TDM Pilot 2 is a time domain Walsh sequence that carries

bandwidth less than or equal to 5 MHz even when the the sector PilotPhase (synchronous mode) or PilotPNactual system bandwidth is larger than 5 MHz. That is (asynchronous mode). PilotPN is the 9-bit identifier of aNFFTPRE=min{NFFT,512}.This significantly simplifies the sector. PilotPhase is defined as (PilotPN+ISF) mod 512acquisition complexity since the access terminal may not where ISF is the superframe index. PilotPhase is used inhave the knowledge of the system FFT size, NFFT, during the synchronous mode as the seed to the scramblinginitial acquisition. sequence (PN sequence) such that each sector not only has

a unique scrambling sequence but also changes from2.1 TDM Pilot 1 superframe to superframe enabling processing gain acrossTDM Pilot 1 is transmitted on the OFDM symbol with superframe. PilotPN, instead of PilotPhase, is used forindex 5 in the preamble, spans the central subcarriers (at asynchronous mode, since two sectors with different timemost 480), and occupies every fourth subcarrier over this bases could have the same PilotPhase at the same time. Thespan resulting in 4 copies of the same waveform in time generator polynomial of the PN sequence is given bydomain. TDM Pilot 1 uses a frequency domain complexsequence that carries preamble FFT size and cyclic prefix g(D)= D20 +D9 +D+ D4 +duration information to modulate the subcarriers. Thesequence is The Walsh sequence length equals to the preamble FFT size,

NFFTPRE, with index equals to the 9 bit PilotPhase/PilotPN

P4k+ expj2nuk(k+l) ko -a < k < k2-N P8P7P6P5P4P3P2P1P0 of value P between 0 and 51 .

2NG 4 4 In the case that the preamble FFT size is less than 9

where bits, the LSBs of the 9 bits are used as the Walsh sequenceAc0 max{16,NGUA~ LEFT NFFT/>2, 1index, i.e., PmodNFFTPRE for NFFTPRE < 512. The MSBsko =max l 6, NGUARD,LEFT, NFFT /2-240 I I

with value LP/NFFT,PRE] are carried by a complex PNk, = min {k,+ 4Nrp NFFT -NGUARD,RIGHT INFFT/2+2401 sequence used to scramble the Walsh sequence with theandda=16+max{0,NFFT / 2 - 2561. It can be shown that 20-bitseedgivenby 0ll0lOll0l0l0ll0llxx where thethe corresponding time domain waveform of each period is last two LSBs are reserved for the first two MSBs of the

nn2/ N{ I A k(k+l) 9-bit PilotPhase/PilotPN. Therefore, xi x0 0 for

p, = e ° C G ) NFFT,PRE 512, Xi 0, XO Ps for NFFT,PRE = 256, and2NG k=- 2NG xl P8, x0 P7 for NFFT,PRE = 128 TDM Pilot 2 sequence

which has a constant magnitude that helps improve peak to is further multiplied by a complex value exp(j271Ios0/3),average power ratio. where IOSI is the Other Sector Interference value for

The relationship between the sequence parameters OSICH. The resulting sequence is transformed to frequencyu, NG, NP, the cyclic prefix duration, and the preamble domain and used to modulate all subcarriers except theFFT size is specified in Table 2. Information on cyclic guard subcarriers.prefix duration and FFT size is necessary for detection of Like TDM Pilot 2, TDM Pilot 3 is also a time domainacquisition information from TDM Pilot 2. Walsh sequence that carries 9-bit acquisition information.

The Walsh sequence length equals to the preamble FFT size,NFFTPRE , with index equals to the 9 bit acquisition

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information a8a7a6a5a4a3a2a1a0 of value A between 0 2.3 PBCCH and SBCCHand 511 which contains information on The Primary Broadcast Channel is carried on the firstsynchronous/asynchronous mode, four LSBs of superframe OFDM symbol in the preamble. Each PBCCH packet isindex (LSB4 (ISF) if asynchronous mode), full/half duplex CRC appended, encoded, channel-interleaved, repeated,modes, etc, which is necessary for decoding the PBCCH scrambled, with the seed containing the sectorpacket. PilotPhase/PilotPN, i.e., h(128P+64+1) for synchronous

Similarly, in the case that the preamble FFT size is less mode and h (128P + 4LSB4 (ISF) + 1) for asynchronousthan 9 bits, the LSBs of the 9 bits are used as the Walshsequence index, i.e., AmodNFFT,PRE for NFFTPRE < 512 . mode, where h is a hash function, defined as

The MSBs with value LA/NFFT,PREI are carried by a h(x) BR((2654435761(xmod232))mod232))mod220complex PN sequence used to scramble the Walsh sequence \(\BR2(2654435761(lx/2032Amod232 mod220with the 20-bit seed given by01 10100 1P8P7P6P5P4P3P2P1P0XIXOx where P8, ..., PO are where BR stands for the bit-reversal operation. The

scrambled data are QPSKi modulated onto usablethe sector PilotPhase/PilotPN bits, the last two LSBs are scaarersoveaon suerfamebutrepated tran iereserved for the first two MSBs of the 9-bit acquisition over16 superfram e bCC pacet tansmte

over 16 superframes. The PBCCH packet contains theinformation. Therefore, x1ix0 0 for NFFTPPR 52 44-bit system information including superframe index, andxI 0, x0 a8 for NFFTPRE = 256, and xj=a8, x0 a7 for deployment-wide static parameters like total number of

N = 128. subcarriers, number of guard subcarriers (in units of 16),FFT,PRE etc., and is updated very 16 superframes. The static nature

The time sequence is further multiplied by a complex of the PBCCH packet allows the transmission of thevalue exp (j4nIost /3), and converted to the frequency PBCCH packet with low effective coding rate without highdomain and used to modulate the subcarriers if the overhead. This is done by updating the PBCCH packetsubcarrier is not a guard subcarrier. every 16 superframes and repeatedly transmitting the same

For system FFT sizes of 128, 256 and 512, TDM PBCCH packet over 16 superframes.Pilots 2 and 3 occupy all usable subcarriers. For system If Preamble Frequency Reuse is disabled, the ithFFT sizes of 1024 and 2048, TDM Pilots 2 and 3 only PBCCH modulation symbol is mapped to the subcarrieroccupies the central 512 subcarriers. with index NFFT/2- NFFT PRE/2 + i if this subcarrier is not

Like TDM Pilot 1, TDM Pilots 2 and 3 have constant a guard subcarrier where 0 < i < NFFT,PRE - 1. That is, themagnitude in time domain. However, if the number of gusable subcarriers is less than the preamble FFT size, the ith modulation symbol is punctured if the subcarrier is aconstant modulus property is distorted and the correlation guard subcarrier If the Preamble Frequency Reuse is(cross/auto) properties of Walsh/PN sequences are also enabled, which is only valid in synchronous mode, theimpaired as a result of the insertion of guard subcarriers. PBCCH packet is modulated only on a subset of subcarriers.Fig. 2 illustrates this effect. Different sectors use different sets of subcarriers determined

by REUSE =PilotPhase mod 8 that changes from1.0 superframe to superframe. The ith modulation symbol is

25% mapped to the subcarrier with index

0.8 _ tL 50% t/ NFFT /2 NFFT,PRE/2+ IREUSE NFFT PRE/8 + i if not a guard75%

subcarrier, where 0 < i < NFFT,PRE /8 - 1.0.6- The Secondary Broadcast Channel (SBCCH) is carried

on the OFDM symbols with indices 1 through 4 in thesuperframe preamble in superframes with an odd value of

0.4 index. A SBCCH packet contains the channel information,cross auto such as number of effective antennas, common pilot

0.2 / /channel hopping mode, number of sub-trees for SDMA, etc.0.2 l lIt is appended with CRC, encoded, channel-interleaved,

repeated, scrambled, with the seed containing sector0.0 0 0 0 0.8 PilotPhase/PilotPN, QPSK modulated onto usable

Correlation06 Magnitude 1.2 1.4 subcarriers. The seed used for scrambling equals toh (216 H(S) + 27P + 64 + 2) for synchronous mode and

Fig. 2 Change of CDF of Walsh sequence cross/auto h('H(S) 2 P+4L5B4 )I)+2) for asynchronouscorrelation as a result of bandwidth reduction. The figure h (26 +YSF /shows no reduction, 250%, 500% and 750% reduction in mode, where H(S) is a 20-bit hash quantity based on thebandwidth. 44-bit system information value S in PBCCH:

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1. Initialize H with zero; Compute n and m 3.1 TDM Pilot 1 Detectionsuch that 32m - n equals to the number of bits of The UMB system acquisition starts from searching forS; Set J to n zeros followed by S; And set TDM Pilot 1. At a given carrier frequency, the accessi = 0 . terminal looks for TDM Pilot 1 signal for each of the 12

2. While i < m, repeat Step 3. hypotheses given in Table 2 over the duration of at least one3. H=H G h(J(32i:32i+31)); i=i+l, where superframe until one candidate is detected. During the

search, one period of the waveformJ (32i: 32i + 31) stands for bits 32i to 32i + 31 {1c,0 n p/ris or th e eeved

{P 0 < n < NFFT,PR-E /4 - 11 iS correlated with the receivedof J.

ln

Like the PBCCH, the mapping between the signal r sampled at the corresponding bandwidth given bymodulation symbol and the subcarrier depends on the the FFT size of the hyothesis to obtain the metricPreamble Frequency Reuse mode. If enabled, the ith NFFTPRE/4-1modulation symbol is mapped to the subcarrier with index Un = E r _pnn=O

NFFT/2-N 2+imodN of the OFDM symbol The correlation can be done more efficiently in frequencywith index Li/NFFT,PRE 1+1 if this subcarrier is a usable domain using the FFT. Only a conservative fraction of the

subcarriers are retained and the rest of subcarriers (withsubcarrier. If disable, the ith modulation symbol is mapped granularity of 16) are treated as guard carriers and zeroedto NFFT/2 NFFT,PRE + 'REUSE NFFT,PRE + i mod NFFT,PRE8 ofgrnlitof1)aeraedsgudcrirsndzodto NFFT /2-NFFT PRE /2 + IREUSE NFFT PRE /8 + i mOdN PRE /8 ° out. This spectrum-shaping removes not only

the OFDM symbol with index Li/NFFT,PRE /8] +1. noise/interference but may also remove signal energy since

The SBCCH packet is updated very superframe. the actual bandwidth is unknown.The correlations from the four periods may be

OFDM symbols 1 through 4 in the preamble are used c be cohrentlyotofom the detection metricfor carrying Quick Paging Channel on superframes with 3 2even index. dn L Un-(NFFT,PRE / 4)k

3. System Acquisition or combined coherently after the frequency offset isThe goal of the system acquisition is to acquire the system removed to reduce the combining loss to form the detectionparameters, necessary to access the system, from the metric:preamble. The acquisition procedure is depicted in 3 2Fig. 3. d = Un(NFFT,PRE/4)k 1n

Stai where )n = Un-(kh )NFFT,PRE/4Un kNFFT,PRE /4

Upon the detection of TDM Pilot 1, the accessterminal gains the knowledge of the TDM Pilot 1 boundary,

Detect TDM Pilot 1 the cyclic prefix duration and the preamble FFT size.

3.2 TDM Pilot 2 DetectionDetect TDM Pilot 2DetectTDMPlot 2

With the knowledge of the TDM Pilot 1 boundary, cyclicprefix (therefore, the TDM Pilot 2 boundary), and thepreamble FFT size, TDM Pilot 2 can be located and

DetectTDM Pilot 3 sampled at the bandwidth based on the preamble FFT size.The sampled data are first transformed to frequency domainvia FFT with the obtained preamble FFT size. As with the

Decode PBCCH TDM Pilot 1, the frequency domain data arespectrum-shaped. The resulting data are then transformed

~<No back to time domain sequence, descrambled, and a fastDcode Succ? Hadamard transform (FHT) is used on the descrambled data

es to detect the Walsh sequence for the PilotPhase/PilotPN.For a preamble FFT size of less than 512, multiple PN

Decode SBCCH descrambling sequences need to be tested to retrieve theMSB(s) of the PilotPhase/PilotPN.

T_Upon the detection of TDM Pilot 2, the access( End )terminal obtains the PilotPhase/PilotPN of the sector.

3.3 1DM Pilot 3 DetectionFig. 3 Flowchart ofsystem acquisition procedure. TDM Pilot 3 is next sampled at the corresponding

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bandwidth, spectrum-shaped and descrambled using the equals to 15. If the current received PBCCH is not the lastPilotPhase/PilotPN detected from TDM Pilot 2. The FHT is of the 16 transmissions, the LLR from the successivethen applied to the descrambled data to detect the transmission of the PBCCH are combined with the LLRacquisition information. Multiple descrambling sequences stored in the LLR buffer and another decoding attempt ismay be tested for detecting the MSB(s) of the acquisition made. Otherwise, the buffer is cleared and LLR data are notinformation if the preamble FFT size is less than 512. combined. This procedure is repeated until a successful

Upon the detection of TDM Pilot 3, the acquisition decoding. The maximum number of transmissions theinformation including synchronous/asynchronous mode, 4 access terminal can combine is 16 since the PBCCH packetLSBs of the superframe index (LSB4 (ISF) if synchronous is updated very 16 superframes. Fig. 5 illustrates the

£ ~~~~incremental redundancy decoding process.mode), full/half duplex modes, preamble frequency reuse,etc, is available to the access terminal.

Fig. 1 shows the detection performance of TDM Pilots a

1,2, and 3.

16 Clear LLR Buffer and Set n=O Yes

PedB, 3km/h-14H -_VehA, 120km/h = a

U)E2'12-

Z~~~~~~~~~~~~~~~~~~~~~~~~~~~~oae anIa pl B C

b i)100

E

6a)

0

2

-14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0

Geometry (dB) Dcd

Fig. 4 TDM Pilots 1, 2, 3 detection performance (95Yepercentile, joint false alarm probability--O.OO01, 5MHzbandwidth). ucs?n<1

Yes

3.4 PBCCH Decoding NoAfter the detection of the TDM Pilot 3, the access terminalis ready to decode the PBCCH packet. With the knowledge E

of the cyclic prefix length and the full/half duplex mode, the Fig. 5 Illustration of incremental redundancy decoding of aaccess terminal is able to locate the PBCCH OFDM symbol PBCCH packet.in the following superframe preamble at

(Ts+Tcp +Tw +Tg)NsNpy where Tw =Ts/32 =3.26 Fig. 6 shows the incremental redundancy decoding

psec is the windowing guard interval, samples at the performance of a PBCCH packet. It is clear that decodingbandwidth determined by the preamble FFT size NFFTPRE of PBCCH rarely takes all 16 transmissions. High geometry

and F tusers are more likely to need less redundancy for lessand performs FFT with the preamble FFT size NFFTPRE processing gain to decode the packet as compared to edgeWith the information of Preamble Frequency reuse, the users. It, therefore, takes less time for high geometry usersfrequency domain data are spectrum-shaped, demodulated, to acquire the system significantly reducing the acquisitiondescrambled with the seed, h (128P + 64 + 1) if time.

A decoding failure may also be the consequence of asynchronous or h (I28P + 4LSB4 (ISF) + 1) if false detection of TDM Pilots 1 to 3. If decoding fails even

asynchronous, de-interleaved, LLR calculated and decoded. if the LLR buffer has combined 16 consecutive PBCCHA failure to decode is most likely due to insufficient SINR. transmissions, the acquisition procedure restarts.Therefore, if the decoding is not successful, the access Upon a successful decoding ofthe PBCCH packet, theterminal determines if the PBCCH carries the last access terminal obtains the system information includingtransmission of the 16 transmissions by checking if the superframe index, system FFT size and number of guardPilotPhase mod 16 equals to 15 (synchronous mode) or if subcarriers, etc. This information is necessary for decodingthe four LSBs of the superframe Index (asynchronous mode) the following SBCCH packet.

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0.1 ___ \ \

0.01-15 -13 -11 -9 -7 -5 -3 -1 1

Geometry (dB)

Fig. 6 PBCCH decoding performance at various levels ofredundancies (channel model: PEDB 3km/h).

3.5 SBCCH DecodingIf the current superframe index is odd, the access terminalstarts to acquire SBCCH. Four OFDM symbols from 1 to 4are sampled and transformed to frequency domain using anFFT. Using the number of guard subcarrier informationfrom the PBCCH as well as the Preamble Frequency Reusemode, the actual guard subcarriers are zeroed out, themodulation symbols are demodulated, descrambled,de-interleaved and decoded. The seed to the descramblingsequence is generated using the procedure described inSection 2.3.

By now the access terminal has all the informationnecessary to access the system and completes the systemacquisition.

4. ConclusionsThe UMB system allows flexible configurations to meetdifferent deployment needs. This flexibility also makes thedesign of UMB preamble challenging as compared toconventional systems. The UMB preamble design meets therequirements and ensures the initial system acquisition foran access terminal is efficient, i.e., low overhead, lowlatency, and low complexity.

1-4244-1074-6/07/$25.OO ©)2007 IEEE 333 Proceedings of IWSDA'07