`
`Fast Acquisition Scheme and Implementation of PRACH in WCDMA System
`
`Ling Qiu, Yongjia Huang, Jinkang Zhu
`University of Science and Technology of China, Box 4. Hefei. Anhui, 230027. China
`
`Abstract: The performance and implementation of
`PRACH(physical random access channel) acquisition in
`WCDMA system is investigated in this paper. The analysis
`shows that
`the conventional methods are not satisfying.
`Thus we proposed the quasi-matched filter acquisition
`scheme of PRACH preamble which based on
`fast
`I-Iadamard transform. And we implement this method by
`
`hard ware in the practical WCDMA field try system. The
`simulation and test results show that the proposed scheme
`achieves
`the
`following
`performance:
`the
`detection
`
`probability with [Eb/NU =7db does not less than 95%, and
`the mean acquisition time is less than I.33ms.
`
`I.
`
`INTRODUCTION
`
`Pesudo—noise(PN) code acquisition is the first action
`of any direct-sequence code division multiple access(DS-
`CDMA)
`system. Acquisition
`refers
`to
`the
`coarse
`synchronization of the received PN sequence and the
`locally generated PN sequence within a fraction of the chip
`duration of
`the
`code
`sequence. Fast
`and effective
`
`in CDMA
`acquisition is one of the key techniques
`communication system. And the acquisition performance
`can even be a limitation of the capacity of a system
`(acquisition-based capacity)[l].
`
`Acquisition can be performed and classified in several
`ways[2-3]. One way to separate acquisition methods is
`based on correlation measurement principle, which is used,
`
`active or passive measurement or a combination of the
`two[4].
`In the passive method, a filter matched to the
`spreading code is used. But
`in the active method,
`the
`received signal
`is multiplied with the locally generated
`replica of the spreading code, and the result is integrated
`over some observation interval. The multiplication and
`
`integration is performed step-by-step for each code phases
`to be tested. Usually. serial‘complete parallel.store parallel
`and hybrid parallel search[5-7] approach can be used for
`the detection of the code phases to be tested. However,
`
`serial search with correlation integration method seldom
`used for fast acquisition.
`
`random access
`the physical
`In WCDMA systems.
`channel (PRACH) has one or more preambles and each
`
`preamble is only 4096 chips. The single acquisition time is
`limited in 5120chips. Thus, there is high requirement for
`
`fast acquisition of RACH preamble. At the same time, there
`are a total of l6 signatures of RACH preamble part. This
`
`further increases the difficulty ofthe acquisition scheme. In
`addition. there need two sets of identical acquisition device
`
`for [/0 branches. Thus, the acquisition efficiency must be
`
`compare to the match filter.
`
`Based on above discussion and aiming at to achieve
`good performance and reduce the hardware resource at the
`same time. we propose a novel acquisition scheme. The
`simulation and test results show that the proposed scheme
`can be easy implement
`in practical WCDMA field try
`system and achieve high performance.
`
`II. WCDMA PRACH DESCRIPTION
`
`In WCDMA system, the random-access transmission
`
`is based on a Slotted ALOHA approach with fast
`acquisition indication. There are 15 access slots per two
`physical frames and they are spaced 5120 chips apart. See
`The
`of
`the
`reference[8].
`structure
`random—access
`
`transmission is also shown in reference[8]. The random—
`access transmission consists of one or several preambles of
`length 4096 chips and a message of length 10 ms or 20 ms.
`Each preamble is of length 4096 chips and consists of 256
`repetitions of a signature of length 16 chips. There are
`maximum l6 available signatures. The baseband modulator
`illustrated
`as
`for RACH is
`figure
`1. The
`binary
`preamblea(k) is modulated to get
`the complex valued
`preamble b(k) .
`
`k
`
`b(k)=a(k)e
`
`43.
`
`Mia
`
`l.k =0’.1,2,...,4095
`
`(i)
`
`0—7803-7005-8/01/51000 © ZOOI IEEE
`
`1701
`
`Qiu, L., Huang, Y. and Zhu, 1., 2001, October. Fast acquisition scheme and implementation of PRACH in WCDMA system. IEEE 54th Vehicular Technology
`(cid:50)(cid:74)(cid:86)(cid:13)(cid:1)(cid:45)(cid:15)(cid:13)(cid:1)(cid:41)(cid:86)(cid:66)(cid:79)(cid:72)(cid:13)(cid:1)(cid:58)(cid:15)(cid:1)(cid:66)(cid:79)(cid:69)(cid:1)(cid:59)(cid:73)(cid:86)(cid:13)(cid:1)(cid:43)(cid:15)(cid:13)(cid:1)(cid:19)(cid:17)(cid:17)(cid:18)(cid:13)(cid:1)(cid:48)(cid:68)(cid:85)(cid:80)(cid:67)(cid:70)(cid:83)(cid:15)(cid:1)(cid:39)(cid:66)(cid:84)(cid:85)(cid:1)(cid:66)(cid:68)(cid:82)(cid:86)(cid:74)(cid:84)(cid:74)(cid:85)(cid:74)(cid:80)(cid:79)(cid:1)(cid:84)(cid:68)(cid:73)(cid:70)(cid:78)(cid:70)(cid:1)(cid:66)(cid:79)(cid:69)(cid:1)(cid:74)(cid:78)(cid:81)(cid:77)(cid:70)(cid:78)(cid:70)(cid:79)(cid:85)(cid:66)(cid:85)(cid:74)(cid:80)(cid:79)(cid:1)(cid:80)(cid:71)(cid:1)(cid:49)(cid:51)(cid:34)(cid:36)(cid:41)(cid:1)(cid:74)(cid:79)(cid:1)(cid:56)(cid:36)(cid:37)(cid:46)(cid:34)(cid:1)(cid:84)(cid:90)(cid:84)(cid:85)(cid:70)(cid:78)(cid:15)(cid:1)(cid:42)(cid:38)(cid:38)(cid:38)(cid:1)(cid:22)(cid:21)(cid:85)(cid:73)(cid:1)(cid:55)(cid:70)(cid:73)(cid:74)(cid:68)(cid:86)(cid:77)(cid:66)(cid:83)(cid:1)(cid:53)(cid:70)(cid:68)(cid:73)(cid:79)(cid:80)(cid:77)(cid:80)(cid:72)(cid:90)(cid:1)
`Conference. VTC Fall 2001. Proceedings (Cat. No. 01CH37211) (Vol. 3, pp. 1701-1705). IEEE.
`(cid:36)(cid:80)(cid:79)(cid:71)(cid:70)(cid:83)(cid:70)(cid:79)(cid:68)(cid:70)(cid:15)(cid:1)(cid:55)(cid:53)(cid:36)(cid:1)(cid:39)(cid:66)(cid:77)(cid:77)(cid:1)(cid:19)(cid:17)(cid:17)(cid:18)(cid:15)(cid:1)(cid:49)(cid:83)(cid:80)(cid:68)(cid:70)(cid:70)(cid:69)(cid:74)(cid:79)(cid:72)(cid:84)(cid:1)(cid:9)(cid:36)(cid:66)(cid:85)(cid:15)(cid:1)(cid:47)(cid:80)(cid:15)(cid:1)(cid:17)(cid:18)(cid:36)(cid:41)(cid:20)(cid:24)(cid:19)(cid:18)(cid:18)(cid:10)(cid:1)(cid:9)(cid:55)(cid:80)(cid:77)(cid:15)(cid:1)(cid:20)(cid:13)(cid:1)(cid:81)(cid:81)(cid:15)(cid:1)(cid:18)(cid:24)(cid:17)(cid:18)(cid:14)(cid:18)(cid:24)(cid:17)(cid:22)(cid:10)(cid:15)(cid:1)(cid:42)(cid:38)(cid:38)(cid:38)(cid:15)
`
`1
`
`SAMSUNG 1015
`
`1
`
`SAMSUNG 1015
`
`
`
`
`
`'—
`16chlps
`Hm: The set ot‘length 16 Walsh Hadamard code
`
`Scrambling code
`'—
`4096chips
`
`+ a(k) X
`
`1 +
`,
`2
`
`X Mk)
`f
`|../.-L—./.1....
`
`Fig.1 Structure of preamble & scrambling code and baseband modulator for RACH preamble
`
`Ill. Quasi-Matched Filter Scheme Based on Fast
`Hadamard Transform
`
`At the bsae station, the code phase of the preamble is
`retarded by a roundtrip delay compared with that of the
`
`pilot signal[8]. Hera we assume that the code phase which
`need to be tested equals 400(200chips). We also assume
`that the correlation period is 1024 chips. In this case, the
`serial search method is not suitable distinctly. [f the hybrid
`
`parallel[7] acquisition approach is adopted here. To limit
`the acquisition time within 5120chips, we appoximately
`assume that the 400 phase to be tested is divided into 14
`
`it should finish the search of
`sub-phase intervals. Thus,
`400/30 e 14 unknown phase within each sub-phase interval.
`
`the total parallel
`16 signatures and NO branches,
`For
`correlators needed to detect
`the unknown phase are
`16x 2x30 =960. It is impractical for implementation of such
`match filters
`huge
`correlator
`arrays.
`If conventional
`method[9] is used for PRACH, there need two sets of 16
`
`match filters for [/0 branches of 16 signatures. As we know,
`a digital match filter actually is a multi-stage parallel
`accumulator. And the number of accumulators relate to
`
`correlation length. At the correlation period is 1024chips, it
`needs 1023 accumulators to implement such a match filter.
`
`is disastrous for system resource to implement
`It
`method (total 16x 2 x1023 = 32700 accumlators).
`
`this
`
`Based on the above discussion and the property of
`PRACH, we propose the
`following new acquisition
`detector structure.
`
`the preamble is
`From figure 1. we can find that
`generated by a signature consists of 256 repetitions of a
`
`length 16 signature and scrambling code. The signature is
`from the set of 16 Hadamard codes of length 16. For the
`same cell. the scrambling code of all mobile terminals used
`
`is the same. Thus. fast Hadamard transform (FHT) can be
`
`used to do the correlation of the preamble. The acquisition
`time when using FHT can be shortened. Furthermore, FHT
`can be implemented by original position operation and this
`
`cannot expend too much hardware resource. FHT is only
`change
`the modality of
`correlation
`operation,
`the
`conventional structure of the acquisition system can still be
`used.
`
`Here we propose the acquisition system of quasi-
`matched filter scheme based on the FHT.
`It
`is shown as
`
`figure 2. According to the figure 1, a non-coherent QPSK
`detector is used for for each F HT branch. The acquisition
`system consists of
`two sets
`of L(16) FHT which
`
`corresponding to the 16 signatures for each [/Q branches
`
`and a maximum selection unit and a threshold comparing
`
`part. If the maximum correlation value Z
`
`111 EX
`
`is larger than
`
`the threshold, then the corresponding phase is considered as
`correct phase and the system turns into tracking state.
`Otherwise.
`it goes back to the state of searching another
`unknown phase. The threshold setting principles we used
`here is Neyman-Pearson criterion. That is at the condition
`of constant
`false alarm rate(CFAR),
`to maximize the
`
`detection probability[5,9].
`
`1702
`
`2
`
`
`
`
`square
`“if "’I accumlatur —>
`
`code generator
`
`
`
`
`uare
`sq
`accumlaioi —'|
`—t
`‘ 7. y.
`W a)
`
`FHTon.
`accumlator —}
`square
`1*_.__
`
`
`1‘- Z I
`- ——+
` l branch scrambling
`Ping»
`
`code generator
`
`Pang
`
` register
`[I
`square —>’
`or
`FHT ->{ accumlator
`
`Maxunuin
`selection
`Q branch scliunliling
`
`
`
`
`
`
`
`
`
`
`
`going to
`racking
`
`I
`
`Search controller
`
`
`
`
`Fig. 2 a non—coherent QPSK acquisition system based on the FHT
`
`In figure 2, r(t) is the received baseband signal in base
`
`guarantee the required performance.
`
`51 is the threshold set for the search mode and 52
`station.
`is the threshold set for the verification mode.
`
`into
`In practical system, the scrambling code is put
`memory. The input data stream is first put into the register
`and the size of the register is chosen as two times as the
`correlation length. Thus,
`the ping-pang operation can be
`done. Every time l6chips data is read out and scrambling
`
`code is de-spreaded. Then the results will be fed into FHT
`and accumulator. Finally the corresponding results of [/0
`branches are squared and summed. The decision variable
`can be obtained.
`
`For one FHT. 4 accumlators are needed. To complte
`
`the detection of400 phase to be tested, the hybrid parallel[7]
`acquisition approach is also adopted here and the 400 phase
`to be tested is divided into 200 sub-phase intervals. Thus, it
`should finish the search of 400/200=2 unknown phase
`
`within each sub-phase interval, Thus, for this system. there
`
`IV.
`
`Performance analysis and simulation results
`
`Based on the above discription, we do the simulation on
`
`mean acquisition time and detection probability. The
`simulation parameters are listed in tablel, The correlation
`length is set as l024 chips. The penalty factor for the false
`alarm is selected as two, i.e.. the penalty time is 21' s. The
`threshold parameters
`(fl and 62
`are adjust
`to meet
`the
`detection probability and minimum mean acquisition time
`of
`the
`requirements. The
`another
`two
`parameters
`verification mode are quoted from [10]. In the conventional
`hybrid parallel method, the parallel branch is set as 100.
`And in each parallel branch. there are 4 phase to be tested.
`
`Table 1 parameters used for simulation
`
`3.84Mcps
`Threshold set method
`CFAR
`
` Chip rate
`
`4 ETSl vehicular A ll
`Multipath model
`
`Multipath fading_
`l Rayleigh
`
`are total
`
`(16x 4 +16)>< 2x 2 = 320 accumlators. This is much
`
`The simulation results are as follows.
`
`less than that used in MF method and conventional hybird
`
`parallel method. Hadamard sequence used here is only 16
`chips. the original position operation doesn‘t need. We can
`use streamline structure to do this operation and the
`acquisition time can be further shortened This streamline
`structure just is a match filter for the Hadamard sequence
`
`implementation. To
`In practical
`with length of lo chips.
`increase the process gain.
`the correlation length should
`greater than léchips. It can select as N times of lbchips and
`N is an integer. The desired correlation result
`is
`the
`accumulation of N consecutive output value of the match
`filter. Here we choose N as 64 and such selection can
`
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`
`1.0
`
`Fig.3
`
`performance of single dwell mode
`
`Fig.4 performance of double dwell mode
`
`We can see from figl that the detection probability(Pd)
`will
`increase and the false-alarm probability(Pt) decrease
`with the inceraseing of signal to noise ratio.
`
`1.0
`0.9
`0.8
`0.7
`0.6
`0.5
`E 0.4
`0.3
`()2
`[H
`0.0
`
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`
`lib/Numb}
`
`We can see from fig.3 and Fig.4 that when the double
`dwell search method is used, a relatively low threshold 5,
`can be used to increase the detection probability, and the
`
`verification mode operation can reduce the probability of a
`false-alarm event; therefore, the mean acquisition time can
`be greatly reduced.
`
`V. Hard ware implementation and performance results
`
`We use Alter FPGA 20K200EFC-3 to evaluate the
`
`hardware scale of the PRACH acquisition system. Total
`11740 logic cells and 142336ESB are used. The diagram
`for the FPGA implementation is shown in figure 5.
`
`ngt
`
`re“)
`
`,i 7,
`
`7 ‘w
`
`lbranch
`data register
`TVA
`‘ lR/w data address
`" '
`- read scrambling r—H—r
`nndaaddmslm scramble code
`generaier
`- Aii
`
`address generater ot‘
`read/write RAM
`A
`”Tm—
`:.\Z.
`.
`R/W data address
`Q branch data
`register
`
`
`
`_____~___
`
` #1:“: J FHT +2 -h()—l
`
`Littlest-um
`""~> compare '—>
`"7 7 decision
`output
`
`»
`
`.
`
`1
`.
`.7 _
`(
`t
`
`' Carmela]
`‘ complex scramble
`7..
`code despremd
`’-i_f‘ " O V
`.
`(muflwmw
`L» FHT -> Z L—>I
`- r~— ‘
`
`Fig.5 the hardware structure of PRACH acquisition scheme
`
`1704
`
`4
`
`
`
`
`
`One of the branches of the process of PRACH
`acquisition
`is
`described
`above.
`It’s
`hardware
`implementation structure mainly contains the following
`modules.
`
`l. DATA_REGISTER
`This module receives signal from A/D, and put the data
`
`into register. The size of the register is two times as
`correlation length. This unit
`is the main limitation for
`FPGA resource.
`
`2. ADDRESS_GENERATOR
`This module generates the R/W(read/write) address for
`both I and Q branches data registers. It also generates the
`read address of scrambling code generator.
`3. SCRAMBING _CODE GENERATER
`This module generates the scramble code that is used for
`current cell according to initial state that CPU informs.
`4. COMPLEX SCRAMBLE _CODE DESPRADING
`The complex scramble code is de-spread in this module.
`The scramble code multiples the input data step-by-step
`and the results are output.
`5. FHT
`
`from
`come
`results of HO branches
`The multiple
`COMPLEX
`SCRAMBING_CODE
`DESPRADING
`module are separate fed into FHT modules. 16 parallel
`
`Reference
`
`1. Mohamed G. El-Tarhuni. Asrar U. H. Sheikh, An
`
`Adaptive Filtering PN Code Acquisition Scheme with
`Improved Acquisition Based Capacity in DS/CDMA.
`proc. ofPlMRC’ 98. ppl486-l490, 1998
`
`Polydoros, On the Synchronization Aspects of Direct-
`Spectrum
`Systems.
`Sequence
`Spread
`Ph.D
`Dissertation. University of Southern California. Los
`Angeles. CA, USA,. p240. 1982
`l. Iinatti. Matched Filter Code Acquisition Employing
`21 Median Filter in Direct Sequence Spread-Spectrum
`Systems With Jamming. Acta Universitatis Ouluensis
`Cl02. Doctoral Thesis. University of Oulu. Oulu,
`
`Finland, pp54-65. 1997
`.l.
`linati, DS Code Acquisition in Slowly Fading
`Multi—path Channel. proc. of [BBB VTC’2000 Fall,
`Boston, Ma USA. pp2408-2413, Sept. 24-28, 2000
`Andrew J. Viterbi, CDMA Principles of Spread
`
`2.
`
`Lt)
`
`4.
`
`5.
`
`four
`chips are needed for FHT. Each FHT unit has
`accumulators with the bit width 10, ll.l2 and 13. The
`
`streamline operation is used here.
`6. ACCUMULATOR
`
`To obtain the high process gain, we accumulate the FHT
`output 32 time in ACCUMULATOR module.
`7. SQUARE
`
`Then accumulator values from l/Q branches are summed
`in this module.
`8. COMPARE
`
`The last module is COMPARE. The correlation results
`
`of 16 signature are compared with the threshold that is set
`by higher layer successively. And the decision variable is
`obtained finally.
`
`V1. Conclusion
`
`investigated the PRACH
`this paper, we have
`In
`acquisition of WCDMA. Considering both acquisition
`performance and hardware scale, we proposed the novel
`fast acquisition scheme that based on FHT. The simulation
`
`ant test results are that the correct detection probability
`
`with Eh/N0 :7dB doesn’t
`
`less than 95% and the single
`
`acquisition time is less than 51200hips.
`
`6.
`
`Spectrum Communication. Louay. 1995
`R.R.Rick and L.B.Milstein. Parallel Acquisition in
`Mobile
`DS-CDMA
`Systems.
`IEEE
`Trans.
`Communications, Vol.45, pp1466-1476. Nov. 1997
`7. W. Zhuang, Non-coherent Hybrid Parallel PN Code
`Acquisition for CDMA Mobile Communications.
`
`lEEE Trans. Vehicular Technology, Vol.45, pp643-
`656. Nov. I996
`
`8.
`
`3G TS 25.2 I l, v3.2.0 (2000-03): Physical channels
`
`and mapping of transport channels onto physical
`channels (FDD)
`
`9.
`
`J. linati, On the Threshold Setting Principles in Code
`Acquisition of DS-SS Signals.
`lEEE JSAC, Vol.18.
`pp62-72. January 2000
`
`10. Polydoros and C.L.Weber. A Uniforied Approach to
`Serial Search Spread-Spectrum Code Acquisition-Part
`l &11.
`113131313 Trans. Communication, Vol.COM-32,
`
`pp542-561. May 1984
`Il. ETSl TR l0]. 102. UMTS
`
`1705
`
`5
`
`