Further Results about RAKE Receiver for
TD-SCDMA Mobile Terminals*
2
Yang Xiao1, Moon-ho Lee
1Institute of Information Science, Beijing Jiaotong University
Beijing 100044, China
E-mail: yxiao@center.njtu.edu
2
Division of Electronics and Information Engineering
College of Engineering, Chonbuk National University
小米路由器hdJeonju 561-756, Korea
E-mail: moonho@chonbuk.ac.kr
Abstract
Abstract—For the sake of the potential ability of overcoming interference in TD-SCDMA(time division-synchronous code division multiple access) systems, pilot signal is adopted, but the prented TD-SCDMA protocol has not considered the Rake technique for their mobile terminals. This paper developed a RAKE receiver algorithm and an implementation circuit, which make u of the pilot signal in the burst structure of the TD-SCDMA ba station to estimate main channel parameter (channel delays) in the downlink of TD-SCDMA wireless network. The algorithm can reduce multipath interference for the mobile units in multiurs’ ca. Theoretic performance analysis prented in the paper and computer simulations confirm that the propod RAKE receiver algorithm achieved a better BER performance under multipath fading propagation and multiurs conditions.许美静
Keywords: W ireless communication, TD-SCDMA, RAKE receiver schemes, channel parameters estimation
1 Introduction
TD-SCDMA standard ems to be a promising approach for implementing cellular communication rvices [1, 2]. Though D-SCDMA has absorbed many new techniques recent propod, it did not consider Pre-Rake and the Rake
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* This project is supported by, Brain Pool Program of Korea under grant: 051S-3-5, MIC (Ministry of Inform. & Comm. Korea, under the ITRC(Information Technology Rearch Center) by the IITA, and the National Natural Science Foundation of China under grant: 60572093.
techniques for its ba stations and mobile stations [3-5] for its implementation of future wireless rvices. One ofthe greatest impairments to mobile radio is the fading nature of the channel [3-10]. When transmitting over a frequency-lective fading channel, the interference rejection property of direct-quence (DS) dictates that any multipath component which is at least one chip out of synchronization with the despreading pudonoi (PN) quence will be en as approximately equivalent to white noi, thus decreasing the multipath component’s ability to degrade system performance. The ability of DS to resolve individual components from the overall multipath signal allows for the reception of many components if a RAKE receiver is ud and improved performance through diversity combining [3-10].
Though the method of channel estimation for mobile terminals to employ the quence of pilot symbols has been involved [6-10], the implementation complexity of Rake receivers and the burst structure of TD-SCDMA have not been considered [5]. To solve the above problems and improve the communication quality of the downlinks of TD-SCDMA, this paper provides an algorithm and a Rake circuit structure, which are of channel parameter estimation in a RAKE receiver bad on TD-SCDMA burst structure. Theory analysis and numerical simulation results are ud to demonstrate the performance of the propod RAKE receiver in terms of bit error probability under multipath fading propagation conditions, even compared with the SIR Rake receivers [11,12].
2 The burst structure of TD-SCDMA
All physical channels TD-SCDMA systems take four-layer structure of superframes, radio frames, subframes and time slots/codes [2]. Depending on the resource allocation, the configuration of subframes or time slots becomes different. All physical channels need guard symbols in every time slot. The time slots/codes are ud in the n of a TDMA component to parate different ur signals in the time and the code domain. The physical channel signal format is prented in Figure 1. The basic physical channel is defined as the association of one code, one time slot and one frequency. The radio frame has a duration of 10 ms and is subdivided into 2 subframes of 5ms each,
and each subframe shown in Fig. 2 is then subdivided into 7 main time slots (TS) of 675 µs duration each and 3 special time slots: DwPTS (downlink pilot) shown in Fig. 3, G (guard period) and UpPTS (uplink pilot). The physical contents of the time slots are the bursts of corresponding length as described in Figure 4.
The DwPTS(SYNC-Synchronization Chips) in each subframe is designed for both downlink pilot and SCH. The ba station would transmit it omnidirectionally or ctorially at the full power level. This DwPTS time slot is usually compod of 64 chips of SYNC and 32 chips of guard period as shown in Fig. 3. The contents in the SYNC are a t of Gold code. The Gold code t is designed to distinguish nearby cells for the purpo of easier cell measurement. The t of code could be repeated in the cellular network. The propod mobile Rake can resort to DwPTS to determine the fingers’ positions. Each time slot of length 675 us consists of two data symbol fields, a midamble of 144 chips and a guard period of 16 chips. The data fields of the burst type are 704chips long. The training quences, i.e. midamble, of different urs active in the same time slot are time-shifted versions of one single periodic basic code. Different cells u different periodic basic codes, i.e. different midamble ts. In this way joint channel estimation for the channel impul respons of all active urs within one time slot can be done by one single cyclic correlation. Thus, the different ur specific channel impul respon estimations can be also obtained quentially in time at propod
mobile Rake. The midamble transmitting power of TD-SCDMA ba stations is the same as the data symbols in the same burst.
Now, we can e that TD-SCDMA mobile Rakes can have two ways to get channel information: DwPTS chips and midamble chips, and can u either of them as training quences to get fingers. In the paper, we mainly study the rake tracing the midamble chips, and in the following ction, we define the data symbol fields as data slot, and midamble field as pilot slot, e Eq. (2) in the following ction.
Figure 1.
Physical channel structure of TD-SCDMA
(96chips)
Gp (96chips) (160chips)Subframe 5ms 1.28Mchip/s where n+m+2=7
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Figure 2.
Sub-frame structure of TD-SCDMA
Figure 3. Burst structure of DwPTS
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Figure 4. Burst structure of the time slot (GP denotes the guard period and CP the chip period)
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Digital Detection Algorithm
小儿智力糖浆For the sake of simplicity, the TD-SCDMA system under consideration is assumed as formed by a single isolated circular cell of radius R with a centrally located ba-station. In particular, the focus here is on downlink communications. It is assumed that the ba station communicates with the K spatially disperd mobile urs by a TD-SCDMA scheme. All the K transmitted signals from the TD-SCDMA ba-station have experienced identical fading when received by a particular mobile ur. The urs are assumed to be uniformly distributed throughout the cell area.
A frequency-lective slowly fading transmission channel has been assumed as the Ref [3-10]
(1)
∑=−=L i k,i θk,i k τt αt h k,i 1j )(e
)(δwhere l k,αis attenuation introduced by the l -path; l k ,θ: pha shift introduced by the -path, defined in the [0,
2l π] interval;l k ,τ: time delay introduced by the -path for l L l ≤≤
1 ,with denoting the number of resolvable paths.
L Different from Ref [6], our TD-SCDMA mobile Rake receivers need not to estimate all the parameters {l k,α,l k ,θ,l k ,τ}, only need to l k ,τ estimate by means of a pilot signal at burst dada frame, which is broadcasted by TD-SCDMA ba station to all the mobile urs in the cell, since in TD-SCDMA burst structure pilot quence and data symbol quence are located in different time slots, which is shown in Fig. 4. We need not cancel the interference of pilot quence to data symbol quence like [6], so the complexity problem of the parameters {l k,α,l k ,θ,l k ,τ} can be solved.
We assumed that the received signal for the kth mobile ur is given by the sum of the output of a linear system having a randomly time-varying impul respon, and an AWGN term
with zero mean and two-sided power spectral density )(t r k )(t n k 20N . The multipath effects are reprented as a quence of replicas of the transmitted signal, each one characterized by a particular delay, pha and attenuation. We have assumed parameter L (i.e., the
number of resolvable paths in our channel model) and considered negligible the intersymbol interference introduced by the downlink channel, i.e., b i
k T 2.0,<τ for L i ≤≤1, with c b NT T = the bit duration.is the spreading chip duration.
c T QPSK modulation scheme is u
d in th
e transmission o
f TD-SCDMA communication system. The baband reprentation of the in-pha and quadrature components of the transmitted signal from the ba station to the kth mobile ur in its cell is
(2) ⎩
⎨⎧+=slot pilot ),(slot dada ),()(j )()()(,,t c t c t d t c t b t s E Q k k I k k k where , , ,
is in-pha data quence of kth mobile ur, with equal to ∑∞=−=0)
()()(n b k k nT t q n b t b ∑∞=−=0)()()(n b k k nT t q n d t d ⎩⎨⎧<≤=otherwi T t t q b 001)()}({n b k )(n b k 1±with equal probability; is quadrature data quence, with equal to )}({n d k )(n d k 1± with equal probability.
The following pudonoi (PN) code quences in Eq.(2) have following forms
∑−=−=10
,,)()(2)(N n c I k I k nT t p n c P t c (3a) ∑−=−=10,,)()(2)(M n c Q k Q k nT t p n c P t c (3b)
剪纸简单又漂亮where
- PN quence associated with the in-pha component, with equal to (binary chip); - PN quence associated with the quadrature component, with equal to (binary chip); M- th number of chips in the PN quence (or chips/bit); P denotes the power of the PN quence with respect to , and
)}({,n c I k )(,n c I k 1±)}({,n c Q k )(,n c Q k 1±)(,n c I k (4) ⎩
⎨⎧<≤=otherwi T t t p c 001)(Additionally, a PN signal (pilot signal) from a TD-SCDMA ba station is added to the downlink transmitted signal in order to perform the channel parameter estimation.
The pilot signal in (2) is defined as
∑−=−=10)()(2)(N n c E E nT t p n c
P t c (5)
where are the binary chips forming the PN quence, pilot signal power P is equal to that of PN quence.
)1)((±=n c E From Eq. (1) and Eq. (2), if K urs are active in a TD-SCDMA cell, it is straightforward to e that the received signal (baband) for the kth mobile is
⎪⎪⎩
⎪⎪⎨⎧+−−+−=∑∑∑===L i k θk,i E k,i k,i Q m m L i K m k,i I m m θk,i k t n τt c ατt c t d τt c t b αt r k,i k,i 1j ,11,j slot pilot ),(e )(slot data )],()(j )()([e )(1 (6) where is the number of fingers of Rake, and 1L L L <1. The digital detection considered here permits to track the channel variations by evaluating the sliding correlation of the received signal
with a locally generated pilot quence as
鲁滨逊历险记)(t r k )(t c E ∫+−=b b T n nT E k b k dt τt c t r T τc )1()()(1
)( (7)
for ],0[c lT ∈∀τ and 1,,0−=N l L . By substituting (6) into (7) and taking into account the assumption of slow fading (i.e., by considering constant the channel parameters over at least two bit intervals) we have
(8)
)()()(,,t c t c t c En k EE k k +=We have the contribution of the auto-correlation between
and every received replica of in the fingers replicas, where is the number of fingers of the Rake. When )(t c E )(t c E L L <11L i k τt ,= as following ∑=−=1,1,j ,,)]τ(e 2
α[)(L i i k θi
k EE k t z P t c i m (9) where is equal to 1 if , el to zero.
)
(x z 0=x When , becomes i τt ≠)(,t c EE k
