3GPPR1-050386-ViewsonOFDMParameterSet.doc
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1、3GPP TSG RAN WG1 Meeting #41 R1-050386Athens, Greece, 9 - 13 May, 2005Source: NTT DoCoMo, Ericsson, Fujitsu, Mitsubishi Electric, NEC, Nortel,Panasonic, Texas InstrumentsTitle:Views on OFDM Parameter Set for Evolved UTRA DownlinkAgenda Item:13.2Document for:Discussion and Decision1. IntroductionAt t
2、he RAN1#40-bis meeting (Beijing), most companies proposed using OFDM-based radio access in the downlink 1-17. This contribution presents our views on the requirements for deciding the OFDM parameter set and the design of OFDM parameter set in the OFDM-based downlink radio access for Evolved UTRA.2.
3、Requirements for OFDM Parameter Set2.1. Backward compatibilityConsidering the simultaneous use of UTRA and Evolved UTRA (i.e., dual-mode usage) and backward compatibility, the radio-frame length of Evolved UTRA should be identical to that of UTRA, i.e., a 10-msec radio frame.2.2. Scalable multiple t
4、ransmission bandwidthAt the RAN Long Term Evolution (LTE) meeting in March 2005 18, multiple transmission bandwidths from 1.25 MHz to 20 MHz were adopted. The same sub-carrier spacing, i.e., the same useful OFDM symbol duration, is desirable for efficiently supporting a multiple transmission bandwid
5、th. Therefore, the number of sub-carriers is changed according to the transmission bandwidth.2.3. RAN latencyIn the requirements for Evolved UTRA and UTRAN agreed to at the RAN LTE meeting in March 2005 18, the requirement for the RAN latency (RAN round trip time (RTT) was decided to be within 10 ms
6、ec. This RAN latency requirement affects the transmission timing interval (TTI) length. The RAN latency is categorized into the air-interface delay, media delay, and delay in the RNC (or corresponding node above base station (BS), such as the Advanced Access Router (AAR) as shown in Fig. 1. Furtherm
7、ore, the air-interface delay in the RAN is classified into the transmitter processing delay, retransmission delay, and receiver processing delay. Here, we assume the delay figures for these three delays as shown in Fig. 2. Figure 1 Categorization of RAN LatencyFigure 2 Air interface delayAssuming th
8、at the transmission delay of an optical fiber is 5 usec/km 19, the media delay, when the distance between the RNC (AAR) and BS is 50 km, becomes 0.25 msec. Moreover, we assume that the delay in the RNC (AAR) is approximately 0.2 msec. Thus, the overall RAN latency can be calculated as 2 x (6.5 TTI +
9、 0.5) msec. (1)Thus, the total RAN latency depends on the TTI length, and the TTI lengths should be designed so that the requirement of the RAN latency of less than 10 msec is satisfied.2.4. High data rateAt the RAN1#40-bis meeting, many companies proposed the use of a Cyclic Prefix (CP) for OFDM-ba
10、sed downlink transmission 1, 2, 4, although other potential techniques such as offset QAM using the Isotropic Orthogonal Transform Algorithm (IOTA) filter 20 were also proposed. In this paper, we assume the use of the CP in the design of the OFDM parameter set. In the case of OFDM-based radio access
11、, it is clear that the achievable data rate is partly dependent on the CP overhead ratio. This means that, according to the increase in the CP overhead ratio, the achievable data rate is reduced. Therefore, naturally, a small CP overhead loss is necessary to improve the achievable data rate. 2.5. In
12、fluence of Doppler effect and phase noiseu Influence of Doppler effectIn the case of OFDM-based radio access, the sub-carrier spacing is designed to be narrower than the channel coherence bandwidth so that the fading of each sub-carrier becomes approximately flat, i.e., frequency-non-selective. Mean
13、while, in Evolved UTRA and UTRAN, the maximum user equipment (UE) speed supported should be approximately 350 km/h implying a maximum Doppler spread of approximately 650 Hz and 840 Hz at a 2 GHz and 2.6 GHz carrier frequency, respectively. To mitigate the influence of the Doppler effect, a sub-carri
14、er spacing at least in the range of 10-20 times the maximum Doppler frequency is necessary. In addition to frequency fluctuation due to the Doppler effect, frequency drift due to the frequency difference in the oscillators between the BS and UE occurs. However, we do not consider the influence of fr
15、equency drift because the frequency drift becomes small as the reference oscillator in the UE tracks the BS carrier frequency. u Influence of phase noisePhase noise is caused by random fluctuations in the frequency of the local-oscillators of the BS and UE. The phase noise has two different kinds of
16、 effects: common phase error and inter-sub-carrier interference. Since the influence of the inter-sub-carrier interference is larger than the common phase error and the common phase error can be compensated by means of pilot-aided channel estimation, we only have to take into account the influence o
17、f the inter-sub-carrier interference. As a result, sub-carrier spacing should be sufficiently wide so that the influence of inter-sub-carrier interference is small.2.6. Wide-area coverage supportWide-area coverage is one of the most important requirements for Evolved UTRA 18. We roughly categorize s
18、upport environments as shown in Fig. 3. In an urban area, a large amount of traffic is gathered in a relatively small site-to-site distance area of less than a few kilometers. However, we do not think that the OFDM parameter set should focus on local areas such as hotspots, very-small cells, and ind
19、oors with a small cell size. The OFDM parameter set must be optimized considering wide-area coverage support.Figure 3 Categorization of environment supported by Evolved UTRA2.7. High rate data provision of Multicast/Broadcast (MBMS)As discussed in many contributions at the RAN1#40-bis meeting, OFDM
20、radio access has a beneficial feature in that soft combining of incoming signals from multiple cell sites is easily achieved, improving the received signal-to-interference plus noise power ratio (SINR) particularly at the cell boundary 1, 2, 4, 6. Using such soft combining, high-data-rate Multicast/
21、Broadcast services can be provided with wide-area coverage. To provide high-data-rate Multicast/Broadcast services, a longer CP duration is necessary for accommodating paths with long time delays from far cell sites and for compensating for inaccuracies in the BS timings. 2.8. Fewer optionsA small C
22、P length is desirable for efficient transmission, e.g., to achieve high data rates and high spectral efficiency. On the other hand, a long CP length is necessary for supporting large time dispersion and broadcast with soft combining in large cells. Thus, to achieve an efficient CP overhead ratio acc
23、ording to cell environments and service types (e.g., both Unicast and Multicast/Broadcast), multiple parameters are needed. In other words, it is very difficult to provide a single optimum OFDM parameter set to support very-small to very-wide area coverage up to several tens of kilometers and both U
24、nicast and Multicast/Broadcast services. However, a small number of OFDM parameter sets is desirable for simplifying the implementation and testing equipment. Thus, we present the need for two OFDM parameter sets with different CP lengths.* Basic short CP length is for typical Unicast environments*
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