IEEE C802.16m-08/086
Project / IEEE 802.16 Broadband Wireless Access Working Group <Title / ULMultiple Access in view of MIMO Support
Date Submitted / 2008-01-16
Source(s) / SungHoPark, HanGyu Cho, Wookbong Lee
LG Electronic Inc. LG R&D Complex, 533 Hogye-1dong, Dongan-gu, Anyang, 431-749, Korea / Voice:+82-31-450-1931
E-mail:
Re: / Multiple Access
Abstract / This contribution shows that uplink MIMO (CSM) link performance of OFDMA is better than that of SC-FDMA. Especially, when the MLD is considered as a MIMO receiver, a significant gain is achievable in the OFDMA.
Purpose / For discussion of comparison between OFDMA and SC-FDMA in view of MIMO support, and approval of OFDMA as the UL multiple access scheme by IEEE 802.16 WG
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ULMultiple Access in view of MIMO Support
SungHoPark, HanGyu Cho, and WookBong Lee
LG Electronics
Abstract
The purpose of this contribution is to find out which scheme is better as the multiple access scheme for 802.16m in view of UL MIMO support. We compare two main candidates, OFDMA and SC-FDMA varying channel model, modulation order, and coding rates.
Simulation results show that for MMSE receiver, OFDMA has about 3.3~4.8 dB gain over SC-FDMA at 10% target BLER. Moreover, if ML receiver is adopted for OFDMA, the gain increases to about 5~6 dB.
Introduction
Multiple access schemes should be considered invarious standpoints such as capacity (throughput / cell coverage), efficiency (resource / power) and system complexity (scheduling overhead, transceiver complexity, etc).Especially, system capacity such as average cell-throughput is emphasized in the 802.16m to meet the high throughput requirement. Since MIMO technique is one of good options to increase the capacity, multiple access schemes should be considered in view of MIMO support – i.e. performance of MIMOtechniques, pilot patternandresource allocation, applicable MIMO receiver, channel estimation efficiency, scheduling overhead, etc.
In this contribution, therefore,we compare OFDMA and SC-FDMA based on link performance in uplink MIMO.Noting that the collaborative spatial multiplexing (CSM) is a baseline uplink MIMO scheme in 16m EVM [1], we show link performance in various channel environments, modulations and channel coding rates. We use the MMSE receiver as the base receiver type for both OFDMA and SC-FDMA, but the ML receiver is also considered for OFDMA, because the computational complexity of the MLD (Maximum Likelihood Detection)in BS is only feasible for OFDMA.
Simulation results show that for MMSE receiver,OFDMA has about3.3 (QPSK) / 3.6 (16QAM) dB gain overSC-FDMA in Ped A channel and about 3.8 (QPSK) / 4.8 (16QAM) dB gain in Ped B channel at the target BLER 10-1. Especially, when the ML receiver is considered for OFDMA, the gain increases toabout 5~5.5 (QPSK) /5.5~6 (16QAM) dB.
Uplink MIMO SC-FDMA System
The single-carrier frequency division multiple access(SC-FDMA) scheme has been employed in the current 3GPP LTE uplink system [2].The structure of SC-FDMA is similar to OFDMA except for the additional DFT spreading.
Fig. 1 describesan uplink CSM operation in the SC-FDMA system.
Fig. 1. A block-diagram of an uplink CSM in the SC-FDMA
The SC-FDMA has better power efficiency than OFDMA, because DFT operation spreads the signal energy in the frequency domain and yields the lower PAPR property in the time domain.In spite of this advantage, SC-FDMA has a number of restrictions by DFT operation, one of which isdistortion oforthogonality.
There are 2 types of subcarrier mapping method in the SC-FDMA– distributed subcarrier mapping / localized subcarrier mapping.The pilot pattern of the SC-FDMA is a kind of block type to maintain single carrier property.This pilot pattern causessevere degradation of channel estimation performance for distributed subcarrier mapping, so the localized subcarrier mapping is considered generally.
Additionally, when there is small number of antennas in the BS side, ML receiveris considerable to improve the performance in uplink. It is infeasible in the SC-FDMA, however, due to excessive computational overhead. If the DFT size is MforN x N MIMO system, the MLD is operated for MN x MN per subcarrier. After all, the SC-FDMA loses opportunity to achieve additional gain by using ML MIMO receiver.
Uplink MIMO OFDMA System
The orthogonal frequency division multiple access (OFDMA) is a baseline access technique in the 802.16 project WirelessMAN-OFDMA. Fig.2 describesan uplink OFDMA system using the collaborative SM.
OFDMA is a representative multi-carrier scheme, which has the following properties:
It is easy to utilize channel selectivity, andachieve frequency diversity gain.
It maintains orthogonality between subcarriers, so there is no inter-carrier interference.
It is available to design flexible pilot pattern, - e.g. block type, comb type and lattice type.
It has flexibility and efficiency in scheduling.
It causes high PAPR due to multi-carrier property.
Moreover, it is feasible to adopt MLD for the performance improvement in the OFDMA, since OFDMA is operated per tone-level andMLDoperational dimension is limited to only NxN.
Fig. 2. A block-diagram of an uplink CSM in the OFDMA
Simulation Environments
The basic MIMO scheme is collaborative SM according to evaluation methodology. For the subchannelization method, PUSC (distributed subcarrier allocation) which is a basic assumption for the CSM in 16m EVM (and legacy system [3]), is used for OFDMA, and the adjacent subcarrier allocation (localized subcarrier allocation) is used for SC-FDMA.
We simulate the collaborative SM using MMSE and ML receiver for OFDMA, and usingMMSE for SC-FDMA.Table Iprovides the simulation parameters.
Table I. Simulation parameters
Center Frequency / 2.5 GHzSystem Bandwidth / 10 MHz
FFT Size / 1024
MIMO Scheme / 1 x2 Open-loop CSM (virtual 2 x 2)
Multiple Access / OFDMA / SC-FDMA (M=64)
Resource Allocation / Legacy PUSC
(Distributed Subcarrier Allocation) / Adjacent Subcarrier Allocation
(Localized Subcarrier Allocation)
Modulation & Channel Coding / Modulation : QPSK, 16 QAM
Channel Coding: CTC R=1/2
Channel Model / ITU with no spatial correlation
Ped A (3km/h), Ped B (3km/h)
Receiver Type / MMSE / ML / MMSE
Channel estimation / Ideal Channel Estimation
Simulation Results
Simulation results show that OFDMA scheme outperforms SC-FDMA. When MMSE receiver is considered, OFDMA shows about 3.3 (Ped A) ~ 3.8 dB (Ped B) and 3.8 (Ped A) ~ 4.8 dB (Ped B) gain for QPSK and 16QAM, respectively. The gain basically comes from channel frequency selectivity, which affects the performance of subcarrier mapping and distorts orthogonality in SC-FDMA. Moreover if ML receiver is used in the OFDMA scheme, the performance gain increase to about 5 ~ 5.6 dB and 5.4 ~ 6 dB gain for QPSK and 16QAM, respectively.
(a) Ped A (3km/h) (b) Ped B (3km/h)
Fig. 3. Link performance of uplink CSM in (a) Ped A and (b) Ped B
Conclusions
This contribution showed that OFDMA has a better link performance than SC-FDMA in view of UL MIMO support due to the fact that OFDMA well utilizes frequency selectivity and is able to obtain additional MLD gain.
References
1)IEEE 802.16m-07/037r1, Draft IEEE 802.16m Evaluation Methodology.
2) 3GPP TR 25.814, "Physical Layer Aspects for Evolved UTRA."
3) IEEE 802.16e-2005 - IEEE Standard for Local and metropolitan area networksPart 16: Air Interface for Broadband Wireless Access Systems.Amendment 2: Physical and Medium Access Control Layers for Combined Fixedand Mobile Operation in Licensed Bands and Corrigendum 1.
Text Proposal for the 802.16m SDD
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11. Physical Layer
11.x. Multiple Access
In view of UL MIMO support, OFDMA shall be adopted as the multiple access scheme for 802.16m.
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