IEEE C802.16m-08/056
Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
Single Carrier Uplink Frame Format
Date
Submitted
2007-01-16
Source(s)
P. Orlik, A. F. Molisch, Z. J. Tao
Mitsubishi Electric Research Lab
201 Broadway
Cambridge, MA 02139
E-mail: {porlik, molisch, tao}@merl.com
Kuze.Toshiyuki@ah.MitsubishiElectric.co.jp
T. Kuze
Mitsubishi Electric Corp.
Re:
In response to 802.16m-07/047 call for SDD contributions
Abstract
This contribution proposes a frame format for uplink single carrier transmissions
Purpose
For discussion, consider single carrier uplink transmissions to reduce PAPR and to better
support roaming.
Notice
Release
Patent
Policy
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Single Carrier Uplink Frame Format
P.Orlik, A. Molisch, Z. J.Tao, T. Kuze
Mitsubishi Electric
Background
The IEEE 802.16 standard employs orthogonal multiple access (OFDMA) in an uplink. In OFDMA, each user
terminal (transceiver) sends data to the base station (BS) on a set of assigned sub-carriers on which the
transmitter modulates data symbols. Thus, each uplink OFDMA symbol contains data from several users
(terminals) on disjoint sets of sub-carriers.
Figure 1 shows a block diagram of a conventional OFDMA transmitter and receiver.
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IEEE C802.16m-08/056
{xn}
Sub-carrier
mapping
M-point
IDFT
Add
CP
DAC/
RF
Channel
Sub-carrier
De-mapping &
Equalization
Detect
M-point
DFT
Remov
e
CP
ADC/
RF
Figure 1 OFDMA Transceiver
An alternative, but similar transmission technique is term Single Carrier Frequency Division Multiple Access
(SC-FDMA).
Figure 2 shows a block diagram for an SC-FDMA transmitter and receiver.
{xn}
N-point
DFT
Subcarrier
mapping
M-point
IDFT
Add
CP/PS
DAC
/
RF
Channel
Detec
t
N-point
IDFT
Sub-carrier
De-mapping
Equalization
M-point
DFT
Remov
e
CP
Figure 2 SC-FDMA Transceiver
2
ADC
/
RF
IEEE C802.16m-08/056
This is essentially the same structure as in Figure 1, except for the presence of an additional N-point DFT in the
transmitter, and an N-point IDFT in the receiver. The effect of the DFT is to spread the user data over all the N
assigned sub-carriers of the OFDM symbol. In contrast, in the OFDMA network of Figure 1, each individual
data symbol xn is carried on a single sub-carrier in the M-point IDFT.
We describe the two transmission techniques, i.e., OFDMA and SC-FDMA, mainly to point out the similarities
between the two techniques. Both OFDMA and SC-FDMA essentially transmit a sequence of OFDM symbols,
where the individual sub-carriers are assigned to multiple user terminals. In both cases, the transmitted signal
can be thought of as a two dimensional signal occupying both the time and frequency domains. Additionally
from Figures 1 and 2, we see that the transmitters and receivers for both networks differ only slightly.
Regulatory domains, e.g., governmental agencies, such as the FCC in the U.S or the ETSI in Europe, may place
restrictions on the type of wireless technologies used in their RF spectrums. Additionally, market acceptance of
competing standards, e.g., WiMAX or 3GPP LTE, may further partition the wireless spectrum into areas where
one service provider supports either OFDMA or SC-FDMA.
Therefore, it is desired to deploy both transmission techniques within the same cellular network. This also has
benefits that would enable easier roaming among carriers and inter-RAT handover.
Uplink sub-frame format
To support SC-FDMA, we suggest adding an additional zone to the uplink sub-frame according to figure 3.
Time (OFDM Symbols)
Frequency (Sub Channel Grouping)
KDl
…
Ko-1 Ko
KS
…
K-1
K
0
1
2
3
4
UpLink OFDMA
UpLink SC-FDMA

C
Figure 3 Suggested uplink sub-frame structure
One zone is used exclusively for legacy OFDMA transmissions from terminals, and another zone is used
exclusively for SC-FDMA transmissions from the terminals. The arrangement (i.e., the ordering of the OFDMA
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IEEE C802.16m-08/056
and SC-FDMA zone), and their relative sizes (i.e., number of constituent OFDM symbols), can be arbitrary, and
can be determined by the capabilities (or proportion of SC-FDMA and OFDMA terminals associated with the
base station). Since capabilities of the terminals are typically exchanged with the base station during the
network entry and re-entry (during hand over), the base station can allocate the size of the SC-FDMA zone
based on the number of terminals that are capable of using SC-FDMA transmission.
The main advantage of SC-FDMA is its ability to reduce the Peak to Average Power Ratio (PAPR) compared to
the PAPR of OFDMA. This reduction in PAPR does come with some constraints on the way in which the subcarrier mapping is performed. Therefore, within the SC-OFDMA zone, sub-carrier mappings is done in such a
way as to achieve a reduction in PAPR. One mapping is an interleaved mapping where allocated are uniformly
spaced. Another mapping is a contiguous mapping where adjacent sub-carriers are allocated to users.
Figure 7 shows the gains in terms of the reduction of PAPR that can be achieved with the use of SC-FDMA and
the two mapping schemes. The graphs in the figure present the results of a simulation of the SC-FDMA for both
mapping schemes and the OFDMA transmission scheme with a random mapping among the M sub-carriers
(this is similar to PUSC or FUSC permutations). The simulated value of the complementary cumulative
distribution function (CCDF), which denotes that probability that the PAPR exceeds a given value PAPR0 is
plotted. That is, Pr{ PAPR > PAPR0}. The plots were generated using QPSK modulation and M = 256 sub
carriers. Each user is assumed to have N = 64 active sub-carriers. We see that SC-FDMA with interleaved
mapping has a PAPR of 0 dB with probability 1, and that PAPR of SC-FDMA with contiguous mapping is
reduced by approximately 4dB.
0
10
OFDMA PUSC-like
mapping
SC-FDMA interleaved
mapping
-1
PrPAPR >PAPR
0
10
SC-FDMA contiguous
mapping
-2
10
-3
10
0
2
4
6
PAPR0 (dB)
8
10
12
Figure 4 CCDF of the PAPR for SC-FDMA and OFDMA. M = 256, N = 64, QPSK modulation
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