ETSI TR 102 443 V1.1.1 (2008-08)

Satellite Earth Stations and Systems (SES); Satellite Component of UMTS/IMT-2000; Evaluation of the OFDM as a Satellite Radio Interface

DTR/SES-00252

General Information

• Status: Published
• Publication Date: 06-Aug-2008
• Technical Committee: SES MSS - SES WG on Mobile Satellite Systems
• Current Stage: 12 - Completion
• Due Date: 18-Jun-2008
• Completion Date: 07-Aug-2008

Standards Content (Sample)

ETSI TR 102 443 V1.1.1 (2008-08)
Technical Report


Satellite Earth Stations and Systems (SES);
Satellite Component of UMTS/IMT-2000;
Evaluation of the OFDM as a Satellite Radio Interface

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2 ETSI TR 102 443 V1.1.1 (2008-08)



Reference
DTR/SES-00252
Keywords
satellite, UMTS
ETSI
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ETSI

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3 ETSI TR 102 443 V1.1.1 (2008-08)
Contents
Intellectual Property Rights.5
Foreword.5
1 Scope.6
2 References.6
2.1 Normative references.6
2.2 Informative references.6
3 Definitions, symbols and abbreviations .7
3.1 Definitions.7
3.2 Symbols.8
3.3 Abbreviations.8
4 OFDM technology and background.9
4.1 OFDM Fundamentals.9
4.1.1 OFDM Definitions.9
4.1.2 OFDM Signal Generation.10
4.1.3 Guard Interval.11
4.1.4 Impact of Guard Interval.12
4.1.5 Impact of Symbol Duration .12
4.1.6 Impact of Inter-Carrier Spacing .12
4.1.7 OFDM Inactive Sub-Carriers.12
4.1.8 Time-Frequency Multiplexing.13
4.1.9 OFDM Signal Reception Using the FFT .14
4.2 OFDM for Mobile Terrestrial and Satellite Scenario .14
5 OFDM and the satellite environment .15
5.1 Non-Linearity Effects and Predistortion Techniques .15
5.1.1 Compensation Techniques.15
5.1.2 Digital Predistortion Techniques .16
5.1.3 Multi-Beam Coverage Using OFDM.16
6 OFDM feasibility .17
6.1 Physical Layer Structure in the OFDM Downlink .17
6.1.1 Physical Channel.17
6.1.1.1 OFDM Physical Channel Definition .18
6.1.2 Channel Coding and Multiplexing.19
6.1.3 Physical Channel Mapping.20
6.1.4 User Traffic Multiplexing Solutions.20
6.1.4.1 Solution based on a generic Costas sequence .20
6.2 Spectrum Compatibility.22
7 OFDM Evaluation Scenario .23
7.1 Reference System Scenario for OFDM S-DMB Analysis.23
7.2 Reference OFDM configurations for the evaluation .24
8 Simulation Results.25
8.1 Uncoded System Performance .25
8.1.1 AWGN Channel.25
8.1.2 Non-linear channel.26
8.2 WCDMA Coding Performance .27
8.2.1 Non selective Rice fading .29
8.2.2 Frequency Selective Channel.30
9 Link Budget Study .36
9.1 System parameters.36
9.1.1 Satellite parameters.36
9.1.2 UE parameters.36
9.1.3 Physical layer configuration and performances .36
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4 ETSI TR 102 443 V1.1.1 (2008-08)
9.2 Link budgets.37
9.2.1 Handset.37
9.2.2 Handheld.38
9.2.3 Vehicular.39
10 Conclusions.39
History .41

ETSI

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5 ETSI TR 102 443 V1.1.1 (2008-08)
Intellectual Property Rights
IPRs essential or potentially essential to the present document may have been declared to ETSI. The information
pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found
in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in
respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the ETSI Web
server (http://webapp.etsi.org/IPR/home.asp).
Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee
can be given as to the existence of other IPRs not referenced in ETSI SR 000 314 (or the updates on the ETSI Web
server) which are, or may be, or may become, essential to the present document.
Foreword
This Technical Report (TR) has been produced by ETSI Technical Committee Satellite Earth Stations and Systems
(SES).
ETSI

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6 ETSI TR 102 443 V1.1.1 (2008-08)
1 Scope
The present document entails a feasibility study that evaluates the use of the OFDM Radio Interface proposed the
3GPP TR 25.892 [i.1] as Satellite Radio Interface on the satellite downlink, presenting physical layer results and link
budget studies. The present document contains informative elements that should serve as a starting point for the
definition and finalization of advanced Satellite Radio Interfaces. The adoption of the OFDM Radio Interface results in
higher link margin under key propagation conditions such as the NLOS propagation case and when CGCs are
considered.
2 References
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific.
• For a specific reference, subsequent revisions do not apply.
• Non-specific reference may be made only to a complete document or a part thereof and only in the following
cases:
- if it is accepted that it will be possible to use all future changes of the referenced document for the
purposes of the referring document;
- for informative references.
Referenced documents which are not found to be publicly available in the expected location might be found at
http://docbox.etsi.org/Reference.
For online referenced documents, information sufficient to identify and locate the source shall be provided. Preferably,
the primary source of the referenced document should be cited, in order to ensure traceability. Furthermore, the
reference should, as far as possible, remain valid for the expected life of the document. The reference shall include the
method of access to the referenced document and the full network address, with the same punctuation and use of upper
case and lower case letters.
NOTE: While any hyperlinks included in this clause were valid at the time of publication ETSI cannot guarantee
their long term validity.
2.1 Normative references
The following referenced documents are indispensable for the application of the present document. For dated
references, only the edition cited applies. For non-specific references, the latest edition of the referenced document
(including any amendments) applies.
Not applicable.
2.2 Informative references
The following referenced documents are not essential to the use of the present document but they assist the user with
regard to a particular subject area. For non-specific references, the latest version of the referenced document (including
any amendments) applies.
[i.1] 3GPP TR 25.892 (V6.0.0): "3rd Generation Partnership Project; Technical Specification Group
Radio Access Network; Feasibility Study for Orthogonal Frequency Division Multiplexing
(OFDM) for UTRAN enhancement (Release 6)".
[i.2] 3GPP TR 25.858 (V5.0.0): "3rd Generation Partnership Project; Technical Specification Group
Radio Access Network; High Speed Downlink Packet Access: Physical Layer Aspects
(Release 5)".
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7 ETSI TR 102 443 V1.1.1 (2008-08)
[i.3] ETSI TS 125 212: "Universal Mobile Telecommunications System (UMTS); Multiplexing and
channel coding (FDD) (3GPP TS 25.212 version 5.9.0 Release 5)".
[i.4] S. Chang: "Compensation of nonlinear distortion in RF power amplifiers", Wiley Encyclopedia of
Telecommunications, J.J. Proakis Ed., 2002.
[i.5] S. Benedetto and E. Biglieri: "Nonlinear equalization of digital satellite channels", IEEE J. Select.
Areas Comm., vol. 1, pp. 57-62, Jan. 1983.
[i.6] J.K. Cavers: "Amplifier Linearization using a digital predistorter with fast adaptation and low
memory requirements", IEEE Trans. Vehic. Tech., vol. 39, pp. 31-40, Nov. 1990.
[i.7] P. Salmi, M. Neri, and G.E. Corazza: "Fractional Predistortion. Techniques with Robust
Modulation Schemes for Fixed and mobile Broadcasting", 13th IST Mobile & Wireless
Communications Summit (IST2004), pp. 990-995, June 2004.
[i.8] S.W. Golomb, and H. Taylor: "Construction and Properties of Costas Array", Proc. IEEE, vol. 72,
pp. 1143-1163, Sep. 1984.
[i.9] S. Cioni, G.E. Corazza, M. Neri, and A. Vanelli-Coralli: "On the Use of OFDM Radio Interface
for Satellite Digital Multimedia Broadcasting Systems", International Journal of Satellite
Communications and Networking, February 2006, Int. J. Satell. Commun. Network. 2006; 24:153-
167, published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/sat.836.
3 Definitions, symbols and abbreviations
3.1 Definitions
For the purposes of the present document, the following terms and definitions apply:
cell: geographical area under Complementary Ground Component coverage
downlink: unidirectional radio link for the transmission of signals from a satellite to a UE
forward link: unidirectional radio link for the transmission of signals from a gateway to a UE via a satellite
guard interval / guard time: number of samples inserted between useful OFDM symbols, in order to combat
inter-OFDM-symbol-interference induced by channel dispersion and to assist receiver synchronization
NOTE: It may also be used to aid spectral shaping. The guard interval may be divided into a prefix (inserted at
the beginning of the useful OFDM symbol) and a postfix (inserted at the end of the previous OFDM
symbol).
inter-carrier frequency / sub-carrier separation: frequency separation between OFDM sub-carriers, defined as the
OFDM sampling frequency divided by the FFT size
OFDM unit: group of constellation symbols to be mapped onto a sub-band, a subset of the OFDM carriers
OFDM samples: discrete-time complex values generated at the output of the IFFT, which may be complemented by
the insertion of additional complex values (such as samples for pre/post fix and time windowing)
NOTE: Additional digital signal processing (such as filtering) may be applied to the resulting samples, prior to
being fed to a digital-to-analog converter.
OFDM sampling frequency: total number of samples, including guard interval samples, transmitted during one
OFDM symbol interval, divided by the symbol period
repeater: device (e.g. CGC) that receives, amplifies and transmits the radiated or conducted RF carrier both in the
down-link direction (from the satellite to the mobile area) and in the up-link direction (from the mobile to the satellite)
return link: unidirectional radio link for the transmission of signals from a UE to a gateway via a satellite
rice factor: power ratio between LOS component and diffuse component
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8 ETSI TR 102 443 V1.1.1 (2008-08)
spot: geographical are under beam coverage
uplink: unidirectional radio link for the transmission of signals from a UE to a satellite
useful OFDM symbol: time domain signal corresponding to the IFFT/FFT window, excluding the guard time
useful OFDM symbol duration: time duration of the useful OFDM symbol
3.2 Symbols
For the purposes of the present document, the following symbols apply:
F OFDM sampling frequency
0
F Maximum Doppler shift.
d
N Total number of IFFT/FFT bins (sub-carriers)
N Number of prefix samples
p
N Number of modulated sub-carriers (i.e. sub-carriers carrying information)
u
T OFDM symbol period
s
T OFDM prefix duration
g
T OFDM useful symbol duration
u
Δf Sub-carrier separation
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
ACI Adjacent Channel Interference
APSK Amplitude and Phase Shift Keying
AWGN Additive White Gaussian Noise
BER Bit Error Rate
C/N Carrier to Noise power ratio
CGC Complementary Ground Component
CRC Cyclic Redundancy Check
CPICH Common Pilot Channel
DC-RF Direct Current to Radio Frequency
DL Down Link
EIRP Effective Isotropic Radiated Power
FDM Frequency Division Multiplexing
FFS For Further Study
FFT Fast Fourier Transform
FIR Finite Impulse Response
GEO Geostationary Earth Orbit
GW GateWay
HARQ Hybrid Automatic Repeat reQuest
HPA High Power Amplifiers
HSDPA High Speed Downlink Packet Access
HS-DSCH High Speed - Downlink Shared CHannel
IBO Input Back-Off
IFFT Inverse Fast Fourier Transform
IMR Intermediate Module Repeater
ISI Inter Symbol Interference
LOS Line-Of-Sight
LTWTA Linearized Travelling Wave Tube Amplifier
LUT Look-Up Table
MAC Medium Access Control
MIMO Multiple Input Multiple Output
NL Non Linear
NLOS No Line-Of-Sight
OBO Output Back Off
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9 ETSI TR 102 443 V1.1.1 (2008-08)
OFDM Orthogonal Frequency Division Multiplexing
PAPR Peak-to-Average Power Ratio
PDSCH Physical Downlink Shared CHannel
PER Packet Error Rate
PhCh Physical ChannelPSK Phase Shift Keying
QAM Quadrature Amplitude Modulation
SCCH Shared Control CHannel
S-DMB Satellite-Digital Mobile Broadcasting
SFN Single Frequency Network
SNR Signal-to-Noise Ratio
T-F Time-Frequency
TPCCH Transmit Power Control CHannel
TTI Transmission Time Interval
TWTA Travelling Wave Tube Amplifier
UE User Equipment
UTRAN UMTS Terrestrial Radio Access Network
WCDMA Wideband Code Division Multiple Access
4 OFDM technology and background
4.1 OFDM Fundamentals
4.1.1 OFDM Definitions
The technique of Orthogonal Frequency Division Multiplexing (OFDM) is based on the well-known technique of
Frequency Division Multiplexing (FDM). In FDM different streams of information are mapped onto separate parallel
frequency channels. Each FDM channel is separated from the others by a frequency guard band to reduce interference
between adjacent channels.
The OFDM technique differs from traditional FDM in the following interrelated ways:
1) multiple carrier multiple carriers (called sub-carriers) carry the information stream;
2) the sub-carriers are orthogonal to each other; and
3) a guard time may be added to each symbol to combat the channel delay spread and inter-symbol interference
induced by linear distortion.
These concepts are illustrated in the time-frequency representation of OFDM presented in figure 1.
5 MHz Bandwidth
FFT
Sub-carriers
Guard Intervals
…
Symbols
Frequency
…
Time

Figure 1: Frequency-Time representation of an OFDM Signal
Since the orthogonality is guaranteed between overlapping sub-carriers and between consecutive OFDM symbols in the
presence of time/frequency dispersive channels the data symbol density in the time-frequency plane can be maximized.
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10 ETSI TR 102 443 V1.1.1 (2008-08)
4.1.2 OFDM Signal Generation
Data symbols are synchronously and independently transmitted over a high number of closely spaced orthogonal
sub-carriers using linear modulation (either PSK, APSK or QAM). The generation of the QAM/OFDM signal can be
th
conceptually illustrated as in figure 2, where ω is the n sub-carrier frequency (in rad/s) and 1/T is the QAM symbol
n u
rate. Note that the sub-carriers frequencies are equally spaced and hence the sub-carrier separation is constant. That is:
ω − ω
n n−1
= Δf , n ∈[1, N −1] .
2π
In practice, the OFDM signal can be generated using IFFT digital signal processing. The baseband representation of the
th
OFDM signal generation using an N-point IFFT is illustrated in figure 3, where a(mN+n) refers to the n sub-channel
modulated data symbol, during the time period mT < t ≤ (m+1)T .
u u
QAM

modulator
.
j ω t
0
.
e
.
QAM
s ( t )
Σ
modulator
jω t
n
.
e
.

.
QAM
modulator
Symbol rate = 1/T ω
u j t
N −1
e
s

symbols/sec

Figure 2: Conceptual representation of OFDM symbol generation

mT
u
( m+1 )T
time u
a( mN + 0 )

mT
u
( m+1 )T
u
time
a( mN + 1 )
a( mN + 2 )
frequency
s (0), s (1), s (2), …, s ( N-1 )
m m m m

IFFT
.
.
s
. m
a( mN + N -1 )

Figure 3: OFDM useful symbol generation using an IFFT
The vector s is defined as the useful OFDM symbol. Note that the vector s is in fact the time superposition of the N
m m
narrowband modulated sub-carriers.
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11 ETSI TR 102 443 V1.1.1 (2008-08)
It is therefore easy to realize that, from a parallel stream of N sources of data, each one modulated with QAM useful
symbol period T , a waveform composed of N orthogonal sub-carriers is obtained, with each narrowband sub-carrier
u
having the shape of a frequency sinc function. Figure 4 illustrates the mapping from a serial stream of QAM symbols to
N parallel streams, used as frequency domain bins for the IFFT. The N-point time domain blocks obtained from the
IFFT are then serialized to create a time domain signal.
QAM symbol rate =
N/T symbols/sec
u
N OFDM
symbol
symbols Useful OFDM
QAM streams
N :1
IFFT
Source(s)
1: N
T symbols
1/ u
Modulator 1/T
u
symbols/s
symbol/sec

Figure 4: OFDM signal generation chain
4.1.3 Guard Interval
A guard interval may be added prior to each useful OFDM symbol. This guard time is introduced to minimize the
inter-OFDM-symbol-interference power caused by time-dispersive channels. The guard interval duration T (which
g
corresponds to N prefix samples) needs to be sufficient to cover the most of the delay-spread energy of a radio channel
p
impulse response. In addition, such a guard time interval can be used to allow soft-handover.
OFDM symbols
m
Prefix length
Useful OFDM symbol duration
copy

Figure 5: Cyclic prefix insertion
A prefix is generated using the last block of N samples from the useful OFDM symbol. The prefix insertion operation
p
is illustrated in figure 5. Note that since the prefix is a cyclic extension to the OFDM symbol, it is often termed cyclic
prefix. Similarly, a cyclic postfix could be appended to the OFDM symbol.
After the insertion of the guard interval the OFDM symbol duration becomes T = T + T .
s g u
The OFDM sampling frequency F can therefore be expressed as:
0
N + N
p
F =
0
T
s
hence, the sub-carrier separation becomes:
F
0
Δf = .
N
It is also worth noting that time-windowing and/or filtering is necessary to reduce the transmitted out-of-band power
produced by the ramp-down and ramp-up at the OFDM symbol boundaries in order to meet the spectral mask
requirements.
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12 ETSI TR 102 443 V1.1.1 (2008-08)
4.1.4 Impact of Guard Interval
The cyclic prefix should absorb most of the signal energy dispersed by the multi-path channel. The entire the
inter-OFDM-symbol-interference energy is contained within the prefix if the prefix length is greater than that of the
channel total delay spread, i.e.:
T >τ
g
where τ is the channel total delay spread. In general, it is sufficient to have most of the energy spread absorbed by the
guard interval, given the inherent robustness of large OFDM symbols to time dispersion, as detailed in the next clause.
4.1.5 Impact of Symbol Duration
The mapping of the modulated data symbol onto multiple sub-carriers also allows an increase in the symbol duration.
Since the throughput on each sub-carrier is greatly reduced, the symbol duration obtained through an OFDM scheme is
much larger than that of a single carrier modulation technique with a similar overall transmission bandwidth. In general,
when the channel delay spread exceeds the guard time, the energy contained in the ISI will be much smaller with
respect to the useful OFDM symbol energy, as long as the symbol duration is much larger than the channel delay
spread, that is:
T >> τ .
s
Although large OFDM symbol duration is desirable to combat time-dispersion caused ISI, however, the large OFDM
symbol duration can reduce the ability to combat the fast temporal fading, especially if the symbol period is large
compared to the channel coherence time. Thus, if the channel can no longer be considered as constant through the
OFDM symbol, the inter-sub-carrier orthogonality loss is introduced and the performance in fast fading conditions are
degraded. Hence, the symbol duration should be kept smaller than the minimum channel coherence time. Since the
channel coherence time is inversely proportional to the maximum Doppler shift f , the symbol duration T needs to be,
d s
in general, chosen such that:
1
T << .
s
f
d
4.1.6 Impact of Inter-Carrier Spacing
Because of the time-frequency duality, some of the time-domain arguments of clause 4.1.5 Impact of Symbol Duration
can be translated to the frequency domain in a straightforward manner. The large number of OFDM sub-carriers makes
the bandwidth of the individual sub-carriers small relative to the overall signal bandwidth. With an adequate number of
sub-carriers, the inter-carrier spacing is much narrower than the channel coherence bandwidth. Since the channel
coherence bandwidth is inversely proportional to the channel delay spread τ, the sub-carrier separation is generally
designed such that:
1
Δf << .
τ
In this case, the fading on each sub-carrier is frequency flat and can be modelled as a constant complex channel gain.
The individual reception of the QAM symbols transmitted on each sub-carrier is therefore simplified to the case of a
flat-fading channel. Moreover, in order to combat Doppler effects, the inter-carrier spacing should be much larger than
the maximum Doppler shift f :
d
Δf >> f .
d
4.1.7 OFDM Inactive Sub-Carriers
Since the OFDM sampling frequency is larger than the actual signal bandwidth, only a sub-set of sub-carriers is used to
carry QAM symbols. The remaining sub-carriers are left inactive prior to the IFFT, as illustrated in figure 6. The split
between the active and the inactive sub-carriers is determined based on the spectral constraints, such as the bandw
...