January 2016

NC-SC-OFDM, a Frequency Agile Waveform for Satellite Transmissions in a Spectrum Scarcity Context

B. Ros, S. Cazalens and C. Boustie French Space Agency

Telecommunications department

Contact : benjamin.ros@cnes.fr

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Available spectrum is becoming scarce

A dynamic sharing of frequency bands between systems is advocated by standardization instances,

as 5G PPP and UE RSPG (Radio Spectrum Frequency Group)

« It will tend to become the norm » -RSPG-

5G-PPP, ETSI-SCN, and ITU (WG4B – 4/40-E) groups advise for the integration of terrestrial and satellite

system

Satellite ensures coverage complement, backup solution in case of disaster, and efficient broadcast

In this context, NC-SC-OFDM shows a good fit to these spectrum scarcity and integrated systems issues

Weak power fluctuations (good fit with on-board non linear amplifier)

Frequency granular access (cognitive radio, spectrum efficient management, jammers avoidance)

o Non-Contiguous (NC)-OFDM waveform has already been proposed as a solution for cognitive radio systems

Near from terrestrial UE architecture (mass market facilities, satellite terrestrial convergence)

Besides, there is a growing interest for SC-OFDM waveform on satellite transmissions

SC-OFDM appears (on forward link) in DVB-NGH standard, ETSI-SCN standardization process, and in

studies on DVB-S2x revolution

ESA ITT was published (ARTES 5.1 ref 3C.011) in june 2015

Context

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I. System scenarios for NC-SC-OFDM waveform

II. NC-SC-OFDM waveform overview

III. Simulation model and parameters

IV. Non linearity effect

V. IMUX filter effect

VI. Signaling frequency holes

VII. Conclusion

Presentation outline

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I. System scenarios

Integrated terrestrial/satellite system

Dynamic resource access depending on the system load

Example : Need of more resources for terrestrial link

Terrestrial and satellite allocation can be interspersed or separated

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I. System scenarios

Wide band system (medium/high throughput or broadcast) dealing with non exclusive bands

Example : avoid an interferer disturbing several terminals

Frequency hole can be created to avoid interferer, and unused power enables increasing

spectral efficiency on remaining subcarriers

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Construction forme d’onde (EW)-SC-OFDM (1) SC-OFDM waveform overview (1/2)

Emitting part

Encoded and

interleaved bits

frequency

M+K carriers

Weighting

frequency

Transmitted carriers

Null carriersNull carriers

Original M carriersAdditional

K/2 carriers

Additional

K/2 carriersExtension

Roll-off = K/M

Half Nyquist filter in

frequency domain

SC-OFDM spectrum EW-SC-OFDM spectrum

Spectrum preview

Temporal signal results from data symbols interpolation with N/M ratio

I/Q

samples

M M+K N

Zero insertion (N-M-K)

Frequency domain

I/Q

constellation

mapping

DFT extension Pilot

insertion weighting IFFT

GI

insertion

Data symbols

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Construction (EW)-SC-OFDM

Typical receiver

Baseband signal

Subcarriers yk

Log Likelyhood

Ratio metrics

Equalized symbols 𝒙 𝒌

Subcarrier yk SNR does not equal data symbol zk SNR

Equalization is MMSE, subcarrier by subcarrier par :

*

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ˆˆ

ˆ ˆ

kk k

k

hx y

h

Data symbols zk

SC-OFDM case

Equalization

De-weighting

Channel

estimation

GI

removal FFT

De-

extension

Pilot

removal IDFT

I/Q

demapping

(EW)-SC-OFDM case

Frequency domain

Waveform envelope

SC-OFDM waveform overview (2/2)

* *

2 22

ˆ ˆˆ

ˆ ˆ ˆ

k k k M k Mk

k k M

h y h yx

h h

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Several ways to insert frequency holes

DFT spreading is applied only over M-L data symbols

Available amplifier power can be spread in (remaining) useful band

Power of remaining subcarriers equals to BWT / (BWT – BWI)

II. NC-SC-OFDM waveform overview

Interferer

Useful signal

Resulting surboost of remaining subcarriers

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Tx Non linear on

board amplifier

Simulation model

Default Parameters

QPSK 2/3 (LDPC + BCH encoding on 16200 bits)

Guard interval = 1/16

426 active subcarriers for a IFFT size of 512

I/Q sampling frequency : 120/7 ~ 17.14 MHz

Useful bandwidth : 14.26 MHz

Payload : amplifier + IMUX with group delay ~ 15 x I/Q

symbol duration

Simulation with gaussian additive noise channel, for 108

information bits

Waveform comparison is done at same Es/N0

Propagation

Rx

Propagation

channel

Interference

model

Gaussian

noise

IMUX filter

III. Simulation model and parameters

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IV. Non linearity effect

Reference simulation

NC-SC-OFDM enjoys good performances through non linear amplifier (target BER=10-5)

At 17 dB C/Im target, OBO of 0.6 dB is needed for SC-OFDM

Only 0.5 dB degradation for NC-SC-OFDM

Gap increases when C/Im target is growing

Degradation remains equal even if frequency hole width is varying

Varying frequency hole width

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IV. Non linearity effect

3 holes, several widths 1 hole, using surboost to counteract loss of

bandwidth

With 3 frequency holes, there are few changes in the degradation

At the most 0.2 dB additional degradation compared to 1 frequency hole

Using surboost enables balancing the loss of bandwidth because of frequency holes

Loss of bandwidth is 11.3% whereas loss of bitrate is 6.2%

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V. IMUX filter effect

Four IMUX filters are considered, with high group delay (~15 x I/Q sample duration):

-3 dB cut-off frequency at 0.4, 0.43, 0.47 and 0.5 * Fech

0.4 and 0.43 filters are impacting signal at the edges (signal has a width of 0.42 * Fech)

Despite group delay >> I/Q symbol duration, filter degradations are closed to 0 because of subcarrier

equalization process and cyclic prefix

With real channel estimation, requiring 0.4 dB more C/N, IMUX filter insertion brings 0.2 dB degradation

Narrower filters bring 0.15 dB additional degradation compared to larger filter

PCE : Perfect Channel Estimation RCE : Real Channel Estimation

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VI. Signaling frequency holes

Several solutions to indicate NC-SC-OFDM frequency plan

Strategy 1 : split spectrum in Na areas; signaling the first and last subcarrier index (Ns) for each area

affected by a frequency hole

Nbits = log2(Na) + Nb_areas_with_hole*[ log2(Na) + 2*log2(Ns)] 56 bit

Strategy 2 : split spectrum in Na areas; signaling the number of the areas to delete (not only the frequency

hole)

Nbits = log2(Na) + Nb_areas_with_hole * log2(Na) 28 bit, but waste of bandwidth

Strategy 3 : signaling the first and last subcarrier index for each hole

Nbits = Nb_holes * [2* log2(Ntotal_subcarriers)] 60 bit

Optimal strategy depends on system specification or targets in term of bandwidth, number of holes, hole

size vs total bandwidth

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VII. Conclusion

Non-Contiguous SC-OFDM show good performances for satellite transmissions

SC-OFDM waveform is efficient thanks to its limited envelope fluctuations

A frequency hole, even quite large, doesn’t degrade so much envelope fluctuations ( 0.5 dB)

Thanks to its frequency granular access and cyclic prefix, waveform is nearly insensitive to IMUX

filtering with high group delay

Frequency hole optimal signalization depends on system specification

Further studies

Assessing the impact of using SC-OFDM on board and ground level (terminal design, resource access

methods) with a satellite and/or ground segment manufacturer (to come in 2016)

Demonstrating synchronization capabilities for NC-SC-OFDM receiver (Tender on going)

Demonstrating hardware broadband transmission (to come in 2016)

Studying and designing signalization for frequency holes : modulation, coding , localization in the PL

frame (Tender on going)

Dissemination

o publication in 2016 on SC-OFDM synchronisation capabilities (HW and/or SW) for fixed and

mobile channels

If a company is interested in the development of this technology in a spatial context, don’t hesitate to

contact us

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References

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