Deliverable 3

4.2Satellite link: SC-OFDM modulation

MERCE proposed a modulation scheme, called Single Carrier-Orthogonal Frequency Division Multiplexing (SC-OFDM) modulation, suitable for the satellite component of the DVB-NGH system.

One of the key components in a satellite is the power amplifier (PA) that is due to bring the incoming signal at a level compatible with the receivers sensibility over large areas. In order to guarantee the durability of the satellite, it is critical to keep low the power consumption of the system and thus to optimize the amplifier power efficiency, i.e. to drive the amplifier with small input back-offs. Single Carrier modulations have long been the reference scheme for satellite transmissions for their suitability to achieve low power fluctuations compatible with small input back-offs. However, OFDM modulation is taking over SC schemes thanks to a better flexibility and a comparatively lower complexity when it comes to compensate for high channel degradations. However, the more sub-carriers are added together the more the signal behaves like a Gaussian noise with large power fluctuations. This means either the use of costly power amplifiers with a large linear region at the expense of a poor power efficiency or significant performance degradations due to the saturation of the peaks in the signal when driving the PA with a small input back-off. As its name stands for, the Single Carrier-Orthogonal Frequency Division Multiplexing (SC-OFDM) modulation was derived to combine the best of the two underlying waveforms and more precisely to preserve a lot of commonalities with pure OFDM while significantly reducing power fluctuations. As shown in [36] that depicts the relationships between the OFDM, SC-FDE and SC-OFDM schemes, this is achieved by applying a Discrete Fourier Transform (DFT) on the symbols to be transmitted prior the actual OFDM modulation. A comprehensive description of the SC-OFDM modulation is given into the ENGINES Deliverable 2.4 that specifically addresses the topics related to satellite transmission Figure . It is shown in this document that the SC-OFDM appears as a sensible solution when it comes to transmit over power amplifier with very small input back-offs.

Channel

+CP

IDFT_N

DFT_M

-CP

Equalizer

DFT_N

IDFT_M

sc mapping

sc de -mapping

OFDM

SC-FDE

sc : Sub-carrier

CP : Cyclic Prefix

SC-OFDM
_{Figure : General architecture of an SC-OFDM system.}

5Study of interference mitigation and PAPR reduction techniques

Multi carrier communications play a key role in digital communications industry but they suffer from a serious drawback from their high Peak to Average Ratio (PAPR) because it reduces the efficiency of the power amplifier.

Two contributions related to PAPR reduction techniques are reported in this document. In Section 5.1, Thomson Broadcast investigates the comparison between several Peak to Average Ratio (PAPR reduction techniques for DVB-NGH, especially Tone Reservation and Active Constellation Extension. Section 5.2 presents the results of a study carried out by INSA-IETR on the optimization of the phase and amplitude of the channel estimation pilots, used jointly for PAPR reduction process.

5.1System considerations

Multi carrier communication is playing a key role in digital communications industry but they are suffering of a serious drawback from their high PAPR. It is impacting the design to cost of transmitters and the power efficiency of the High Power amplifier on the transmitter site. Indeed, in one hand, network operators are requiring higher Modulation Error Ratio of the signal being transmitted to the end user and in the other hand they need better power efficiency of their systems to improve the operating expenditure of the entire network. In the past years, it is commonly accepted that the output of transmitter used in the broadcast UHF bandwidth exceeds 35dB but these requirements are clearly made against the efficiency of the transmission system. However, techniques enabling to reduce the PAPR are helping to maintain good performances while improving the efficiency of the transmitter. Moreover, like standard clipping techniques, PAPR techniques help to protect the HPA against overdriving the amplifier.

Exciter

High Power Amplifier
_{Figure : System under consideration}

Choosing a technique is not an easy task and a figure of merit of the techniques has been put in place in order to estimate the gain for the system. Techniques are being also checked for their impact on the receiver design trying to put the complexity of the signal processing into the transmission system and not on the receiver chipset.

Measurements made on Tone Reservation techniques applied to fixed transmission have shown that:

• 
  Power may be increase up to 10% or 0.4dB as well as power effciciency
  
• 
  MER is increased by 2dB for operating the transmitter at operational transmission
  

<sub>Figure : MER gain(dB) versus operational MER(dB) for TR implementation

Figure : MER verus HPA IBO</sub>
From the chart above, we observe that the gain in MER of tone reservation technique is linearly increasing with the the input back off of the transmitter HPA resulting with higher MER performance for higher MER operatioanl requirements. For instance, in a system requiring a MER of 35dB , an HPA with a back off inferior to 0.4dB can be used improving system efficieny by 10%.

Tone reservation technique is matching the needs for fixed transmission but better gain can be achieved where lower constellations order are available. Better results for mobile transmission with Active Constellation Extension are achieved in QPSK mode (1.5dB power gain has been achieved in previous B21C project). Other techniques (SC-OFDM or Joint PAPR) are being investigated in the project may potentially show superior efficiency for a mobile system.

Input Back Off IBO dB	MER operation dB	MER gain dB	MER with PAPR dB	Out Of Band products (oob) dB	Figure Of Merit Fom
6.6	31.38	1.9	33.28	1.87	0.156875
6.7	31.78	2.07	33.85	2	0.2164375
6.8	32.3	2.23	34.53	2.1	0.2254375
6.9	32.66	2.4	35.06	2.2	0.235
7	33.1	2.6	35.7	2.3	0.24625
7.1	33.6	2.8	36.4	2.4	0.2575
7.2	34	3.1	37.1	2.48	0.274375
7.3	34.6	3.3	37.9	2.5	0.285625
7.4	35.1	3.5	38.6	2.7	0.296875
8	38.4	4.8	43.2	2.9	0.37

_{Figure : Performance table for Tone Reservation}

Input Back Off IBO dB	MER operation dB	MER gain dB	MER PAPR dB	Oob dB	Fom
6.7 (QPSK)	33.1	15	48.1	2.3	1.54375
6.7 (16QAM)	33.1	5	38.1	3.3	0.68125

_{Figure : Performance table for Active Constellation Extension on QPSK – 16 QAM}