October 12, 2015

The new standard, DVB-S2X, has extended its operation range down to signal to noise ratio of -10dB. The standard defines a range of robust modulation and coding schemes, within a frame compatible with DVB-S2 and DVB-S2X frames. A very robust header enables acquisition and synchronization in the very low SNR operation conditions as well as in a burst mode. Operation at this low SNR range makes it possible to use low-cost terminals, both in terms of antenna size and equipment and opens a new range of applications and markets, such as mobile broadband.
We describe the implementation and operation of the VL-SNR waveform by SatixFy’s SX-3000 ASIC. The generic design of the ASIC is based on embedded general purpose and digital signal processors supported by hardware accelerators along the data path makes it possible to implement the DVB-S2X VL-SNR waveform as well as proprietary and programmable waveforms, with even higher robustness.

Introduction

Many applications, such as sensor networks, remote infrastructure monitoring, emergency services etc. require ubiquitous coverage, while requiring low data rates. Satellites can provide an excellent solution for their communication needs. One of the key requirements of such systems is the ability to operate in very low signal to noise ratio (SNR) conditions. This ability is required not only from the point of view of cost, namely antenna size, power consumption and components costs but also from
the point of view of link reliability and availability. In the recently published DVB-S2X standard, a new protocol was introduced for operation in a very low SNR (VL-SNR), enabling operation in SNR as low as -10dB. A Higher level of symbol spreading, and compression makes it possible to operate in even lower SNR’s if needed.

Consider Shannon’s famous formula for capacity in an additive white Gaussian noise channel:

In the low SNR region, the capacity is power limited and is not dependent on the allocated bandwidth. It is also known from coding theory that sophisticated codes are not efficient in that region. Simple repetition codes, which are in effect equivalent to bandwidth reduction, are the most efficient.

The modulation and coding (modcod) scheme defined by the standard frames in DVB-S2 and DVB-S2X, support operation as low as -2.5 dB. The VL-SNR mode of the DVB-S2X standard bridges the gap within the power limited region, of SNRs as low as -9.9 dB. we describe the standard VL-SNR mode and the way it is implemented within SatixFy’s SX-3000 ASIC. some protocol modifications that are compatible with the standard are presented as well, yet enable operation in even lower SNR, down to -30 dB, as we call it Extremely Low SNR (EL-SNR).

The VL-SNR Protocol in DVB-S2X

The VL-SNR protocol was introduced to the DVB-S2X standard due to the requirement to extend the operating range of the protocol down to -10dB SNR. One of the requirements for the operation mode was that VL-SNR frames can be seamlessly interlaced within a stream of other DVB-S2/ DVB-S2X frames. In such a way receivers that enjoy higher SNR can decode the S2/S2X frames and skip the VL-SNR frames. Terminals for which the SNR is not sufficient for receiving the standard frames should be able to detect and synchronize to a VL-SNR frame within a stream of non-VL-SNR frames. Reception can be considered as done in a burst mode.
This post is not intended to explain the VL-SNR mode in detail. Details can be found in DVB-S2/ DVB-S2X standards including use cases and design considerations. However, we will point out some of the additional features introduced

However, we will point out some of the additional features introduced into it.

A Choice of Two Constant- Length Frames

Nine MODCODs were defined for VL-SNR. 6 of which for a frame of 33282 symbols, and the other three for a frame of length 16686 symbols. These lengths are the same as those for QPSK and 16APSK normal frames of DVB-S2, and thus can replace them within a stream.

Robust Modulations

The π/2-BPSK modulation was added for the data symbols for 6 of the 9 defined MODCODs. This modulation is also used for the pilot and header of the standard frames. 2 other MODCODs were defined using π/2-BPSK with a spreading factor of 2. Namely, the information bits are repeated, and each bit actually modulates two consecutive symbols for extra robustness.

Low rate LDPC codes

To further increase robustness new LDPC codes for rates as low as 1/5 were introduced. Another frame type – medium frame – for which the number of the LDPC output bits is 32400 (in addition to the normal frame of 64800 LDPC bits and the short frame of 16200 bits) was introduced as well. However, in order to fit the total number of symbols to the fixed frame size, shortening and puncturing are included so the effective LDPC coding rates are somewhat higher. If we include the BCH encoder and the bit spreading as a type of repetition code, the effective coding rate ranges between 0.081 to 0.35.

Long frame header

The physical header of the standard DVB-S2/S2X frame contains 90 symbols. It is designed for decoding at SNR as low as -2.5dB. This SNR is still too high for the VL-SNR mode. In addition, it should be decoded as a burst, namely without a reliable frequency and symbol synchronizations. For these reasons a 900 symbols header was defined for it, using Walsh-Hadamard encoding to convey the specific modcod information. It was shown in the standard that this header can be decoded with SNR as low as -10dB, with a frequency error of 1% of the symbol rate. This header is transmitted in addition to the standard S2/S2X header, so terminals which can decode it can identify that a VL-SNR frame is being received.

Additional Pilots

In order to facilitate synchronization and channel estimation in the difficult conditions, additional pilots were added to the frame. These pilots are mandatory. The frame structure with pilots is given in the image below, is taken from DVB-S2X standard (ETSI EN 302 307-2 V1.1.1)

The segments designated with P are the pilot signals. The yellow ones appear each 1440 symbols, similar to a standard DVB-S2/S2X frame, and thus can be used by a DVB-S2 receiver. The additional ones are depicted in red, magenta or blue and contain a different number of symbols.

Dummy frames

In order to make it possible to maintain synchronization in case of long periods without data, dummy frames are inserted. The dummy frames are of the same length as the short VL-SNR frame, and thus can be interlaced within a standard DVB-S2/S2X stream as a normal one.

Implementation of VL-SNR within SX-3000

As describes the SX-3000 ASIC designed to comply with the entire DVB-S2X implementation. The ASIC is designed as a flexible software modem supported by hardware acceleration circuits for high rate computation-intensive operations. When implementing the VL-SNR mode, we faced the following challenge:

• Different frame structure
• Requirement for reliable acquisition
• Need to extend the operation range to even lower SNR values

To overcome the challenges our design focused on Flexible Segments Definitions and a very powerful Acquisition Module.

Extremely Low SNR Operation

SNR of -10dB is indeed rather low, but for some applications, it is still not low enough. By reducing the operational SNR, equipment size and cost can be reduced dramatically, thus opening the door for a plethora of use cases and applications. Unfortunately, considering the equation above, this can be achieved only at the expense of channel capacity. In this section, a waveform for Extremely Low SNR (EL-SNR) operation is described, with low data rates is described.

At levels of SNR of -20dB down to -30dB, the most effective coding is repetition. Hence the idea of symbol spreading can be further extended for operation at the low level of SNRs. The idea can be applied by repeating each symbol in a frame N times. At the receiver, an integration of N symbols would make it possible to increase the symbol energy by N thus enabling operation of the same modulation and coding at SNR lower by 10*log10(N) dB at the expense of N times reduction of the data rate.

Conclusions

Operation at low SNR values creates new opportunities for the satellite communications markets and enables new applications that could make use of the ubiquitous coverage provided by satellites using low cost appliances and terminals. The DVB-S2X standard has included a VLSNR mode that extends the operational SNR range down to -10dB. Satixfy’s SX-3000 ASIC that supports it facilitates the implementation of such low cost terminals. The DVB-S2X VL-SNR mode was highlighted, focusing on the main modifications compared to the other MODCODs defined by the standard, and also presented some key points in the implementation of the VL-SNR mode within the standard.
In addition a waveform for operation at even lower SNR has been introduced. This waveform, based on the idea of frame repetition, can make use of standard frames as building blocks while flexibly adapting itself to the requirement and operation conditions.

This paper is one of a series of SatixFy’s technical application for the SX-3000 SDR (Software Defined Radio) ASIC device. Authors:  Doron Rainish (doron.rainish@www.satixfy.com) and Avraham Freedman (avi.freedman@www.satixfy.com) both of: SatixFy Ltd., 12 Hamada st. Rehovot, Israel 74140, Tel: +972-89393210, Fax: +972-89393223.

*  This paper was previously published by: Yoel Gat and Yoram Ben-Ami, at the Ka conference, Bologna, Italy, 2015 (http://www.kaconf.org/2015/)