08-09-2016, 10:01 AM
Design and Implementation of the 155Mbps Adaptive MODEM for
Broadband Satellite Communications
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Abstract
In this paper, we presented the design and implementation
of 155Mbps satellite Modem adaptive& compensated
against the rain attenuation. In order to compensate the
rain attenuation over high-speed satellite link, the adaptive
coding schemes with variable coding rates and the pragmatic
TCM that can be decoded both the QPSK and TC-
8PSK using same Viterbi decoder was studied and analyzed.
The pragmatic TCM with rate 213, selected to the
optimal parameters for implementation, was modeled by
VHDL in this paper. The key design issues are how to
achieve a high data rate and how to integrated into a single
ASIC chip various finctions such as the diflerent data
rates, Scrambler/descrambler, Interleaver, Encoder/decoder,
and BPSHQPSU8PSK modulator/demodulator.
The implemented 155Mbps adaptive
MODEM has the simpliJed interface circuits among the
many functional blocks, and parallel processing architecture
to achieve the high data rate. This 155Mbps adaptive
MODEM was designed and implemented by single ASIC
chip with the 0.25 CMOS standard cell technology.
INTRODUCTION
In high-speed information communication networks, the
satellite communications play an important role to implement
nation-to-nation network, emergency network, backup
network, backbone network, private network. National
backbone network like ATM satellite communications require
the high reliability(very low bit error rate). But due to
rain fading and other channel impairments in satellite channel,
especially Ka-band application of error correcting code
scheme is necessary. Furthermore, the trend of satellite
communication is transformed ftom narrow-band service to
broadband multimedia service. Therefore during poor
channel conditions, QPSK with (n,k,m) convolutional code
is employed in order to maintain the performance. As channel
conditions are improved, higher rate schemes such as
TC-SPSK(Trel1is Coded 8PSK) are used for satellite communications.
In this paper, we implemented the 155Mbps Adaptive Modem for the Broadband Satellite Communications.
Adaptive channel coding schemes are defined as
European standard[ 11 which is DVB(Digita1 Video Broadcasting)
specification. The Modem transmission frame is
synchronous with the MPEG-2 transport stream(TS). The
Modem was implemented BPSK, QPSK and 8PSK modulations,
and the concatenation of convolutional and ReedSolomon
codes. For SPSK, “pragmatic” trellis convolutional
code is able to be configured flexibly, allowing the
optimization of the system performance for a given satellite
transponder bandwidth. Satellite systems can be affected by
power limitations, therefore ruggedness against noise and
interference has been one of the design objectives of the
system. On the other hand, when larger power margins are
available, spectrum efficiency can be increased to reduce
the cost of the space segment. Therefore our adaptive modem
offers many transmission modes(inner coding and
modulations), giving different trade-offs between power
and spectrum efficiency. All the modes are appropriated for
operation in quasi-linear satellite channels. The specifications
of the 155Mbps adaptive modem are Table 1. Figure 1
shows a 155Mbps Adaptive Satellite Modem.
TRANSMITTER STRUCTURE
Randomization for energy dispersal
This modem input stream shall be organized in fixed length
packets, following the MPEG-2 TS. The total packet length
of the MPEG-2 TS is 188bytes. This includes 1 sync-word
byte(i.e.,47HEX). In order to ensure adequate binary transitions,
the data of the input MPEG-2 TS shall be randomized.
To provide an initialization signal for the descrambler, the
MPEG-2 sync byte of the first transport packet in a group
of eight packets is bit-wise inverted from 47HEX to
BSHEX. This process is referred to as the “Transport Multiplex
Adaptation”. Figure 2 shows a block diagram of the
Scrambler in this modem.
RECEIVER STRUCTURE
inner decoder
This unit performs first level error protection decoding. It
should operate at an input equivalent “hard decision” BER
in the order of between 10-1 and 10-2(depending on the
adopted code rate), and should produce an output BER of
about 2*10-4 or lower. This unit makes use of “soft decision”
information. This unit is in a position to try each of
the code rates and puncturing configurations until lock is
acquired. Furthermore, it is in a position to resolve phase
ambiguity.
Decoding process for rate 2/3(Pragmatic TCM)
As shown in figure 3, the decoder of pragmatic code consists
of high-speed Viterbi decoder, sector phase quantizer,
and soft decision logic. Sector phase quantizer is used for
decoding the uncoded bit and soft decision logic makes the
3 bit soft decision inputs of the Viterbi decoder, which allows
the decoder to accept not only a hard decision but also
a relative 3 bits weight indicating the likelihood that received
coded bit was a zero or a one. The Viterbi algorithm
is well suited to use this information , the use of soft decisions
will result in a performance gain of 2 dB over hard
decision. In adapting the binary decoder, the use of soft
decision is very important. The particular Viterbi decoder
used in this system accepts soft decision inputs on a scale of
0 thru 7, with a soft decision 7 indicating the strongest binary
1, and a soft decision 0 indicating the strongest binary
0. With this in mind, the signal vector space is quantized,
and a pair of soft decisions (one for each code bit), are assigned
to each quantization point. Through simulation, the
soft decision assignments of figure 4 were found to yield the
best performance among reasonable alternatives.