Introduction to satellite communication « Rageshwarrier’s Blog
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CDMA
On the average, a number of users occupy all of a transponder bandwidth all of the time.
• The signals are encoded so that information from an individual transmitter can be detected and recovered only by a properly synchronized receiving station that knows the code being used (“address”).
*Decentralized satellite network: only the pairs of stations that are communicating need to coordinate their transmissions.
*No frequency (as in FDMA) or time slot (as in TDMA) coordination with any central authority is required.
*Suited for military tactical communications environment, where many small groups of mobile stations communicate briefly at irregular intervals
CDMA is the application of a spread spectrum technique (which is the inverse of the bit packing technique, where as
many bits as possible are packed in 1Hz of bandwidth).
• One bit is spread over some or many hertz.
• 2 types of spread spectrum: direct sequence (DS, one bit is spread in phase), or frequency hop (FH, one bit is spread in frequency).
• In FH CDMA, multiple users can transmit on the same frequency with only some minimal interference one to
another, due to the powerful antijam properties of spread spectrum.
FDMA-Frequency Division Multiple Access
An earth station is permanently assigned a carrier frequency (or several) and a bandwidth around that
carrier. The station modulates all of its outgoing traffic (whatever the destination) on that carrier.
• An originating station’s traffic capacity is limited by its allocated bandwidth and the C/N that it can
achieve on the down-links.
• The carrier frequencies and bandwidths assigned to all the earth stations constitute a satellite’s frequency
plan.
MULTIPLE ACCESS
Multiple access is the “ability of a large number of earth stations to simultaneously interconnect their respective
voice, data, teletype, facsimile, and television links through a satellite”
• The basic problem involved is how to permit a changing group of earth stations to share a satellite in a way that
optimizes the
– satellite capacity – adaptability to multimedia traffic
– spectrum utilization – cost
– satellite power – user acceptability
– interconnectivity – flexibility
• There are three basic techniques: FDMA, TDMA, CDMA
In all the three classical multiple access techniques some resource is shared.
• If the proportion allocated to each earth station is fixed in advance, the system is called fixed access
(FA) or pre-assigned access (PA).
• If the resource is allocated as needed in response to changing traffic conditions, the multiple access
system is termed demand assignment (DA).
TURBO CODES
A turbo encoder is a combination of two simple encoders.
• The input is a block of K information bits sent uncoded.
• The two encoders generate parity symbols from two simple recursive convolutional codes, each with a small number
of states.
• Key innovation of turbo codes is an interleaver P which permutes the original K information bits before input to the
second encoder.
• The permutation P allows that input sequences for which one encoder produces low-weight (few non-zeroes)
codewords will usually cause the other encoder to produce high-weight (many non-zeroes) codewords.
CONVOLUTIONAL CODES
Sometimes called VITERBI codes because the algorithm for decoding was studied by Viterbi.
• Are generated by a tapped shift register and two or moremodulo-2 adders wired in a feedback network.
• The name is given because the output is the convolution of the incoming bit stream and the bit stream that represents the impulse response of the shift register and its feedback network.
• Each incoming information bit propagates through the shift register and influences several outgoing bits, spreading the information content of each data bit among several adjacent bits.
CODING TECHNIQUES
Coding techniques are applicable to satellite transmissions in order to improve the system performance.
• There are two basic alternatives to be explored when coding is considered: block coding and convolutional
coding.
• Before to treat them, we must introduce the concept of channel capacity and the Shannon bound.
BER of digital signals can be improved by the use of error detection techniques, that add redundant bits to
a data stream in such a way that an error in the data stream can be detected (error detection codes) and
corrected (forward error correction, FEC).
• Unless one redundant bit is added for every data bit, the exact position of a single bit error cannot be
determined.
• Usually one redundant bit is added for every N data bits, thus allowing a single error within that block of
N bits to be detected.
TYPES OF DATA CODINGS
- BLOCK CODES
- 8-bit ASCII coding Linear block codes
- Binary cyclic codes
- BCH (Bose-Chaudhuri-Hocquenghem) codes
- Golay codes
- Convolutional codes
- Punctured codes
- Turbo codes
DESIGN OF A COMPLETE SATELLITE LINK TO ACHIEVE A SPECIFIED PERFORMANCE (C/N) (LINK BUDGET)
The carrier power level at a receiving ES depends on the EIRP at the satellite, the ES antenna gain, and the path
loss, but it is reduced by any losses that occur along the way.
• For frequencies above 10 GHz rain causes attenuation of the carrier, and the attenuation rises rapidly with
frequency.
• In both analog and digital radio systems there is an almost linear relationship between the S/N ratio (or the
BER) at the demodulator output and the C/N at the demodulator input, provided that C/N is above a certain
threshold.
Performance objectives of the satellite link: must be
specified either in terms of minimum allowable S/N (or BER for a digital signal), or as a minimum allowable (not
lower than a certain value) C/N, which is generally specified for a percentage of time.
• Factors that contribute to noise in an ES receiving channel include:
– the receiver thermal noise
– losses in waveguides and waveguide components
– sky noise
– interference entering the ES receiving antenna
– interference entering the satellite receiving antenna
– thermal noise generated in the satellite
– intermodulation noise throughout the system
– many other sources of noise may exist in a particular system
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