CODE DIVISION MULTIPLE ACCESS

CODE DIVISION MULTIPLE ACCESS

•NO restriction on time or bandwidth•Sometimes referred to as “Spread-spectrum Multiple Access”•Transmissions can spread throughout the entire allocated bandwidth

CHIPSETA unique binary word that each earth station’s transmission are encoded.Each station has a unique chip code.To receive a particular earth station’s transmission, a receive station must know the chip code for that station.

Code Division Multiple Access ENCODER

DECODER

If half the bits within a code were made the same and half were made exactly the opposite, the resultant would be zero cross correlation between chip codes. Such a code is called an ORTHOGONAL CODE.The primary difference between spread-spectrum PSK transmitters and other types of PSK transmitters is the additional modulator where the code word is multiplied by the incoming data. Because of the pseudorandom nature of the code word, it is often referred to as pseudorandom noise (PRN).

The PRN must have a high autocorrelation property with itself and a low correlation property with other transmitter’s pseudorandom codes.The code word rate must exceed the incoming data rate by several orders of magnitude. The code rate must be statistically independent of the data signal. When these two conditions are satisfied, the final output signal spectrum will be increased by a factor called the Processing Gain.

Processing gain is expressed mathematically as :

3 popular techniques used to produce the spreading function

Direct SequenceFrequency HoppingHybrid Direct-sequence Frequency Hopping (hybrid-DS/FH)

Direct Sequence Spread Spectrum (DS-SS)Is produced when a bipolar data-modulated signal is linearly multiplied by the spreading signal in special balanced modulator called SPREADING CORRELATOR. The spreading code rate or:Rcw = 1/TCWhere: Tc = the duration of single bipolar pulse

Gold CodesThe spreading (chip) codes used in spread-spectrum systems are either maximal-length sequence codes, sometimes called m-sequence codes or Gold codes.Invented by Magnavox Corporation in 1967.

Frequency-Hopping Spread Spectrum

Is a form of CDMA where a digital code is used to continually change the frequency of the carrier.The carrier is first modulated by the data message and then up-converted using a frequency-synthesized local oscillator whose output frequency is determined by an n-bit pseudorandom noise code produced in spreading code generator.

Channel Capacity

2 methods used to interface terrestrial voice-band channels with satellite channels:

Digital Noninterpolated Interface (DNI)Digital Speech Interpolated Interfaces (DSI)

Digital Non interpolated Interface

Assigns an individual terrestrial channel (TC) to a particular satellite channel (SC) for the duration of the call.Once a TC has been assigned an SC, the SC is unavailable to the other TCs for the duration of the call.DNI is a form of preassignmentEach TC has a permanent dedicated SC.

Digital Speech Interpolated Interfaces (DSI)

Assigns a terrestrial channel to a satellite channel only when speech energy is present on the TCDSI is a form of demand assignmentSCs are randomly assigned on an as-needed basisTC:SC ratio is 2:1

Time-Assignment Speech Interpolation (TASI)

Is a form of analog channel compression that has been used for sub oceanic cables for many years.TASI is very similar to DSI except that the signals interpolated are analog rather than digital

Satellite Radio Navigation

NAVIGATIONIs the art of science of plotting, ascertaining or directing the course of movements, in other words, knowing where you are and being able to find your way around.The most ancient and rudimentary method of navigation is WANDERING.

Navigation

WANDERING

It is the most ancient and rudimentary method of navigation and simply continuing to travel about until you reach your destination, assuming of course that you have one.

Navigation

CELESTIAL NAVIGATION

Earliest effective means of navigation wherein direction and distance are determined from precisely time sightings of celestial bodies, including the stars and moon.

Navigation

PILOTING

Method of navigation by means of fixing a position and direction with respect to familiar, significant landmarks such as railroad tracks, water towers, barns, mountains and bodies of water.

Navigation

Dead Reckoning

A navigation technique that determines position by extrapolating a series of measures velocity increments.The term “dead” is derived from the word “deduced”

Navigation

Dead Reckoning

A navigation technique that determines position by extrapolating a series of measures velocity increments.The term “dead” is derived from the word “deduced”

Navigation

Charles Lindbergh

He used dead reckoning successfully in 1927 during his historic 33-hour transatlantic journey

Navigation

Amelia Earhart

She attempted to make the first around-the-world flight in 1937 using the dead reckoning technique.

Navigation

Radio or Electronic Navigation

Navigation technique wherein the position is determined by measuring the travel time of an electromagnetic wave as it moves from a transmitter to a receiver

Navigation

DECCA - A radio navigation system for terrestrial surface broadcast

OMEGA - Radio navigation system that provide global coverage and terrestrial surface broadcast

LORAN – Also a terrestrial surface broadcast

Navy Transit GPS – Low-orbit satellite broadcast, provides global coverage

Navstar GPS – Medium-orbit satellite broadcast also provides global coverage

Navigation

LORAN (Long Range Navigation)

The most effective, reliable and accurate means of radio navigation.Means of radio navigation in which receivers acquire coded signals from two pairs of high-powered, land based transmitters whose location are precisely knownLORAN-A was developed during World War IILORAN-C is the most recent version, surfaced in 1980

Navigation