Spread Spectrum TECHNOLOGY
BACKGROUND - MINIATURISED VSAT
One basic limitation of satellite antenna aperture using traditional modem technology has been the amount of energy emitted off axis (off-axis EIRP) as the antenna aperture is reduced. This is because as the antenna aperture is reduced (all other things being equal) the antenna directivity (beam-width gets wider).
The off-axis EIRP is important in maintaining proper (geo-stationary) inter satellite co-ordination, as the off-axis energy will inadvertently create interference on adjacent satellites. The limits of off-axis emission are specified in terms of Power Spectral Density (PSD) at a given angle off antenna bore-sight (EIRP PSD). In other words, the amount of energy in a given bandwidth is the key concern when attempting to reduce the antenna aperture (which will increase off-axis gain).
In order to overcome this issue whilst still using a small antenna, the approach as developed by THISS and implemented in our satellite modems is to apply a spread spectrum waveform. In effect this reduces the PSD of a modulated signal by spreading the desired energy over a wider bandwidth (hence spread-spectrum).
MULTIPLE ACCESS TECHNIQUES
The THISS modem employs a combination of CDMA, TDMA and FDMA for inbound (terminal to hub) transmission. That is, a unique radio resource is specified as a unique combination of spreading code, time slot and frequency allocation. This allows the satellite bandwidth to be shared amongst users (terminals) in a flexible way.
For use with small satellite antennas, the inbound transmission may require the use of a spread spectrum waveform to meet off-axis EIRP constraints as described above. This bandwidth can used (shared) by multiple terminals in the time domain (TDMA) and in the code domain (CDMA).
TIME DIVISION MULTIPLE ACCESS
TDMA allows a separation of radio resource in the time domain. Time is divided into timeslots, and a terminal can transmit a (spread spectrum) burst into a timeslot. Terminals transmitting with the same frequency and code, will not interfere with each other if transmitting into different timeslots.
CODE DIVISION MULTIPLE ACCESS
CDMA describes a multiple access scheme where the modulated signals of each individual user are overlapping both in the time and frequency domains (referred to as simultaneous users).
This is achieved by the assignment of a unique signature code to each of the users, which is applied in the transmitter in the process of transforming a traditional narrowband modulated signal into a spread spectrum signal. This code is synonymously referred to as a spreading code or spreading sequence.
With knowledge of the unique spreading sequence of an individual user, the signal of that particular user can be retrieved at the receiver. As additional simultaneous users are overlaid on the same frequency (in the same timeslot), the signal quality of each individual user is marginally degraded, due to the imperfect code division statistical separation of each. This is known as Multiple Access Interference (MAI). This allows the possibility in a CDMA scheme to trade-off signal quality (perceived Eb/No / BER / link margin) with the amount of users sharing a segment of bandwidth at the same time.
FREQUENCY DIVISION MULTIPLE ACCESS
FDMA allows a separation of radio resource in the frequency domain. The available spectrum for the system is divided into multiple bands (carrier frequencies). Terminals transmitting into the same timeslot, with the same code, will not interfere with each other if transmitting on different frequencies.
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