The inter-modulation products generated within the satellite transponder as a result of the amplification of multiple carriers by the TWTA that exhibits both amplitude non-linearity and phase nonlinearity cause inter-modulation interference. In order to avoid excessive modulation interference the TWTA must be operated with a large output back off and this will result in downlink-limited system. This in turn reduces the transponder’s capacity. The inter-modulation interference once calculated can be used as an additional interference source to obtain the total carrier to noise and interference ratio.

Inter-modulation occurs when a multi-carrier signal passes through a device that exhibits nonlinearity characteristics. The most commonly used high power amplifier TWTA exhibits both amplitude and phase non-linearity’s, loading to the occurrence of inter-modulation products. The effect of inter-modulation products caused by amplitude nonlinearity is more dominant than that caused by phase non-linearity.

Inter-modulation distortion occurs when two or more frequencies mixed across a non-linear device. The resultants are either the difference frequency, or the sum frequency. A mixing of two discrete frequencies can generate undesirable frequencies. Spurious radiation, arising from close spacing of transmitter and receiver, is a prime source of undesirable frequency that can cause inter-modulation distortion in an electronic circuit. This is particularly the case when the antenna couplers are employed. Cross modulation and parasitic generation are two other sources of undesirable frequencies that may cause inter-modulation distortion.

Cross Modulation occurs when the signal from the adjacent channel crosses over into a second channel that modulates the frequency of the second channel.Parasitic Generation occurs when regenerative feedback is sufficient to cause a circuit to oscillate, even though it is not designed to oscillate. Both types of distortion are common in systems that are misaligned.

Inter-symbol Interference:

The inter-symbol interference is generated within the channel itself, as a result of filtering and non-linear characteristics of the satellite at TWTA, which is operated normally close to saturation especially in single carrier per transponder satellite systems.

For a linear channel were bandwidth is available for transmitting data at a rate of R bits per second is between R/2 and R hertz, inter-symbol interference can be eliminated by the use of Nyquist pulse shaping criteria. For a nonlinear satellite channels, such interference cannot be eliminated and the degradation of CNR results.

Terrestrial Interference:

The terrestrial microwave links are allocated the 6/4 GHz frequency bands the same as given to satellite communications. In areas of heavy population density the terrestrial link congestion can be so great that it may be impossible to locate an earth station. Since the uplink and downlink frequencies of the earth station are 6 and 4 GHz respectively it is susceptible to interference to interference from terrestrial microwave transmission and reception.

The interfering power within the bandwidth of the satellite signal received by the earth station depends on the spectral density of the terrestrial interfering carrier. The interference between the earth station and a terrestrial microwave system is a function of the carrier power, the carrier spectral density and the frequency offset between the two carriers. For a broadband satellite carrier bandwidth includes the interfering carrier frequency, the interfering carrier power is used. For a narrow band satellite carrier, the interference reduction factor is the ratio between the total carrier power and the power in the narrow bandwidth.

For a narrow transmit earth station the amount of interference is determined by by the frequency separation between the interfering carrier frequency and the terrestrial carrier frequency. The spectral densities of the interfering carrier and the terrestrial carrier can obtain the interference reduction factor.

Cross – Polarization Interference:

The coupling of energy from one polarization state to the other orthogonal polarization state results in the cross polarization interference. The frequency reuse satellite employs orthogonal linear and orthogonal circular polarizations, which encounters another major source of interference called cross polarization interference. This result from the finite cross polarization discrimination of the earth station and satellite antennas and by the depolarization caused by the rain.

The cross polarization discrimination is defined as the ratio of the power received in the principal polarization to positive power received in the orthogonal polarization from the same incident signal. In other words it represents the carrier-to-cross polarization interference ratio when the two polarized signals have the same power. The net cross polarization discrimination of the satellite link is the combined effect of the earth station and satellite and satellite antennas for both uplink and downlink.

Adjacent Channel Interference:

In band-limited satellites the adjacent channel interference is more common. This type of interference arises when some of the energy of a band limited signal falls into adjacent channel because of the overlapping amplitude characteristics of the channel filters. The interference arises when some of the energy of a band limited signal falls into adjacent channels because of the overlapping amplitude characteristics of the channel filters.

The adjacent channel interference is more severe in a single channel per transponder TDMA system using QPSK modulation where the earth station HPA and/or the satellite TWTA operates near or at saturation. The nonlinear characteristics this power amplifier regenerates the side lobes of the filtered QPSK spectrum and thus carries interference into adjacent channel. This phenomenon is termed as spectrum spreading.