Line of Sight Systems, TDM, FM, and Radio Wave Propagation

Line of Sight (LDV) Communication Systems

Key Telecommunication Organizations and Definitions

• Line of Sight System (LDV)
• International Telecommunication Union (ITU)
• International Radio Consultative Committee (CCIR)
• Consultative Committee International Telegraph and Telephone (CCITT)

LDV System Advantages

Line of Sight (LDV) systems offer several key benefits:

• Flexible, high channel capacity (ranging from a few voice channels to several TV channels).
• Expandability.
• Short installation time.
• Excellent adaptation to difficult terrain and natural barriers.

Applications of LDV Systems

LDV systems are utilized in various telecommunication scenarios:

• Fixed Telephony and Television: Used for integrated or multi-channel television and fixed telephony systems, both nationally and internationally.
• Non-Integrated Systems: Employed for fixed telephony and television that are not interconnected to the national or international network.
• Mobile Systems: Used in mobile systems, whether connected or not connected to national and international networks.

Multiplexing Techniques: TDM and FDM

TDM Advantages and Limitations

The main advantages of Time Division Multiplexing (TDM) are:

• Low cost and small size of terminal equipment.
• Immunity to amplitude nonlinearities in the link.
• Individual voice channels can be easily inserted and extracted.

TDM systems face limitations:

• TDM systems cannot interface with Frequency Division Multiplexing (FDM) systems of similar capacity.
• They do not allow the transmission of large groups of telephone channels, as they require very narrow pulses and wide bandwidth.

Frequency Modulation (FM) vs. Amplitude Modulation (AM)

There are specific reasons for preferring FM over amplitude modulation (AM):

1. Crosstalk Prevention: Transmission using FDM telephone multiplexing requires a highly linear relationship between the input and output amplitudes of baseband signals to prevent crosstalk. This linearity is more easily achieved with FM.
2. Signal-to-Noise Ratio (SNR): Using FM with a deviation ratio greater than unity results in substantial improvements in the signal-to-noise ratio.

However, the use of FM imposes restrictions on the uniformity of group delay and frequency characteristics of the intermediate frequency (IF) section and microwave components of the system to avoid crosstalk. In addition, levels of economic decoupling from the signal at feeders or in the path must not be excessive.

Radio Wave Propagation Phenomena

Refraction

Refraction is the change of direction of a ray as it passes obliquely from one medium to another, where the media have different speeds of propagation. The speed at which an electromagnetic wave propagates is inversely proportional to the density of the medium in which it is spreading.

Electromagnetic Reflection

Reflection occurs when an incident wave strikes a barrier between two media, and part of the incident power does not penetrate the second material.

Diffraction

Diffraction is defined as the modulation or redistribution of energy from a wave front as it passes near the end of an opaque object. Diffraction is the phenomenon that allows light waves or radio waves to propagate around corners.

Radio Wave Interference

Interference occurs when two or more electromagnetic waves combine, resulting in the degradation of system performance.