Chapter 18: MMDS and LMDS

MMDS (Multichannel Multipoint Distribution Service) is a Multichannel broadcast service that operates in the 2.1 to 2.9 GHz. Frequency range. It’s designed to be a POINT to MULTIPOINT broadcast service that is capable of delivering multiple channels of television programming for digital or analogue mode together with internet access, telephone and data transfer services to individual receive sites. Channels containing video, audio and data transmitter from a central site to individual residences, multiple dwelling units, and business locations. MMDS systems, which can be configured to offer just television are commonly referred to as “ Wireless Cable Television”.

HOW DOES MMDS SYSTEM         

                    WORK?

The architecture of a typical wireless cable system can be divided into two basic nodes: transmitting sites and receiving sites. Transmitting sites are the main or “HUB” node of MMDS system.

Programming information is generally delivered to an MMDS system from satellites, it can also be generated locally or come from pre-recorded video tapes, these signals are fed into discrete transmitters where they are modulated up converted and amplified. Transmitter powers range 1 to 200 watts per channel and 15 to 100 watts average power per digital channel, individual microwave channels are then multiplexed or combined together and are passed to a transmitting antenna via low loss coaxial cable or waveguide. A centrally located transmitting antenna radiates the MMDS signal to the desired receiving sites, generally in an omni directional (360 degrees) pattern.

LMDS (Local Multipoint Distribution System)

LMDS (Local Multipoint Distribution System) is a system for broadband microwave transmission direct from a local antenna to homes and businesses within a line-of-sight radius, a solution to the so-called last-mile problem of economically bringing high-bandwidth services to users. LMDS is an alternative to installing optical fiber all the way to the user or to adapting cable TV for broadband Internet service. Depending on the implementation, LMDS offers a bandwidth of up to 1.5 Gbps downstream to users and 200 Mbps upstream from the user. A more typical data rate is 38 Mbps downstream. Some services offer both downstream and upstream service (symmetrical service); others offer downstream only (asymmetrical service) with upstream being obtained using wire connections. 

In addition to the investment by service providers for transmitters, users need to install transceivers costing about $125-225. However, the cost of installing LMDS is considered far lower than installing fiber optic cable or upgrading cable TV systems. The first markets for LMDS are seen as:

·         High-speed data transmission for businesses

·         Interactive television and streaming multimedia from Web sites

·         Voice service (usually as a supplement to other services) 

Because LMDS requires a more expensive and possibly larger transceiver than can conveniently be packaged in a handheld device, LMDS is not viewed as a replacement for or alternative to mobile wireless technologies such as cellular and GSM. On the other hand, LMDS offers much higher data rates because of its use of a higher range of frequencies with their wider bandwidths. In general, LMDS is for fixed locations and offers higher data rates; cellular digital such as GSM is for mobile users at lower data rates.

References:

http://searchnetworking.techtarget.com

http://wikipedia.com

http://multi-session.net

Broadband Telecommunication Handbook, 2nd edition by Regis J. "Bud" Bates

Chapter 17: Microwave and Radio-based system

The term microwave refers to electromagnetic energy having a frequency higher than 1Gigahertz (billions of cycles per second), corresponding to wavelength shorter than 30 centimeters.

Microwave signals propagate in straight lines and are affected very little by the troposphere. They are not refracted or reflected by ionized regions in the upper atmosphere. Microwave beams do not readily diffract around barriers such as hills, mountains, and large human-made structures. Some attenuation occurs when microwave energy passes through trees and frame houses. Radio-frequency (RF) energy at longer wavelengths is affected to a lesser degree by such obstacles.

The microwave band is well suited for wireless transmission of signals having large bandwidth. This portion of the RF electromagnetic radiation spectrum encompasses many thousands of megahertz. Compare this with the so-called shortwave band that extends from 3 MHz to 30 MHz, and whose total available bandwidth is only 27 MHz. In communications, a large allowable bandwidth translates into high data speed. The short wavelengths allow the use of dish antennas having manageable diameters. These antennas produce high power gain in transmitting applications, and have excellent sensitivity and directional characteristics for reception of signals.

Uses of Microwave
Wireless transmission of information

  §  One-way and two-way telecommunication using communication satellite

  §  Terrestrial microwave radio broadcasting relay links in telecommunications networks        

including e.g. backbone or backhaul carriers incellular network linking BTS-BSC and BSC-MSC.

Wireless transmission of power

• Proposed systems e.g. for connecting solar power collecting satellites to terrestrial power grids

Properties

§  Suitable over line-of sight transmission links without obstacles

§  Provides large useful bandwidth when compared to lower frequencies (HF, VHF, UHF)

§  Affected by the refractive index (temperature, pressure and humidity) of the atmosphere, rain , snow and hail, sand storms, clouds, mist and fog, strongly depending on the frequency.

Advantage

·         The high frequency of microwaves gives the microwave band a very large information-carrying capacity; the microwave band has a bandwidth 30 times that of all the rest of the radio spectrum below it.

Disadvantage

·         Microwaves are limited to line of sight propagation; they cannot pass around hills or mountains as lower frequency radio waves can.

Microwave Transmission

      Microwaves are widely used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna. This allows nearby microwave equipment to use the same frequencies without interfering with each other, as lower frequency radio waves do.

      Microwave radio transmission is commonly used by communication systems on the surface of the Earth, in satellite communications, and in deep space radio communications. Other parts of the microwave radio band are used for radars, radio navigation systems, sensor systems, and radio astronomy.

       The next higher part of the radio electromagnetic spectrum, where the frequencies are above 30 GHz and below 100 GHz, are called "millimeter waves" because their wavelengths are conveniently measured in millimeters, and their wavelengths range from 10 mm down to 3.0 mm. Radio waves in this band are usually strongly attenuated by the Earthly atmosphere and particles contained in it, especially during wet weather. Also, in wide band of frequencies around 60 GHz, the radio waves are strongly attenuated by molecular oxygen in the atmosphere. The electronic technologies needed in the millimeter wave band are also much more difficult to utilize than those of the microwave band.

Radio

Radio is the transmission of signals through free space by modulation of electromagnetic wave with frequencies significantly below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic field that pass through the air and the vacuum of space.

Information is carried by systematically changing some property of the radiated waves, such as amplitude, frequency, phase, or pulse width. When radio waves pass through an electrical conductor, the oscillating fields induce an alternating current in the conductor. This can be detected and transformed into sound or other signals that carry information.

Radio frequencies occupy the range from a few hertz to 300 GHz, although commercially important uses of radio use only a small part of this spectrum. Other types of electromagnetic radiation, with frequencies above the RF range, microwave, infrared, visible light, ultraviolet, X-rays and gamma rays. Since the energy of an individual photon of radio frequency is too low to remove an electron from an atom, radio waves are classified as non-ionizing radiation.

References:

http://www.bssbook.com

http://en.wikipedia.com

http://searchnetworking.techtarget.com