A study conducted by US National Cancer Institute found that employees in seven industries in eastern US who were exposed to microwave signals at workplace for about 20 years experienced a ten fold increase in brain tumors . FCC’s official position FCC claims that EMR exposure on ground level from base stations is safe. Cellular and Personal Communications Service (PCS) base stations in the US are required to comply with the RFR limits that are recommended by expert organizations and government agencies responsible for health and safety.
Also when antennas are mounted on rooftops, the Radio Frequency (RF) exposure levels on the roofs are higher than those on the ground. However, FCC states that unsafe RF level would be encountered only if the person is very close to or directly in front of the antenna. FCC admits that it is not able to monitor the emissions from all the transmitters that are under the FCC jurisprudence. However, it does have measurement instrumentation for evaluating RF levels in areas that may be accessible to the public or workers. It also conducts an investigation if there is enough evidence to indicate a lack of compliance with the FCC laws .
The Malaysian Communications and Multimedia Commission (MCMC) Guidelines on permissible radiation levels, released in August 1998, are based on international standards developed to minimize the possible impact of radiation on health. The MCMC continues to monitor the latest safety standards recommended by world organizations such as the International Commission on Non-ionizing Radiation Protection (ICNIRP) and the World Health Organization (WHO) to ensure that the guidelines adopted for Malaysia remain current . Cellular System Infrastructure
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Infrastructure-based wireless networks have base stations, also called access points, deployed throughout a given area. These base stations provide access for mobile terminals to a backbone wired network. Network control functions are performed by the base stations, and often the base stations are connected together to facilitate coordinated control. This infrastructure is in contrast to ad hoc wireless networks, described in Chapter 16, which have no backbone infrastructure. Examples of infrastructure-based wireless networks include cellular phone systems, wireless LANs, and paging systems.
Base station coordination in infrastructure-based networks provides a centralized control mechanism for transmission scheduling, dynamic resource allocation, power control, and handoff. As such, it can more efficiently utilize network resources to meet the performance requirements of individual users. Moreover, most networks with infrastructure are designed so that mobile terminals transmit directly to a base station, with no multihop routing through intermediate wireless nodes. In general these single-hop routes have lower delay and loss, higher data rates, and more flexibility than multihop routes.
For these reasons, the performance of infrastructure-based wireless networks tends to be much better than in networks without infrastructure. However, it is sometimes more expensive or simply not feasible or practical to deploy infrastructure, in which case ad hoc wireless networks are the best option despite their typically inferior performance . Frequency-multiple access (TDMA)">division multiple Access (FDMA) In frequency-division multiple access (FDMA), the available bandwidth is divided into frequency bands. Each station is allocated a band to send its data.
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In other words, each band is reserved for a specific station, and it belongs to the station all the time. Each station also uses a band pass filter to confine the transmitter frequencies. To prevent station interferences, the allocated bands are separated from one another by small guard bands . Time Division Multiple Access (TDMA) In time-division multiple access (TDMA), the stations share the bandwidth of the channel in time. Each station is allocated a time slot during which it can send data. Each station transmits its data in is assigned time slot.
The main problem with TDMA lies in achieving synchronization between the different stations. Each station needs to know the beginning of its slot and the location of its slot. This may be difficult because of propagation delays introduced in the system if the stations are spread over a large area. To compensate for the delays, we can insert guard times. Synchronization is normally accomplished by having some synchronization bits (normally referred to as preamble bits) at the beginning of each slot . Code Division Multiple Access (CDMA) Code-division multiple access (CDMA) was conceived several decades ago.
Recent advances in electronic technology have finally made its implementation possible. CDMA differs from FDMA because only one channel occupies the entire bandwidth of the link. It differs from TDMA because all stations can send data simultaneously; there is no timesharing . Advantages FDMA CDMA TDMA N stations can transmit in parallel .
There is no need for time synchronization between the N transmitters All terminals can use the same frequency, no planning needed Huge code space (e. g. 232) compared to frequency space Interferences (e. g. white noise) is not coded forward error correction and encryption can be easily integrated It is easier to achieve asymmetric bandwidth assignments in TDMA than in FDMA: using multiple time-slots is much simpler than transmitting on multiple frequencies in parallel TDMA tends to have better transmission delays than FDMA can be easily derived from simple queuing theory The protocol does not require tunable receivers Disadvantages FDMA CDMA TDMA Need for tunable transmitters and receivers =) increased complexity A station gets the bandwidth of only one channel – when more is wanted multiple transceivers are required
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If one station does not transmit, its sub-channel cannot be used by other stations – this prevents statistical multiplexing Tight time-synchronization between stations required High expected access delay even in otherwise idle systems Higher complexity of a receiver (receiver cannot just listen into the Medium and start receiving if there is a signal) all signals should have the same strength at a receiver Types of Antenna Omni-directional antennas The omnidirectional antenna radiates or receives equally well in all directions. It is also called the “non-directional” antenna because it does not favor any particular direction.
An omnidirectional antenna has four cardinal signals. This type of pattern is commonly associated with verticals, ground planes and other antenna types in which the radiator element is vertical with respect to the Earth’s surface. The omnidirectional characteristics of radio waves make them useful for multicasting, in which there is one sender but many receivers. AM and FM radio, television, maritime radio, cordless phones, and paging are example of multicasting . Directional Antennas Directional Antennas are designed for use on point-to-point links, or as client antenna in point-to-multipoint applications.
Usually, they have the narrowest possible beamwidth and significantly higher gain than other antenna types. Essential rule to be applied is the higher the gain, the lower the beamwidth. Directional Antennas are usually constructed in form of grid or parabolic dish antennas . Unidirectional Antennas Microwaves are unidirectional. When an antenna transmits microwave waves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. The unidirectional property has an obvious advantage. A pair of antennas can be aligned without interfering with
another pair of aligned antennas . They are used in cellular phones, satellite network, and wireless LANs. Adaptive array In the last fifty years, extensive studies have been carried out worldwide in the field of adaptive array systems. However, far from being a mature technology with little research left to tackle, there is seemingly unlimited scope to develop the fundamental characteristics and applications of adaptive antennas for future 3G and 4G mobile communications systems, ultra wideband wireless and satellite and navigation systems, and this informative text shows you how! 1.
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Provides an accessible resource on adaptive array fundamentals as well as coverage of adaptive algorithms and advanced topics 2. Analyses the performance of various wideband beam forming techniques in wideband array processing 3. Comprehensively covers implementation issues related to such elements as circular arrays, channel modeling and transmit beam forming, highlighting the challenges facing a designer during the development phase 4. Supports practical implementation considerations with detailed case studies on wideband arrays, radar, sonar and biomedical imaging, terrestrial wireless systems and satellite communication systems 5.
Includes examples and problems throughout to aid understanding 6. Companion website features Solutions Manual, Mat lab Programs and Electronic versions of some figures Adaptive Array Systems is essential reading for senior undergraduate and postgraduate students and researchers in the field of adaptive array systems. It will also have instant appeal to engineers and designers in industry engaged in developing and deploying the technology. This volume will also be invaluable to those working in radar, sonar and bio-medical applications . Advantages 1.
Both beam smart and adaptive arrays provide high efficiency and thus high power for the desired signal. When a large number of antenna elements are used at a higher frequency, Beam Smart antennas use narrow pencil beams. Thus high efficiency is obtained in the direction of the desired signal. If a fixed number of antenna elements are used the same amount times the power gain will be produced with the help of adaptive array antennas. 2. Another advantage is in the amount of interference that is suppressed. Beam smart antennas suppress it with the narrow beam and adaptive array antennas suppress the interference by adjusting the beam pattern.
Disadvantages Cost The cost of such a device will be more, not only in the electronics section, but also in the power. That is the device is way too expensive [especially if MIMO methods are used. ], and will also decrease the life of battery of mobiles. The receiver chains that are used must be reduced in order to reduce the cost. Also the costs rise up due to the RF electronics and A/D converter used for each antenna. Size For this method to be efficient large base stations are needed. This will increase the size. Apart from this multiple external antennas are needed on each terminal.
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This is not practical. But companies re trying methods like dual polarization to reduce the size. Diversity When multiple mitigation is needed, diversity becomes a big problem. The terminals and base stations must have multiple antennas. There are mainly three types of diversities. They are spatial, polarization, and angle. Spatial separation of the antennas that are used is practically impossible when it is applied on mobile phones. It is also difficult to be achieved in point-to-point systems where a near line-of-sight exists between the transmitter and receiver.
By using polarized diversity, the above problem can be avoided to a certain point. Dual polarization can be easily instigated without the use of spatial separation. Angular diversity is the most commonly used method nowadays. The signals, which have the maximum signal power, are selected from multiple beams and are used to maintain diversity. But the gain depends on the angular spread. That is, if the spread is small, the diversity will also be small. •Tracking •Spatial-temporal processing •Hooks in international •Standards to include provisions for smart antennas.
Vertical integration 4th Generation A descendant to 2G and 3G technologies aiming to provide the very high data transfer rates. 4G technology provides very speedy wireless Internet access to not only stationary users but also to the mobile users. This technology is expected to trounce the deficiencies of 3G technology in terms of speed and quality. 4G can be best described in one word “MAGIC”, which stands for Mobile multimedia Anytime Anywhere Global mobility support, integrated wireless and personalized services. Technology used in 4G
4G is a multipurpose and versatile technology hence it can utilize almost the entire packet switched technologies. It can use both orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA).
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OFDM mechanism splits a digital signal into different narrowband and frequencies. The reason why 4G make use of this technology lies in its ability to minimize the intervention among symbols and channels associated to data streaming. 4G is also capable of using multiple inputs / multiple output technology (MIMO).
This antenna technology is used to optimize the data speed and reduce the errors in the networks. Universal Mobile Telecommunication Service (UMTS), which is basically a broadband 3G technology, is also a part of 4G. This broadband technology transfers data in the form of frames or packets. Hence it is capable of carrying voice, video, text and other types of multimedia datagram with the speed of 2Mb. UMTS is part of 4G because it can enables 4G to make use of international mobile phone roaming via using GSM (Global system for Mobile Communications).
Another wireless telecommunication technology known as time division synchronous code division multiple accesses (TD-SCDMA) provide support to 4G to transfer both circuit switched data like video and voice and packet switched data . During my research on the Internet and other resources, I found out that 4th generation network is still growing and was only available very few places. At the moment they cover Bandar Sunway, Shah Alam, Putrajaya, Danau Kota, Jalan Imbi, Jalan Pudu, Bangsar, Jalan Tun Razak, KLCC, Subang.