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Antenna is an electronic device mostly known as transducer that is designed to receive and transmit electromagnetic waves and can be very effective in the telecommunication systems. It was the most neglected section of all the personal communication systems. But with the rapid evolution of the technology, communication is expected to be transmitted and received quickly and with fewer errors whether it is through wireless networks or otherwise, thus antennas can solve this problem.
Actually it is not the Antennas which are smart but the smart system that makes it work are smart. Smart antennas consists of many other antennas which when combine together would produce or receive signals faster and more efficient. Normally this set of antennas is called MIMO which means multiple inputs -multiple outputs. . Beamforming is a technique where signals are processed and send to the direction of the receiver or vice-versa. The signal is either a fix pattern or adaptive pattern that is modified to make maximum usage of signal.
Smart Antennas can be referred as a new technology which is used with the mobile communication, but in fact the first Smart Antennas were introduced to use by the government in the military application in 1960s. The transmission of the signal used directed beams to avoid environmental interferences such as noise and at the same time hiding transmitted data from enemies. During those days the antennas structure was very large and it was time-consuming to calculate the exact transmission rate and correctness of the signal.
In 1987, the concept of wireless communication has been invented by Marconi and since then there has been an extensive and continuing increase in the use of mobile communication. Newer technologies and approach were explored and deployed and when the latest one that is the Space Division Multiple Access (SDMA) has been emerged it brings a promising future to the wireless communication. SDMA is based on the use of Smart Antennas and researchers have worked on this technology to apply it on personal communication.
Types of Smart Antennas
There are two main types of Smart Antennas
Switched beam is the simplest structure of Smart Antennas; it forms fixed multiples beams to serve users in a predefined way. The signal gives its best performance by switching from one beam to another when the user of the mobile moves through their cell. The table below shows how this approach works:
But the problem with this beam occurs is the user is not in the middle of the main beam.
The other type is the Adaptive Array Antennas. It is the most advanced approach of the Smart Antenna System as it automatically adjusts the beam with the condition of environmental factor that is the movement of the user and any interferers. Furthermore it also identifies, track and reduce any interfering signals and it maximize the signal radiation in the direction of the mobile user. The adjustment is attained the incoming signal is multiplied with complex weights and then summing them together to obtain the desired radiation pattern.
Categories of Smart Antennas
The Smart Antennas fall into three categories SIMO, MISO, and MIMO.
Single-Input and Multiple-Output (SIMO) uses a single antenna for transmission to be received by multiple antennas in wireless communication. A benefit of this category is that the antennas are combined to reduce errors and improve the transmission distance but in some cases this gives rise to problem with multipath effect. As discussed in SearchMobileComputing, 2005 website when an electromagnetic field (EM field) is met with obstructions such as hills, canyons, buildings, and utility wires, the wavefronts are scattered, and thus they take many paths to reach the destination. The late arrival of scattered portions of the signal causes problems such as fading, cut-out (cliff effect), and intermittent reception (picket fencing). In digital communications systems such as wireless Internet, it can cause a reduction in data speed and an increase in the number of errors. Thus two or more antennas as transmitted can solve this problem.
Multiple-Input, Single-Output (MISO) means that there are multiple antennas to transmit but a single antenna to receive. This has practically the same benefits and limitations as that of SIMO, but this technology has widespread application in Digital TeleVision (DTV), Wireless Local Area Networks (WLANs), Metropolitan Area Networks (MANs) and mobile communications.
Multiple-Input, Single-Output (MIMO) used multiples antennas for transmitters and as well as receiver for wireless communication. Antennas which were not a great subject of discussion recently become the most important category in wireless communication. Especially when Researchers finally proved that with MIMO antennas system there are lots of advantages such as greater receiver gain, increased data rates, larger network throughput, and improved reliability through antenna diversity. IEEE 802.11n wireless standard which is expected in 2009 uses MIMO to increase maximum speed to 100 Mbps and beyond, double the 802.11a and 11g wireless standards.
This article was extracted from Techworld.com which clearly shows the use of MIMO system in wireless communication. Cox, 2008 says that “MIMO has made its way into chipsets and could be in WLAN devices by year-end. The 3rd Generation Partnership Project, a collaboration of telecom standards groups, is also evaluating MIMO techniques for use in cellular networks. MIMO doubles the spectral efficiency compared with that of current WLANs. So far, the only company with MIMO chipsets is Airgo Networks, which launched its products in August 2003. MIMO is unique because it multiplies bandwidth by essentially providing multiple channels between devices, says Ben Manny, director of the radio communications laboratory in Intel’s corporate technology group.”
MIMO can be classified into three categories Precoding, Spatial Multiplexing and Diversity Coding. Precoding is multi-layer beamforming to support MIMO radio system. It is used for single-user MIMO or multiple-user MIMO in a linear or non-linear algorithm. Spatial multiplexing is a way to transmit and separate high rate signal into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. Diversity methods is a single stream (unlike spatial multiplexing) to transmit data and is coded using techniques called space-time coding.
Overview Architectures of Smart Antenna (MIMO)
One of the biggest problems in wireless telecommunication is fading and inter-symbol interferences as shown in the diagram, below. Fading refers to the distortion of intensity over certain propagation media and Inter-symbol interference (ISI) occurs when there is reflection caused by other remote objects. The transmitted signal, at the receiver end are overlapped and delayed. To overcome this problem we use MIMO architecture, Orthogonal Frequency-Division Multiplexing (OFDM) modulation and Low-Density Parity-Check (LDPC) coding.
MIMO operate in two modes diversity mode and spatial multiplexing mode to solve the problem.
The use of two or more antennas that are spaced sufficiently apart such that they can receive signals from independent signal paths are involve in a Simple Receive Diversity. A basic way to select an optimal receive antenna from an array of antennas is Selection Combining, whereby the receiver switches to another antenna whenever it detects weak signals or a high noise level from the current receiving antenna. More sophisticated techniques such as Maximum Ratio Combining (MRC) receive on multiple antennas simultaneously and apply advanced signal processing algorithms to combine the different versions of the received signals to maximize SNR and minimize receive errors. Selection Combining and MRC can be implemented on just the receive side of the link as shown in the figure below. (Video54, 2008, p3)
Spatial Multiplexing Mode
In rich multipath environments with multiple uncorrelated signal paths as figure 5, Spatial Multiplexing (SM) allows the sender to transmit different portions of the user data on multiple paths in parallel to increase capacity. The target receiver must implement a corresponding de-multiplexing algorithm to recover the original information stream from multiple antennas. In an ideal multipath environment, SM can increase the capacity of a single frequency channel linearly with the number of transmit antennas used. (Video54, 2008, p4)
Figure 5: Spatial diversity (Video54, 2008, p4)
Spatial Multiplexing requires the same multiplexing algorithm on both sides of a communications link. Therefore it is not interoperable with existing 802.11a/b/g devices. Until 802.11n is defined, only SM client and SM network devices from the same vendor can communicate with each other. (Video54, 2008, p4)
In contrast, Selection Combining and Maximum Ratio Combining are diversity techniques that can be implemented on just one side of a communications link; therefore they can benefit all existing 802.11a/b/g devices even when diversity is only implemented on the Access Point. (Video54, 2008, p4)
Thus the above explanation shows how MIMO which is being a category of Smart Antennas is designed.
Benefits/Limitations of Smart Antennas
Smart Antennas System, being one the latest technology in progress has brought lot of benefits. It serves as a dual purpose that is while enhancing the signal quality; it also increased the reuse of the frequency. The benefits are in more details below:
As Smart Antennas receive signal from several antennas, this increase the power for higher gain of signal. The Antennas also focuses on the communication device which allows the range of operation and the service by the antennas to increase. Thus reducing the cost by lowering the amplifier costs, power consumption and result in a higher reliability. To be more precise, with Smart Antennas the network providers will not require as many antennas/base stations to provide coverage. (Lehne et al. 1999, p. 5)
Furthermore, Smart Antennas provide the facility to reuse the frequencies which can increase the number of users. This also add to the factor of cost reduction, that is if more users are on the same frequency there will be less operating cost for purchasing the frequency space. Interference will also be suppressed with the introduction of the Antennas system. The transmission pattern will be generated directly as before it was radiated from all the direction and increases interferences. Thus the suppression will also increase the ability to reuse and achieve a better coverage.
With Smart Antennas signal are transmitted while targeting the communication device which can also be used to determine the accurate geographic information of the device. This can allow network providers to offer new services to the devices for example guiding emergency services to the location or the locality information. This system also increases security, as radiation of the signal is direct and not as in the traditional way when it was omni-directional. If someone wants to intercept the transmissions they would need to be at the same location or between the two communicating devices. (Gadh et al. 2003)
Moreover, with the frequency reuse there in an increase in the bandwidth as Smart Antennas provide diversity. The adaptive array utilize multi-path signal to reach a device and reduce the effective delay spread of the channel and allowing higher bit rates to be supported without an equalizer. And as Smart antennas are not a new protocol or standard, it can easily be integrated with existing non smart antennas and devices.
As all other system, Smart Antennas also has some constraints. Lots of faults or problems can occur which can be very hard to diagnose as Smart Antenna is much more complicated than the traditional one (Lehne et al. 1999, p. 5). And also though there will be a reduction in the cost factor with the frequency reuse when the system is already implemented but while implementing this complex system, they are far more expensive than the traditional antenna. Furthermore due to the antenna arrays which are utilized by smart antenna systems, they are much larger in size than traditional systems. This can be a problem in a social context as antennas can be seen as ugly or unsightly. (Lehne et al. 1999, p.6) Smart Antennas need to have the best location to operate because of the direct radiation and not as the traditional one. For example in a road context, smart antennas are better situated away from the road, unlike normal antennas which are best situated along the road. (Lehne et al. 1999, p. 6)
Need of Smart Antennas
Smart Antennas has been required to accommodate with the future of mobile communication that is to significantly improve the performance of wireless performance and increase the number of users. So Spatial Division Multiple Access (SDMA) which is a satellite communication mode has exploited this technology to optimize the use of radio spectrum and minimized cost. SDMA also known as SDM (spatial-division multiplex) has also used Smart Antennas or adaptive array to adjust dynamically with the changing traffic and the user requirements by providing frequency reuse and antennas that are highly directional, unlike the previous wireless system like FDMA, TDMA or CDMA.
Frequency Division Multiple Access (FDMA) was the first generation of wireless networks communication. FDMA technique was a basic technology in analog that was used by the Advanced Mobile Phone Service (AMPS) and was the most widely technology installed in mobile phone in North America. FDMA assigned each channel to only one user at a time and is also used in the Total Access Communication Systems (TACS). But The Digital-Advanced Mobile Phone Service (D-AMPS) which also uses FDMA adds TDMA to get three channels for each FDMA channel, tripling the number of calls that can be handled on a channel. (Sanchez, 2003)
Time Division Multiple Access (TDMA) was first used BY D-AMPS then Global System for Mobile communications (GSM) in 2G cellular system, and Personal Digital Cellular (PDC). Each of them implements TDMA differently and in incompatible way (Search Networking, 2006). TDMA is a channel access method to share medium network and used in digital mobile communication dividing signal into three time slots in order to increase the amount of data that can be carried. It also allows multiple stations to share the same transmission medium while using only the part of its bandwidth they require. TDMA is used for Digital Enhanced Cordless Telecommunications (DECT) and also in the satellite system and combat-net radio. (Wikipedia, 2008)
Code Division Multiple Access (CDMA) refers to the second generation (2G) and third generation (3G) wireless communication. It can be said to be an alternative to TDMA and FDMA, but CDMA has a much higher bandwidth than the data being communicated in contrast with TDMA which divides access by time and FDMA divides by frequency. CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth and also allows roaming (Search Telecom, 2007) but it does not guaranteed the future of mobile communication and this is where SDMA arise and become the solution.
Even though Smart Antennas has some limitations, it has proved that it is the latest technology to bring an excellent future to wireless communication. With the adaptive array system the communication would be clearer with better signal and also the frequency reuse is of great advantage to cost reduction. Furthermore, MIMO is one of the latest discussed and the IEEE 802.11n wireless standard is expected in 2009. So it can be concluded that Smart Antennas has a great future.
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