15-03-2011, 04:37 PM
presented by:
Vinith Chauhan
Pradeep Kumar Nathaney
Krishna Mohan Rai
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Abstract: ---- This paper presents a tutorial on the concepts of smart antenna system and benefits of smart antenna system design over conventional omni-directional approaches. The adoption of smart antenna techniques in wireless system is expected to have a significant impact on the efficient use of the spectrum, the minimization of the cost of establishing wireless networks, the optimization of service quality and increase the multipath rejection .It enables operator of PCS, Cellular and wireless local loop (WLL) networks to realize significant increases in signal quality, capacity and coverage. Operators often require different combinations of these advantages at different time’s .As a result, these systems offering the most flexibility in terms of configuration and upgradeability. Smart antenna systems are applicable with some modifications to all major wireless protocol and standards.
1. Introduction:
In truth, antenna are not smart, antenna system are smart. Generally co-located with a base station, a smart antenna system combines an antenna array with a digital signal-processing capability to transmit in an adaptive spatially sensitive manner. Such a system can automatically change the directionality of its radiation patterns in response to its signal environment. Current wireless services include transmission of voice, fax, low speed data and more bandwidth consuming interactive multimedia services like video on demand and internet access will be supported in the future. Wireless networks must provide these services in a wide range of environments, spanning dense urban, suburban and rural areas. Varying mobility needs must also be addressed. Wireless local loop networks serve fixed subscribers. Micro-cellular networks serve pedestrians as slow moving users, and macro cellular networks serve high speed vehicles- borne users. Several competing standards have been developed for terrestrial networks .AMPS (advance mobile phone system) is an example of first generation frequency division multiple access analog cellular system. Second generation standards includes GSM and IS-136, using TDMA, IS-95 using CDMA Increased service and lower cost have results in an increased air time usage and number of subscribes. Since the radio resources are limited, system capacity is a primary challenge for current wireless network designers, other challenges are 1) an-unfriendly transmission medium due to the presence of multipath, noise interference and time-variations. 2) The limited battery life of the user’s hand-help terminal. 3) Efficient radio resource management to offer quality of services.
Current wireless modems use signal processing in the time dimension alone through advanced coding, modulation and equalization techniques. The primary goal of smart antenna in wireless communications is to integrate and exploit efficiently the extra dimension offered by multiple antennas at the transceiver in order to enhance the over all performance of the network. Smart antennas systems use modems which combine the signals of multi-element antennas both in space and time. Smart antenna can be use for both receive and transmit end, both at the base station and the user terminal. The use of smart antenna at the base alone is more typical since practical constraints usually limit the use of multiple antennas at the terminal. Multiple antenna capture more signals energy which can be combined to improve the signal to noise ratio. Spatial diversity obtained from multiple antennas can be use to combat channel fading and space time processing can help mitigate inter-symbol interference and co-channel interference.
2. Type of smart antenna system:
Smart antenna systems are categorized as either beam or adaptive array system.
2.1 Switched Beam Antennas:
Switched beam antenna system forms multiple fixed beam with heightened sensitivity in particular direction. These antenna systems detect signal strength, choose from one of several predetermined fixed beam and switch from one beam to anther as the mobile thought out the sector. Instead of shaping the directional antenna pattern with the metallic properties and physical design of a single elements (like a sectorized antennas), switched beam systems combine the output of multiple antenna in such a way as to form finally sectorized ( directional) beam with spatial selectivity than can be achieved with conventional, signal element approaches, in terms of radiation patterns, switched beam is an extension of current micro Cellular or cellular sectorized method of splitting a typical cell . The switched beam approached further subdivides macro sector into several micro sector as a means of improving range and capacity.
Each micro sector contains a predetermined fixed beam pattern with the greatest sensitivity located in the center of the beam and less sensitivity else where. The design of such systems involves high gain, narrow azimuthal beam width antenna elements. The switched beam system select one of several predetermined fixed-beam patterns (based on weighted combination of antenna outputs) with the greatest output power in remote user's channels .These choice are driven by RF as baseband DSP hardware and software. The system switches its beam in different direction throughout space by changing the phase differences of signal used to feed the antenna elements as received from them. When the mobile user enters a particular macro sector, the switch beam system selects the micro sector containing the strongest signal. Throughout the call, the system maintains signal strength and switches to other fixed micro sector as required. Smart antenna system communicates directionally by forming specific antenna beam patterns. When a smart antenna directs its main lobe with enhanced gain in the direction of the user, it naturally form side lobes and nulls as area of medium and minimal gain respectively in directions away the main lobe. Different switched beam and adaptive smart antenna systems central the lobe and nulls with varying degree of accuracy and flexibility.
2.2 Adaptive smart antennas:
Adapting antenna technology represents the most advanced smart antenna approach, using a variety of new signal processing algorithm. The adaptive system take advantage of its ability to effectively locate and track various type of signal to dynamically minimize interference and maximize intended signal reception .Both system attempts to increase gain according to the location of the user. However, only the adaptive systems provide optimal gain while simultaneously identifying, tracking and minimizing interfering signal.
3. Architecture of smart antenna:
The data block to be transmitted is encoded and modulated to symbols of a complex constellation. Each symbol is then mapped to of one of transmit antennas (spatial multiplexing) after space-time weighting of the antenna elements. After transmission through the wireless channel, de-multiplexing and decoding is performed at the receiver in order to recover the transmitted data. A large number of transmission schemes over MISO or MIMO Channels have been proposed in the literature, designed to maximize spectral efficiency and page link quality through the maximization of diversity, data rate and signal to interference and noise ratio (SINR).Each of these schemes relies on a certain amount of channel state information (CSI) available at the transmitter and/or receiver side .CSI at transmitter can be made available through the feedback or can be obtained based on estimation of receiver side. The former approach introduces the trade-off between feedback channel bandwidth and CSI accuracy, whereas in the latter channel reciprocity issue in frequency division duplex (FDD) system should be accounted for CSI at the receiver can be obtained using training based or blind techniques, which exploit other properties of the received signal, such as constant envelope and finite alphabet.
Transmission schemes that do not require CSI at the transmitter exploit the spatial dimension by either introducing coding on the spatial domain or employing spatial multiplexing gain. The former approach, space-time coding, increases redundancy over space and time, as each antenna transmits a differently encoded fully redundancy version of the same signal. The received signal is detected using a maximum likelihood (ML) decoder .Space -time codes were originally developed in the form of space-time trellis codes (STTCs), which required a multidimensional Viterbi algorithm for decoding at the receiver. These codes can provide diversity equal to the code without loss in bandwidth efficiency. Space-time block codes (STBCs) offer the same diversity as STTCs but do not provide coding gain. However, STBC are often the preferred solution over STTCs, as their decoding only requires linear processing. STBCs for two transmit antennas (proposed by Aiamouti) have been adopted as part of third-generation (3G) standards .Space-time coding technique assumes in principle prefect CSI at the
