SMART ANTENNA SYSTEM FEATURES AND BENEFITS

Vinith Chauhan Department of Electronics and communication Engineering, St. Margaret Engineering College, Neemrana (Raj.)Pradeep Kumar Nathaney Department of Electronics and communication Engineering, St. Margaret Engineering College, Neemrana (Raj.)G. J. Foschini Department of Electronics and communication Engineering, Delhi College of engineering,Delhi.

Keywords and phrases: Smart antenna, Switched beam system, Adaptive array

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.

Fig.1 Switched beam system coverage pattern and Adaptive Array coverage pattern

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.

(1) (2)

Fig2.1 Switched beam system coverage pattern Fig2.2 Adaptive array coverage pattern

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 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 receiver. Nevertheless, unitary and differential space-tome coding has been proposed, which does not require CSI at either side of the communication link.

Fig.3

Traditional switched beam and adaptive array systems enable a base station to customize the beam they

generate for each remote user effectively by mean of internal feedback control.

3.1 Uplink Processing:

It is assumed here that a smart antenna is only employed at the base station and not at the handset or subscriber unit. Such remote radio terminals transmit using omnidirectional antennas, leaving it to the base station to separate the desired signals from interference selectively. Typically, the received signal from the spatially distributed antenna elements is multiplied by a weight, a complex adjustment of amplitude and a phase. These signals are combined to yield the array output. An adaptive algorithm controls the weights according to predefined objectives. For a switched beam system, this may be primarily maximum gain; for an adaptive array system, other factors may receive equal consideration. These dynamic calculations enable the system to change its radiation pattern for optimized signal reception.

3.2 Downlink Processing:

The task of transmitting in a spatially selective manner is the major basis for differentiating between switched beam and adaptive array systems. As described below, switched beam systems communicate with users by changing between preset directional patterns, largely on the basis of signal strength. In comparison, adaptive arrays attempt to understand the RF environment more comprehensively and transmit more selectively. The type of downlink processing used depends on whether the communication system uses time division duplex (TDD), which transmits and receives on the same frequency (e.g., PHS and DECT) or frequency division duplex (FDD), which uses separate frequencies for transmit and receiving (e.g., GSM). In most FDD systems, the uplink and downlink fading and other propagation characteristics may be considered independent, whereas in TDD systems the uplink and downlink channels can be considered reciprocal. Hence, in TDD systems uplink channel information may be used to achieve spatially selective transmission. In FDD systems, the uplink channel information cannot be

used directly and other types of downlink processing must be considered.

4. Benefits of smart antennas:

Multipath propagation, defined as the creation of multiple signal paths between the transmitter and the receiver due to the reflection of the transmitted signal by physical obstacles, is one of the major problems of the mobile communication. It is well known that the delay spread and resulting inter symbol interface(ISI) due to multiple signal paths arriving at the receiver at different time have a critical impact on communication link quality. On the other hand, co-channel interface is a major limiting factor on the capacity of wireless systems, resulting from the reuse of the available network resources (e.g., frequency, time) by number of users.

Smart antenna systems can improve link quality by combating the effects of multipath propagation or constructively exploiting the different paths, and increase capacity by mitigating interference and allowing transmission of different data streams from different antennas. More specifically, the benefits of smart antennas can be summarized as follows-

Increased range/coverage: The array or beam forming gain is the average increase in signal power at the receiver due to a coherent combination of the signals received at all antenna elements. It is proportion to the number of receive antennas and also allow for lower battery life.

Lower power requirement and/or cost reduction: Optimizing transmission toward the wanted user (transmit beam forming gain) achieves lower power consumption and amplifier costs.

Improved link quality / reliability: Diversity gain is obtained by receiving independent replicas of the signal through independently fading signal components. Based on the fact that it is highly probable that at least one or more of these signal components will not be in a deep fade, the availability of multiple independent dimensions reduces the effective fluctuations of the signal. Forms of the diversity include temporal, frequency, code, and spatial diversity obtained when sampling the spatial domain with smart antennas. The maximum spatial diversity order of non-frequency-selective fading MIMO channel is equal to the product of the number of receives and transmits antennas. Transmit diversity with multiple transmit antennas can be exploited via special modulation and coding scheme, whereas receive diversity relies on the combination on independently fading signal dimensions.

Increase spectral efficiency: Precise control of the transmitted and received power and exploitation of the knowledge of training sequence and/or other properties of the received signal (e.g., constant envelop, finite

alphabet) allow the interference reduction/mitigation and increased numbers of users sharing the same available resources (e.g., time, frequency, codes) and/or reuse of these resources by user served by the same base station/access point. The latter introduces a new multiple access scheme that exploit the space domain, space-division multiple access (SDMA). Moreover, increased data rates-and therefore increased spectral efficiency-can be achieved by exploiting the spatial multiplexing gain, that is, the possibility to simultaneously transmit multiple data streams, exploiting the multiple independent dimensions, the so called spatial signatures or MIMO channel Eigen modes.

Traditionally, smart antenna systems have been designed focusing on maximization of one of the above mentioned gains (beam forming, diversity and multiplexing gains). Nevertheless, the trade off between these gains has been recently studied, and smart antenna approaches have been proposed that combine the resulting benefits.

5. DEPLOYMENT OF SMART ANTENNAS IN FUTURE SYTEMS IMPLEMENTATION:

Smart antennas are already part of current releases 3G standards and more sophisticated approaches are considered for future releases. Furthermore, there is currently increasing interest in the incorporation of smart antenna techniques for IEEE wireless LAN/ MAN. However, implementation cost can vary considerably, and cost effective implementation is still the major challenge in the field. At the base station of particular importance is the development of improved antenna structure (possibly employing micro- electromechanical system, MEMS), improved cabling structures and efficient low cost radio frequency/ digital signal processing (RF/DSP) architectures. At the terminal the application of smart antenna techniques can have significant impact, in terms of not only system performance but also cost and terminal physical size. Promising areas for further research are efficient smart antenna algorithm design, small low power RF structures, and viable low power DSP implementations. Moreover antenna structures, RF architectures, and DSP implementations are expected to operate efficiently with in a wide variety of air interface scenarios, both separately and in parallel. To this end, innovative development flow methodologies jointly covering the RF and baseband part of complex wireless system-on-a –chip should be studied.

The financial impact of the deployment of smart antenna technologies in future wireless systems was studied for CDMA 2000 and UMTS. The result shows that smart antenna technologies are keys to securing the financial viability of operators business, while at the same time allowing for unit price elasticity and positive net present value. There are hence crucial for operators

that want to create for high data uses and/ or gain high market share.

Conclusion: In this paper an overview of the benefits and features of smart antenna architectures is given. Smart antenna system used in wireless communication to increase the range or coverage, spectral efficiency and reduced power requirements and cost reduction. Smart antennas constitute a promising but still emerging technology.

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