Smart Antenna

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The rapid growth of the Internet user base and of bandwidth-hungry applications in recent years has created a need for ?last mile? broadband access for residential and business consumers. This demand for high-speed access is becoming a market force for advanced broadband access technologies and networks.
We define ?broadband? access as one that provides at least 5 Mbps peak (burst) rate per user in the downlink direction and 500 Kbps peak (burst) rate in the uplink. The average bit rates may be significantly lower in many applications. This bit rate asymmetry arises because applications such as web browsing are asymmetric. The growing demand for streaming audio and video will increase downlink throughput and quality of service (QoS) requirements. Other applications such as telephony and video conferencing need symmetric and constant bit rate services. Internet services and content are evolving in ways hard to predict. The only predictable trend is that bit rates and QoS requirements will increase rapidly.
Early applications of broadband access technologies were ?big pipe? applications aimed at large offices and business campuses offering 10 to 100 Mbps connectivity. However, new deployments increasingly target ?small pipe? volume markets such as medium-sized businesses, SOHOs (Small Office/Home Office) and residential customers. Broadband data access services are currently offered through a range of competing wired (Digital Subscriber Line -xDSL, fiber to the home (FTTH), hybrid fiber coax (HFC) and cable) and wireless (Multichannel Multipoint Distribution Service (MMDS), Local Multipoint Distribution Service (LMDS), High Altitude Long Operation (HALO) and satellite) technologies. Each approach has different cost structures, performance and deployment trade-offs.
While cable and DSL are currently gaining momentum in the broadband access marketplace, BWA is emerging as a third access technology with several advantages over its wired counterparts. These include rapid deployment, high data (Mbps/sq.mile) scalability, low maintenance and upgrade costs of the wireless facilities, and granular investment to match market growth.
A typical BWA system uses radio hubs called base transceiver stations (BTS) to serve a group of subscribers. The customer premises equipment (CPE) uses a rooftop directional antenna. The licensed frequencies for BWA lie in the 24-48 GHZ band (e.g., LMDS) or below the 5 GHz (e.g., MDS, WCS and MMDS bands). There are also a number of unlicensed bands at 2.4, 5, 5.7, 24 and 38 GHz.
Despite the advantages of wireless access, there remain a number of critical issues to be resolved before BWA can successfully penetrate the market. The chief concerns are spectrum efficiency, network scalability, self-installable CPE antennas, and reliable non-line of sight operation. Smart antennas (SA) offer a powerful tool to address these problems.
SA is an emerging technology that has gained much attention over the last few years for its ability to significantly increase the performance of wireless systems. SA is being inserted into 2.5 generation (GSM-EDGE) and third generation (IMT 2000) mobile cellular networks [1]. In this paper we outline why smart antennas constitute a particularly good match for emerging BWA systems.

The rest of the paper is organized as follows. In the next section, we describe BWA architectures and its challenges with emphasis on spectrum efficiency and on scalability. In section 3, we give an overview of the leverages offered by smart antennas in fixed BWA. We present a classification of smart antenna techniques and describe some applications and performance value. Section 4 concludes the paper.

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INTRODUCTION:

What is a smart antenna?
A smart antenna is an array of antenna elements connected to a digital signal processor. Such a configuration dramatically enhances the capacity of a wireless link through a combination of diversity gain, array gain, and interference suppression. Increased capacity translates to higher data rates for a given number of users or more users for a given data rate per user.

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Multipath paths of propagation are created by reflections and scattering. Also, interference signals such as that produced by the microwave oven in the picture, are superimposed on the desired signals. Measurements suggest that each path is really a bundle or cluster of paths, resulting from surface roughness or irregularities. The random gain of the bundle is called Multipath fading.
PRINCPLE OF WORKING:
The smart antenna works as follows. Each antenna element "sees" each propagation path differently, enabling the collection of elements to distinguish individual paths to within a certain resolution. As a consequence, smart antenna transmitters can encode independent streams of data onto different paths or linear combinations of paths, thereby increasing the data rate, or they can encode data redundantly onto paths that fade independently to protect the receiver from catastrophic signal fades, thereby providing diversity gain. A smart antenna receiver can decode the data from a smart antenna transmitter this is the highest-performing configuration or it can simply provide array gain or diversity gain to the desired signals transmitted from conventional transmitters and suppress the interference.
No manual placement of antennas is required. The smart antenna electronically adapts to the environment by looking for pilot tones or beacons or by recovering certain characteristics (such as a known alphabet or constant envelope) that the transmitted signal is known to have. The smart antenna can also separate the signals from multiple users who are separated in space (i.e. by distance) but who use the same radio channel (i.e. center frequency, time-slot, and/or code); this application is called Space-division multiple access (SDMA).
BEAM FORMING BASICS:

Beam forming is the term used to describe the application of weights to the inputs of an array of antennas to focus the reception of the antenna array in a certain direction, called the look direction or the main lobe. More importantly, other signals of the same carrier frequency from other directions can be rejected. These effects are all achieved electronically and no physical movement of the receiving antennas is necessary. In addition, multiple beam formers focused in different directions can share a single antenna array one set of antennas can service multiple calls of the same carrier.

It is no coincidence that the number of elements in the above diagram equals the number of incoming signals. A beam former of L antenna elements is capable of accepting one signal and reliably rejecting L-1 signals. A greater number of interfering signals will diminish the performance of the beam former. Beam forming presents several advantages to antenna design .Firstly, space division multiple access (SDMA) is achieved since a beamformer can steer its look direction towards a certain signal. Other signals from different directions can reuse the same carrier frequency.
Secondly, because the beamformer is focused in a particular direction, the antenna sensitivity can be increased for a better signal to noise ratio, especially when receiving weak signals. Thirdly, signal interference is reduced due to the rejection of undesired signals. For the uplink case of transmitting from the antenna array to a mobile telephone, system interference is reduced since the signal is only transmitted in the look direction. A digital beamformer is one that operates in the digital domain. Traditionally, beam formers were implemented in analog; the weights were determined and applied to the antenna inputs via analog circuitry. With digital beam forming, the antenna signals are individually translated from Radio Frequencies (RF) to Intermediate Frequencies (IF), digitized and then down-converted to base-band I and Q components. A beam forming algorithm implemented on one or more digital signal processors then processes the I and Q components to determine a set of weights for the input signals. The input signals are then multiplied by the weights and summed to output the signal of interest (SOI).

One of the foremost advantages offered by the software radio technology is flexibility. Because beam forming is implemented in software, it is possible to investigate a wide range of beam forming algorithms without the need to modify the system hardware for every algorithm. Consequently, researchers can focus their efforts on improving the performance of the beam forming algorithms rather than on designing new hardware, which can be a very expensive and time consuming process. A complete description of the RLS algorithm can be found in .This algorithm was chosen for its fast convergence rate and ability to process the input signal before demodulation. While the first reason is important especially when the environment is changing rapidly, the later reason decreases the algorithm dependency on a specific air interface.

Applications in Mobile Communications:

  • Increased antenna gain?
  • Decreased inter-symbol-interference (ISI)
  • Decreased co-channel-interference (CCI)

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