TERM PAPER – COMMUNICATION SYSTEMS AND TECHNIQUES (EC – 312)

Digital Beamforming: Basics and its Implementation in Smart Antennas for WLAN

April 2011 E&CE Deptt. IIT Roorkee

Enrollment No. Student’s Name

08116010 Chandranshu Garg

08213002 Ankur Aggarwal

08116020 Jatin Bajaj

08116027 Nitin Agarwal

08116028 Pankaj Garg

08116031 Prashant Upadhyay

08116041 Shubham Gupta

08213003 A Tandava Krishna Yadav

08213005 Gaurav Mittal

08213008 M. Revanth

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I. INTRODUCTION he beamforming is the combination of radio signals from a set of small non-directional

antennas to simulate a large directional antenna. The simulated antenna can be pointed electronically, although the antenna does not physically move. In communications, beamforming is used to point an antenna at the signal source to reduce interference and improve communication quality. In direction finding applications, beamforming can be used to steer an antenna to determine the direction of the signal source. In this introduction to we will cover the basics of Digital Beamforming Technique. 1.1 Directional Antennas

A directional antenna is one designed to have a gain in one direction and a loss in others. An antenna is made directional by increasing its size. This spreads the radiating conductors of the antenna over a larger distance, so that the constructive and destructive interference can be better controlled to give a directional radiation pattern. A simple directional antenna consists of a linear array of small radiating antenna elements, each fed with identical signals (the same amplitude and phase) from one transmitter. As the total width of the array increases, the central “beam” becomes narrower. As the number of elements increases, the “side lobes” become smaller. Directions in which the signal strength is zero are called “nulls.” The following figure is the radiation pattern for a line of 4 elements (small antennas) spaced 1/2 wavelength apart.

1.2 Electronically Steered Arrays

By varying the signal phases of the elements in a linear array, its main beam can be steered. The simplest way of controlling signal phase is to systematically vary the cable lengths to the elements. Cables delay the signal and so shift the phase. However, this does not allow the antenna to be dynamically steered. In an electronically steered array, programmable electronic phase shifters are used at each element in the array. The antenna is steered by programming the required phase shift value for each element. The

Digital Beamforming: Basics and its Implementation in Smart Antennas for WLAN

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beam pattern below is for an 8-element linear array with a progressive phase shift of 0.7π per element. The central beam has been steered about 45 degrees to the left. A phase shift of 2π corresponds to one wavelength or one carrier wave period, and more positive values are equivalent to saying that the signal is transmitted earlier.

II. DIGITAL BEAMFORMING The digital beamforming is an integration between antenna technology and digital technology.A beamformer is a processor used in conjunction with an array of antennas to provide a versatile form of spatial filtering. In digital beamforming, the operations of phase shifting and amplitude scaling for each antenna element, and summation for receiving, are done digitally. Either general-purpose DSP’s or dedicated beamforming chips are used. Digital processing requires that the signal from each antenna element is digitized using an A/D converter. Since radio signals above shortwave frequencies (>30 MHz) are too high to be directly digitized at a reasonable cost, digital beamforming receivers use

analog “RF translators” to shift the signal frequency down before the A/D converters. The following figure shows a translator that shifts the entire cellular telephone uplink band at 824-849 MHz down to the 1-26 MHz range. Once the antenna signals have been digitized, they are passed to “digital down-converters” that shift the radio channel’s center frequency down to 0 Hz and pass only the bandwidth required for one channel. The down-converters produce a “quadrature” baseband output at a low sample rate. For beamforming, the complex baseband signals are multiplied by the complex weights to apply the phase shift and amplitude scaling required for each antenna element.

III. SMART ANTENNAS A Smart Antenna is defined as an array of antennas with a digital signal processing unit that can change its pattern dynamically to adjust to noise, interference and multipath. The objective of the smart antenna system is to provide spatial filtering capability over a range of potential mobile station scan angles.Such a configuration dramatically enhances the capacity of a wireless link through a combination of diversity gain, array gain, and interference suppression.

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Increased capacity translates to higher data rates for a given number of users or more users for a given data rate per user. This is done so that the main beam tracks the desired user and/or nulls are placed in the direction of interferers and/or side lobes towards other users are minimized.

Applications of beamforming technology

Applications Description

RADAR Phased array RADAR; air traffic

control; synthetic aperture RADAR

SONAR Source location and classification

Communications

Smart antenna systems; Directional transmission and reception; sector broadcast in

satellite communications

Imaging Ultrasonic; optical; tomographic

Geophysical Exploration

Earth crust mapping; oil exploration

Biomedical

Neuronal spike discrimination; fetal heart monitoring; tissue hyperthermia; hearing aids

It should be noted that the term “smart” refers to the whole antenna system and not just the array antenna alone.

The smart antenna works as follows; assume that there is a user sending a

signal to the base station. Then each element of smart antenna array in the base station will receive the signal but at different time instance since the distance between the user and each element of array is different from other elements. By using this time delay and the distance between antenna elements the location of the user can be calculated. Therefore, the transmitter can send a signal to the exact location of

that user. This strategy can be applied for the system with multiple users as well. We can classify smart antennas in two main types: • Switched beam antennas • Adaptive beamformer antennas 1. Switched Beam Antenna Systems A switched beam antenna system consists of several highly directive, fixed, pre-defined beams which can be formed by means of a beamforming network. Multiplexed beams are formed by shifting the phase of each element's signal by a predetermined amount (this is done by the beamforming network). The transceiver can then choose between one or more beams for transmitting or receiving. While providing increased spatial reuse, switched beam systems cannot track moving nodes which therefore experience periods of lower gain as they move between beams. The system detects the signal strength and chooses one beam, from a set of several beams. The problem of this approach is that the user of interest may not be in the centre of the main beam.

Fig. Switched Beam Antenna System 2. Adaptive beamformer Antenna Systems In Adaptive beamformer antenna systems, the main lobe can be pointed virtually in any direction, and often automatically using the received signal from the target using sophisticated “direction-of-arrival" techniques. Adaptive system uses advanced signal processing algorithms to locate and track the desired and interfering signals to dynamically minimize interference and maximize intended signal reception. It performs both beam steering and interference nulling. However the receiver complexity and associated hardware increases the implementation costs.

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Fig. Adaptive Beamformer Antenna System

IV. WLAN Among various near-term application opportunities for smart antennas, wireless local area network (WLANs) is discussed in this paper. Smart Antennas enhances the capacity of a wireless network. They can used to increase desired coverage, lower power emission, improve link quality, and increase spectral efficiency of the network. But implementing such a system in real-time, through digital signal processing alone is complex and expensive for an indoor WLAN. For WLAN purposes, switched beam antenna is an appropriate choice because of the less in-expensive hardware and easy implementation of it. The current IEEE 802.11n Multiple-Input Multiple-Output (MIMO) technology increases the channel capacity by making use of the rich multipath environment. However, in an outdoor environment with little multipath, beamforming technology retains its advantage for increasing targeted coverage and link quality by steering the main lobe in the desired direction. Using smart antennas, we can electronically steer a high gain beam and perform real-time mobile station location, interference mitigation and physical layer signal formation. This application of smart antenna technology requires the system to locate arbitrary (typically hostile) WLAN users and disassociate them from the network. The fundamental idea behind this system is to use the smart antenna to scan a region of interest, spatially isolate a targeted mobile user, and transmit packets in the direction of this user in order to disassociate the user from the

network. In thi way, smart antenna systems can be used for WLAN surveillance and user tracking. Also, GPS (Global Positioning System) is unreliable when it comes to indoor or urban environments. We can have a real-time indoor WiFi localization system that utilizes smart antennas to triangulate the location of the mobile target. Figure below shows an indoor WLAN design of a building using a switched array beam antenna. .

Smart Antennas, because of their increased range and coverage can also be used to provide high speed wireless internet in rural areas more effectively. In an ad-hoc network, smart antenna has a tremendous potential. But there is a need for defining standardized interfaces between the link and the physical layer for networking protocols to be able to use directional beamforming smart antennas rather than omnidirectional ones. 4.1 Advantages of Smart Antennas in WLANs The major difference is that the usage of omnidirectional antennas causes co-channel interference when two users use the same band of frequency that eventually limits the user capacity in a system. But smart antennas can focus their beams towards desired user reducing interference to other users using the same frequency band. Other advantages as seen from various types of smart antennas studied include robustness against multipath fading and co-channel interference which

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improves reliability of received signal; reduced power consumption; low probability of interception and detection; enhanced location estimates and enhanced range of reception. More specifically, the benefits of smart antennas can be summarized as follows. (i) Increased range/coverage The array or beamforming 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 proportional to the number of receive antennas and also allows for lower battery life. (ii) Lower power requirements and/or cost reduction Optimizing transmission toward the wanted user (transmit beamforming gain) achieves lower power consumption and amplifier costs. (iii) 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. (iv) Increased 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 allows for interference reduction/ mitigation and increased numbers of users. 4.2 Disadvantages of Smart Antennas One of the major existing disadvantages of smart antennas is in their design and implementation in hardware. For a WLAN, implementation of Smart Antenna is still an expensive issue. Multiple RF chains can increase the cost and make the transceiver bulkier. Also, most of the devices such as mixers amplifiers and ADCS used are nonlinear devices. Using smart antennas can increase the number of such

components used. This can affect the performance of the array if not checked periodically. Furthermore since antenna arrays use more than one source of signal the data bandwidth required for digital processing increases linearly with number of antenna elements used. This can limit data rates for different applications. ------------------------------------------------------------- Note that the technological challenges in terms of hardware and processing load can be satisfactorily met by resorting to present-day miniaturized RF components and faster and low power processors. The accommodation of the antenna array, itself within a small factor device however remains a challenge. However, as the operating frequency continues to increase, the antenna sizes will shrink. Keeping all the advantages smart antenna provides, the future looks bright for applying beamforming technology to WLANs -------------------------------------------------------------

REFERENCES

[1] Yikun Huang “Beamforming Antennas for Wireless Communications” presentation.

[2] S. Jeon, Y. Wang, Y. Qian, and T. Itoh, “A novel smart anlenna system implementation for broad-band wirelcss communications,” IEEE Trans. Antennas Propagat., vol. 50, pp.600-606, May 2002.

[3] T. W. Nuteson, G. S. Mitchell, D. S. Haque, and J. S. Clark “A Smart Antenna Employing Digital Beamforming for WLAN Surveillance”.

[4] Wenjiang Wang, Sivanand Krishnan, Khoon Seong Lim, Aigang Feng, Boonpoh Ng “A Simple Beamforming Network for 802.11b/g WLAN Systems”.

[5] Chin-Heng Lim, Yahong Wan, Boon-Poh Ng, Chong-Meng Samson See “A Real-Time Indoor WiFi Localization System Utilizing Smart Antennas”.

[6] “A Primer on Digital Beamforming” Toby Haynes, Spectrum Signal Processing March 26, 1998.

[7] Vida Vakilian “Digital beamforming implementation of switch-beam smart antenna system by using integrated digital signal processor and Field-Programmable-Gate-Array” Faculty of Electrical Engineering, Universiti Teknologi Malaysia, May 2008.