Wireless broadband network design issues with ...

WIRELESS BROADBAND NETWORK DESIGN ISSUES WITH REGARD TO IEEE 802.11B WIRELESS TRANSMISSION S . Omar, J. Chen lomar, icic041)@cse.unsw.edu.au The University of New South Wales Sydney, NSW 2052, Australia

ABSTRACT

concerns associated with wireless are security, reliability, and speed.

As the world is becoming increasingly mobile, people are moving ever faster than before. Over the past few years the popularity for wireless networks has soared, and it is now considered one of the fastest growing industries in today’s global marketplace. Numerous research effort has been invested in order to improve the current technologies. Furthermore several new technologies are being introduced to accommodate the ever-increasing demand for ubiquitous wireless Internet access by mobile consumers. Nevertheless there are technological limitations and in this paper two major challenges faced by wireless networks will be examined. Firstly, the need for more capacity and bandwidth in existing systems to ensure faster speed and a larger number of users. Secondly, the requirement for greater coverage areas to reduce infrastructure and maintenance cost.

This paper first provides a brief overview of the wireless network. Then the current technical challenges will be identified, and proposed solutions will be analyzed. Finally, results of experiments which analyze the range of access points and the effect of microwave interference will be described. The conclusion will summarize the current progress of wireless communications technologies and speculate on the future of wireless networks.

2. BACKGROUND As the growth of wireless technology shows no signs of slowing down, the development of regulatory structures is increasingly vital. When designing a wireless solution, there is a wide range of technologies to choose from, each with its own set of advantages and limitations.

1. INTRODUCTION The wireless communications industry has experienced extraordinary growth in recent years. It offers several significant advantages over the traditional wired network. The increase in mobility increases the productivity of workers; employees can access real time information anytime, anywhere. Costs of implementing a wireless network in the long run are significantly lower than for wired networks, and the Return on Investment (ROI) is often higher [I.: [?j [7j. Wireless networks are not only faster and simpler to deploy, they also eliminate the need for complex cabling and construction. Even with such advantages it is still relatively uncommon to replace the conventional network with a wireless network as there are still certain limitations associated with it. One of them is the restricted spectrum frequency 121 [SI. With the 802.11 technologies the network relies on the unlicensed Industrial, Scientific, and Medical (ISM) radio spectrum, which is easily congested and can be interfered with from other technologies. Other prime

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Radio devices operate in bands of designated frequency range. They may operate in the unlicensed ISM radio frequency (RF) bands (figure l), however all radio waves travel at the speed of light and have a characteristic wavelength and frequency 1 7 1 [ ’ 7 I.

A radio setup has two parts, the transmitter and the receiver. By modulating the transmitted data onto the radio carrier, data can be accurately extracted at the receiving end 131. Both the transmitter and receiver use antennas to radiate and capture the radio signals. The goal of sending data over RF is to send as much data as far and as fast as possible. It is done in two ways: use more frequency spectrum or use more complex modulation techniques. To improve one aspect usually means making a sacrifice for the other. Ideally techniques are required so that a system will be able to cover wider ranges and yet not forfeit high bandwidth. The propagation characteristics of waves are a function of frequency. Radio signals can be attenuated by atmospheric

and terrain conditions that may cause signal distortions i 2 1 I4 I. Signal distortions include diffraction, refraction, reflection and absorption. These path attenuations are due to distance or delay factors. Multi-path propagation is when multiple copies of a signal arrive at different phases, and if phases add destructively signal detection will be more difficult.

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area network (LAN), Bluetooth for personal area network (PAN), and GPRS for wide area networks (WAN). Bluetooth is a short-range radio technology in tended to transmit signals over short distances of up to ten meters in the unlicensed 2.4GHz band. Because 802.1 1 Wireless Local Area Networks (WLANs) also operate in the same band, there are interference issues to consider L6J. The 802.11 family has three main specifications, a, b and g. 802.1 la provides up to 54Mbps in the 5GHz band, and has 13 non-interfering channels versus 3 for 802.1 lb. The more widely deployed standard is 802.1 lb, also known as Wireless Fidelity (Wi-Fi) 141. It operates in the 2.4GHz band and has a maximum raw data rate of 11Mbps. 802.1 Ig has 54Mbps raw data rate with 802.11b backward compatibility. The higher speed comes from using the Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme (which is also used in 802.1 la). 802.1 l g operates in the 2.4GHz band.

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Figure 1 : ISM Unlicensed Frequency Band [I41 Most wireless network systems use spread spectrum technology. It is a wideband radio frequency technique for use in reliable and secure communications systems. More bandwidth is consumed with this technology than with narrowband technology, but it produces a signal that in effect is louder and easier to detect. If a receiver is not tuned to the right frequency, a spread spectrum signal looks like background noise. There are two types of spread spectrum used in wireless network defined under IEEE 802.1 1 standard, Frequency Hopping Spread Spectrum (FHSS), and Direct Sequence Spread Spectrum (DSSS). For FHSS, the frequency shifting spreads the transmission over a wide frequency band. When it is properly synchronized, it functions as a single logical channel [SI 11 21. To a receiver that does not know the hopping pattern, it appears to be a short duration impulse noise. There are 26 defined hopping patterns in three different sets. They are called orthogonal patterns.

In DSSS, a carrier is modulated by a digital signal. Each data bit becomes a string of chips transmitted in parallel across a wide frequency range. It generates a redundant bit pattern called a chip or chipping code. The longer the chip is, the greater the probability that the original data can be recovered. The disadvantage is that it requires more bandwidth. To an unauthorized receiver, DSSS appears as a low power wideband noise and is rejected. There are currently three commonly used wireless technologies, the IEEE 802.1 1 family for wireless local

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General Packet Radio Service (GPRS) is a packetswitched service that allows data communications to be sent and received over the existing Global System for Mobile (GSM) communications network. The user will experience data rates of up to 54 Kbps 115 1, which is significantly faster than a GSM circuit-switched data connection 17'.

3. DESIGN ISSUES Designing a wireless network is a challenging task. Wireless networks require extensive planning because of the composition of the radio channel. Every building has its own characteristics with respect to radio transmission, and unexpected interference can arise almost everywhere i4j [g,. As a consequence, each wireless network deployment is unique in many ways and careful planning and meticulous site surveys are generally required. A wide range of techniques have been developed for enhancing coverage and spectral efficiency. Through the use of microcell's "reuse pattern", the spatial separation ensures the signal in one cell is sufficiently attenuated before it reaches another cell using the same channel, and thus does not create significant interference. The use of this technique results in higher user capacity. Individual microcells (figure 2 ) overlap to allow continuous communication within the network. They handle low-power signals and as the user moves, the signal is passed or "hand off' from cell to cell so that users can roam across a geographic area [SI 191. Since users in each cell are closer to the cell hub, a higher percentage can be serviced through either a line of sight (LOS) or a non-LOS link. This results in increased system coverage, and higher revenue earning potential.

In this experiment, various positions are at marked distances away from the AP. Distances vary from 1 to 10 meters. Interference was kept to a minimum for this range testing, so that optimal results could be achieved. In figure 3, the graph shows the signal-to-noise ratio (SNR) recorded at the 3 positions over a period of time. The average SNR at the 1 meter distance is 60dB, whereas at the 5 meter distance the SNR is reduced to 26dB, more than half the signal quality. This shows that the quality of the signal, even though only 4 meters apart, has greatly attenuated. From 5 meter to 10, the signal quality stays at a similar level. At 5 meters the signal peaks and troughs, whereas in 10 meters the signal quality stays around 20dB.

Figure 2: Microcell Model [ 14 I m

Adaptive, “smart” antennas have been now widely investigated for wireless communications applications !IO]. The system utilizes multiple antennas at base stations to better focus radio energy and thus improve the signal quality. The main feature of the smart antenna is the sophisticated signal processing that is applied to an array of antennas to dynamically control transmission and reception. In contrast, traditional cellular systems that waste energy by broadcasting over an entire cell instead of directing it at the intended recipient. The rest of the RF energy is wasted and generates noise that interferes with other users in the system 11 11. The unique approach is to create a “personal cell” for each user. A personal cell is created for each user, follows that user while he or she communicates, and is dismantled when the user is finished. These personal cells reduce interference, enhance coverage area, improve capacity, reduce costs, and enable higher quality calls and data transmissions. To effectively improve the capacity, spectral efficiency is calculated at each base station. Spectral efficiency measures the ability of a wireless system to deliver information with a given amount of radio spectrum, and is directly related to system capacity Ill]. It determines the total throughput each base station can support in a given amount of spectrum. It is measured in units of bitdsecond per Hertdcell (b/s/Hz/cell). 4. EXPERIMENTS

It is important to conduct site surveys and field studies in order to investigate unexpected interference that could impede the wireless network performance. Experiments are conducted to gain a better understanding of the range of the “Access Point” (AP) and how interference would affect the signal quality 1131.

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Figure 3: Distance vs. Signal Strength There are a number of objects that can cause interference with RF signals; some of these objects include radios, microwaves, and phones. To find out how much signal quality is affected by these objects, a signal log session is kept in the laptop. A hypothetical test run under the situation where no interference is present is recorded as the control. Then interfering objects are introduced and the results recorded. Firstly when the radio is introduced in this experiment, the signal quality was not affected by much. The average SNR between the two signals are not significantly altered. One reason could be that as the radio is tuned in the frequency of 108MHz, and the wireless transmission is running on the 24GHz band, being different frequencies, they do not interfere with each other. In figure 4, microwave interference is introduced. From the graph it is clearly shown that the signal quality is greatly reduced. The average SNR between the control and the interfered signal is 55dB to 30dB.

[3] T.K. Sarkar, Z . Ji, K. Kim, A. Medouri, M. Salazar-Palma. “A Survey of Various Propagation Models for Mobile Communication.” IEEE Antennas and Propagation Magazine, Vol. 45, No. 3, June 2003.

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