Adaptive Intelligent Ad-Hoc Networks
The Wireless network has seen an exponential growth due to its ability to free its user from the expenses and hassles in setting up their infrastructure. Such growth is more spectacular in the emerging economies where there was minimal or no presence of wired communication infrastructure in the past. Reputable market research firms have identified that the wireless network users will grow by a factor of 8 by 2007 (Gartner, Inc., March 26, 2003), while wireless product shipments grew by 73% in 2002 (Dataquest Inc., September 19, 2002). Even a small fraction, such as 1%, of the broadband network market is a significant figure for a viable medium sized business. Despite many new advances in technology, consumers are dependent on the infrastructures of large companies to satisfy their need. The performance of centralized and large-scale networks could be significantly curtailed at a time of congestion, or completely lost due to natural or unnatural disasters. There are numerous applications of a communication network whose operation does not depend on such large-scale infrastructure and does not require any network administrator to configure the system at the end-user premises.
However, not much breakthrough has been made in this area and the ad-hoc networking industry is in its infancy. Self-organizing wireless networks can extend the power of computing technology to dynamic and harsh environments. To achieve this, network protocols need to extract relevant information from the environment (i.e., sense the environment), and make intelligent decisions about how best to support the requirements of an individual user or application. This is done while maximizing overall system utilization. This significantly extends current network technology, which was designed for more predictable and stable environments. Applications such as remote community communications, battle management, emergency responses and tough terrain communications in oil-field exploration need to take advantage of self-organizing networks.
What is Ad-Hoc?
In the last decade, developments in wireless data communication, such as GSM, GPRS, WLAN and mobile devices, including PDAs and mobile phones, were combined into a trend—mobile communication. The movement of mobile nodes in the network could be random or predictable, static or fast moving. They may move as individual or as a group.
Ad-hoc networks are formed by users or devices wishing to communicate without the necessity or existence of any centralized administration or infrastructure. Total Mobility is one of the most common reasons to apply ad-hoc topologies.
Wireless Ad-Hoc Networks:
Wireless Ad-Hoc networks are mobile by virtue of their characteristic. Each node in a wireless ad-hoc network has a wireless access interface, e.g. Bluetooth, WLAN, UWB, etc., and is free to enter or leave the network at any time. Ad-hoc networks can function as standalone networks meeting direct communication requirements of their users, or as an addition to existing infrastructure based networks to extend or enhance their coverage. This kind of communication becomes a valuable solution, especially in situations of missing or incomplete network. Regardless of application and technology, the following are the main features of ad-hoc networks:
Dynamic network topology: Due to node mobility and wireless radio propagation, network topology is constantly changing. This requires specific designed network protocol functions for topology construction and maintenance
Distributed nature: As there is no permanent central administrator or authority, all networking functions have to be distributed across participating nodes
Multi-hop communications: Due to limited range of wireless interfaces, usually it is not possible to setup direct communication links between all nodes. The nodes must run routing algorithms to establish routes in the network and to forward packets destined for other nodes; as well as route packets with real-time constraint in some cases (e.g. voice transmission)
Limited bandwidth: Wireless technologies that are envisaged to be suitable for ad-hoc networks provide.
Wideband Orthogonal Frequency Division Multiplexing (W-OFDM)
Orthogonal Frequency Division Multiplexing (OFDM) has been successfully applied to a wide variety of digital communications applications over the past several years and has been adopted as the wireless LAN standard. This paper presents the challenges associated with implementing OFDM for high speed wireless data communication and how Wide-band OFDM (W-OFDM), a variation of OFDM improves bandwidth and noise tolerance.
Just what is OFDM, and what makes it better? To answer this question, we need to review some basic ideas about wireless telecommunications systems, and how OFDM fits into the overall picture.
In what follows, we will review the following concepts needed to understand OFDM; digital messages, carrier waves, modulation and multiplexing. Then we will explain OFDM and why it is used.
Wireless communications systems are used to send messages between two locations using radio waves which travel across free space. Messages of all types (voice, music, image, video, text) are usually converted to digital form and are represented as a stream of 1's and 0's called bits (binary digits). Voice messages can be represented by about 10,000 bits per second, CD quality music needs about 100,000 bits/sec, and TV quality video messages require about 1,000,000 bits per second, plus or minus. Text messages can be sent at any speed, depending on how long you are willing to wait.
Radio waves are electromagnetic waves used to carry a message over a distance. Thus radio waves are also called a carrier waves. A carrier wave looks like a sine wave, and moves like a train at the speed of light. The frequency of the carrier wave is the number of times per second that the wave train goes up and down and back up as it moves past you, and is measured in units of cycles per second or Hertz.
Carrier (electromagnetic) waves of different frequencies and wavelengths have different properties. For example, radio waves can travel through walls, but light waves cannot. Lower frequency waves tend to travel further, and can bend around corners. Higher frequency waves travel more or less only via line of sight. Thus certain parts of the radio spectrum are better suited for certain types of telecommunications. For indoor wireless communications through walls over a distance of several hundred feet, or outdoor communications over several miles mostly over line of sight with perhaps some trees in the way, carrier frequencies in the range of 1 to 5 GHz (gigahertz or billion cycles per second) are used.
Modulation is the process whereby a carrier wave of a particular frequency is modified or modulated by the message signal, so that the modulated carrier wave can be used to carry the message over a distance. For digital messages (a stream of 1's and 0's), there are three basic kinds of modulation:
Amplitude Shift Keying (ASK) (digital AM) in which the amplitude of the carrier wave is modulated in step with the message signal.
Frequency Shift Keying (FSK) (digital FM) in which the frequency of the carrier wave is modulated in step with the message signal.
Phase Shift Keying (PSK) (digital PM) in which the phase of the carrier wave is modulated in step with the message signal.
ASK and PSK may also be used at the same time on one carrier, which is called Quadrature Amplitude Modulation (QAM) or Amplitude/Phase Keying (APK). The receiver is designed to receive the carrier wave, detect these amplitude and phase shifts in the carrier (demodulation), and thus retrieve the digital message.
When a carrier wave is modulated, it is no longer a single frequency but is spread out over a range of frequencies. The bandwidth of the modulated carrier wave is the range from lowest to highest frequency, with the original carrier frequency in the center. The bandwidth is approximately equal to the speed of the digital message, e.g. 10,000 Hz (10 KHz) for voice or 1,000,000 Hz (1 MHz) for video.
OFDM (Orthogonal Frequency Division Multiplexing) is a method of using many carrier waves instead of only one, and using each carrier wave for only part of the message. OFDM is also called multicarrier modulation (MCM) or Discrete Multi-Tone (DMT). We first describe Multiplexing, then Frequency Division and then Orthogonal. It is important to stress that OFDM is not really a modulation scheme since it does not conflict with other modulation schemes. It is more a coding scheme or a transport scheme.
Multiplexing is a way to split a high speed digital message into many lower speed ones. A useful analogy is a highway with a toll collection point. Where each car is one bit of the message, and the number of cars passing a given point in one second is the speed of the message, which represents bits per second. The single lane highway may be split into 10 different lanes for paying tolls. At a point beside the single lane highway, the cars will pass at high speed, whereas at the toll booths, the cars will pass slowly. Thus the single high speed message (flow of cars past a point of single lane highway) is divided into many low speed messages (flow of cars past many toll booths). In a perfect system, the first car will take the first toll lane, the second car takes the second toll lane, etc. The 11th car takes the first toll lane again, and follows the first car. A multiplexer is a switch that assigns each car to one of the many toll booths.
Demultiplexing is the opposite, where many low speed messages are combined into one high speed message. Following the analogy, demultiplexing is where the many low speed messages (cars) passing slowly through the toll booth lanes are merged back into a high speed message travelling quickly on a single lane highway.
Wide-Band Orthogonal Frequency Multiplexing (W-OFDM) Technical
Orthogonal Frequency Division Multiplexing (OFDM) is a multi carrier transmission technique whose history dates back to the mid1960's. Although, the concept of OFDM has been around for a long time, it has recently been recognized as an excellent method for high speed bidirectional wireless data communication. The first systems using this technology were military HF radio links. Today, this technology is used in broadcast systems such as Asymmetric Digital Subscriber Line (ADSL), European Telecommunications Standard Institute (ETSI) radio (DAB:Digital Audio Broadcasting) and TV (DVBT:Digital Video Broadcasting---Terrestrial) as well as being the proposed technique for wireless LAN standards such as ETSI Hiperlan/2 and IEEE 802.11a. There is also growing interest in using OFDM for the next generation of land mobile communication systems.
OFDM efficiently squeezes multiple modulated carriers tightly together reducing the required bandwidth but keeping the modulated signals orthogonal so they do not interfere with each other. Any digital modulation technique can be used on each carrier and different modulation techniques can be used on separate carriers. The outputs of the modulated carriers are added together before transmission. At the receiver, the modulated carriers must be separated before demodulation. The traditional method of separating the bands is to use filters, which is simply frequency division multiplexing (FDM). Fig. 1 shows a representative power spectrum for three sub channels of a FDM system.
In a classic FDM system, the sub channels are non-orthogonal and must be separated by guard bands to avoid inter channel interference. This results in reduced spectral efficiency.
Another method to achieve frequency separation, but is more spectrally efficient than FDM is to overlap the individual carriers, yet ensuring the carriers are orthogonal is to use the discrete Fourier Transform the (DFT) as part of the modulation and demodulation schemes. This is where the name orthogonal FDM (OFDM) arises. High speed, fast Fourier transform (FFT) chips are commercially available, making the implementation of the DFT a relatively easy operation. Fig. 2 shows the spectrum of an OFDM signal with three sub carriers. The main lobe of each carrier lies on the nulls of the other carriers. At the particular sub carrier frequency, there is no interference from any other sub-carrier frequency and hence they are orthogonal. In Fig.2, the sub carriers are 300 Hz apart.
The orthogonal nature of the OFDM sub channels allows them to be overlapped, thereby increasing the spectral tightly efficiency. In other words, as long as orthogonality is maintained, there will be no inter channel interference in an OFDM system. In any real implementation, however, several factors will cause a certain loss in orthogonality.
Designing a system which will minimize these losses therefore becomes a major technical focus. Another advantage to OFDM is its ability to handle the effects of multipath delay spread. In any radio transmission, the channel spectral response is not flat. It has fades or nulls in the response due to reflections causing cancellation of certain frequencies at the receiver. For narrowband transmissions, if the null in the frequency response occurs at the transmission frequency then the entire signal can be lost.
Multipath delay spread can also lead to inter symbol interference. This is due to a delayed multipath signal presents overlapping with the following symbol. This problem is solved by adding a time domain guard interval to each band OFDM symbol. Inter carrier interference (ICI) can be width avoided by making the guard interval a cyclic extension of, the OFDM symbol. There are, however, certain negatives associated with this technique. It is more sensitive to carrier frequency offset and sampling clock mismatch than single carrier systems. Also the nature of the orthogonal encoding leads to high peak to average ratio signals: or in other words, signals with a large dynamic range. This means that only highly linear, low efficiency RF amplifiers can be used.
We present here WOFDM technology, which is less sensitive to inherent OFDM problems such as frequency offset, sample clock offset, phase noise and amplifier non-linearities. WOFDM is also able to tolerate strong multipath and fast changing selective fading by using a powerful equalization scheme combined with a forward error correction scheme.
Wireless IP Surveillance
Today’s heightened requirements for security, public safety, and crime prevention have created an unprecedented worldwide demand for cost-effective, flexible, and reliable video surveillance systems.
This paper introduces the benefits of using wireless technology based on Internet Protocol (IP) for video surveillance.
We begin by describing the benefits of IP-based networking compared to traditional Closed Circuit TV (CCTV) technology, and the benefits of wireless networking. We point out that by combining IP and wireless technologies for their video surveillance solutions, companies and organizations can realize the benefits of both. We go on to introduce some of the important technical aspects of surveillance technology, and describe the main challenges involved in delivering surveillance services—challenges that have been addressed by EION’s own wireless surveillance solutions. Next, we describe the opportunities created by the new technologies for enterprises and security companies. Finally, we describe how EION, a leader in wireless IP-based networking, is spearheading the movement to wireless IP surveillance with innovative solutions based on their best-of-breed wireless networking products.
Advantages of wireless IP video surveillance:
Less expensive than wired solutions
Can use existing IP network for video surveillance
Can be used to monitor remote locations
Can be set up, reconfigured, expanded or disassembled quickly to add video surveillance to special events
Video images can be transmitted over secure
Internet connection or private IP network for little or no cost.
Scalable—can be expanded at little cost without having to lay wire or cable
Can be integrated with solutions that provide surveillance in high-speed
Wireless IP Surveillance—the Benefits of Both IP Networking and Wireless
The trend toward using IP networks for surveillance purposes is part of a larger drive to move more and more types of services (video, voice over IP, in addition to data services) to IP. By now, the benefits of IP-based networking are probably familiar:
Earlier technical challenges regarding quality of service, throughput, and processing performance have been addressed, making IP a sound alternative to traditional analog and privately owned or controlled communication mediums.
Transmitting video, data, and voice messages over the Internet or a Virtual Private Network (VPN) costs much less than traditional alternatives, allowing enterprises to reduce their telecommunications costs, and service providers to add more subscribers and deliver more diverse services for a relatively low rate.
The benefits of wireless communications are also widely recognized:
Wireless networking has allowed many companies, organizations, and even countries to make Internet access, as well as applications such as telephony widely available to both urban and remote rural areas without assuming the expense involved in laying cable lines or copper wires.
Wireless technology has advanced to the point where the quality of the services delivered over wireless networks is equivalent to that of wired alternatives.
Now wireless technology can be combined with IP-based networking to deliver advanced data, video, and voice services wirelessly, simultaneously achieving the benefits of both IP and wireless technology. In addition, this powerful convergence of wireless and IP is revolutionizing surveillance services as well—making cost-effective new solutions available for users and providers of video surveillance.