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Wireless Communications

Wireless Multimedia Communications

With the recent technological advances, cellular phones and other personal telecommunication systems are being widely deployed. By the end of the year 2003, for example, it is predicted that there will be more than 830 million wireless subscribers and over 700 million Internet subscribers. This translates into a huge market potential for mobile Internet solutions, and ubiquitous mobile extensions to all basic desktop applications and data files at anytime and anywhere. This rapid growth in wireless communications, together with recent developments in hardware miniaturization, has also opened up new exciting applications. In addition, it would allow the integration of computers and communication in the form of highly intelligent small sized multimedia terminals that are well endowed with computing power and advanced networking facilities to serve business and personal users on the move.

These future services will need to support a wide range of high-quality services that require high speed data transmission or high data rates that can be more than 1000 times faster than present mobile systems. Unfortunately, the ability to achieve high bit rates while achieving good and reliable system performance is severely restricted by the propagation characteristics of the harsh wireless and time dispersive mobile radio environment.

In the last few years, we have developed various enabling technologies for providing high speed and high rate data transmission for wireless personal communications systems. In particular, these techniques are currently being utilized to further develop tools and enabling technologies for reliable multimedia wireless communications. These include adaptive modulation, combined source and channel coding, multimedia compression algorithms, low power VLSI (Very Large Scale Integration) and RF (Radio Frequency) implementations along with wideband CDMA (Code Division Multiple Access) technologies.
 

Interference Cancellation for CDMA Communication Systems

Digital cellular systems such as GSM (Groupe Spéciale Mobile) have been progressively introduced throughout the world for mobile communications. To further improve quality, coverage and capacity, there has been a lot of work into the development of the Third-generation or 3G mobile systems in the past few years. Such systems will be able to provide end-user services data rates that are more than 100 times faster than present mobile systems. They will also significantly expand the range of options available to users and allow communication, information, multimedia and entertainment services to be delivered via wireless terminals. For example, 3G systems will allow users to make video calls to friends and colleagues from a mobile terminal, whilst simultaneously accessing a remote database from that same terminal, or receiving emails and phone calls.

Code Division Multiple Access (CDMA) is a digital wireless technology that offers improved call clarity and provides greater network capacity and service reliability. It also offers a host of calling features and multimedia service capabilities such as high speed data transmission, high quality wireless voice, multimedia services (including image and video) services as well as intelligent networking capability. In particular, such technology is expected to become the worldwide standard for UMTS (Universal Mobile Telecommunication System) and IMT-2000 (International Mobile Telecommunication-2000).

Because of its inherent multiple access interference, the performance of CDMA can be severely degraded as the number of subscribers increases. To fully utilize the potential of CDMA systems and significantly improve the system performance, we have developed multiuser successive interference cancellation technologies and demonstrated that substantial capacity improvement (in terms of the number of subscribers that can be supported) can be achieved.
 

Compact Antenna Design

There has been a tremendous revival in the need for antenna design over the last five years. This has not only occurred because of the huge demand for wireless communication devices but also because the antenna is now seen as an area where dramatic improvements in wireless communication performance and quality can be derived. For example, one current trend by mobile telephone manufacturers is to incorporate the antenna into the body of the handset (known as an in-built or integrated antenna) rather than have it located externally. Not only does this reduce the possibility of the antenna being broken off but it makes the handset smaller, more beautiful and also reduces the possible health hazard of electromagnetic radiation. The recently released Nokia 8810 mobile telephone is one of the first examples using this type of in-built antenna and commands respect as one of the premium mobile telephones in the market place. This trend will likely be followed by other major manufactures and also incorporated into the design of many other wireless devices. For instance, wireless phones for the home and wireless LANs (Local Area Networks) will most likely use this in-built antenna technology in the near future.

Another very significant development in antenna technology is in the area of smart antennas. Smart antennas make use of two or more antennas to dramatically improve the performance of the overall system. While smart antennas are still in the development phase it is likely that there will be a need to put two or more antennas into the handset or wireless LAN. Consequently, novel antennas designs will need to be developed that are compact and that will allow several antennas to be incorporated into a small space. The so called active antennas are also attracting much interest. In the past, the antenna has usually been seen as a simple add on element that is included in the final design. However, it has been recognized that if the antenna is thought of as an integral part of the entire circuit significant improvements in the overall performance may be achieved. For example, antennas with amplifiers incorporated in them are being developed and these not only have reduced overall size but also have improved performance because the antenna performance can be matched perfectly to the amplifier.

During the past few years, we have developed various small sized or compact antenna designs that allow the integration and in-built antennas within wireless devices. In particular, further developments in the form of active antennas and smart antenna designs are currently being pursued.
 

Integrated Circuits Technology for Wireless Radio Frequency Devices

Research in wireless integrated circuits has several clear goals. First, power consumption, a key parameter in determining operating time and range of the wireless device, must be reduced. Other equally important goals are the reduction of the size and cost of the device. One way to reduce both the power consumption and size of a wireless system is by using non-conventional RF (Radio Frequency) front-end architectures, which allow more components to be integrated on the same chip. We have developed novel techniques for implementing a key RF front-end component, the I/Q (In-phase/Quadrature) down-converter, which is used in two promising front-end architectures for single-chip receiver implementation. The techniques have been demonstrated in a fabricated chip.
 

Orthogonal Frequency Division Multiplexing

Advances in digital signal processing in recent years are transforming Audio-Visual (A/V) signals into digital representations and A/V consumer electronic products into digital electronic systems. Audio CD (Compact Disc) is the highly successful example and it has been the standard in the music industry. Recently, the invention of the minidisk, using digital compression technology, has further reduced the size of the disc as well as the player. In the video area, the recent introduction of the DVD (Digital Video Disc) has started a similar transformation in the video industry. This transformation also takes place in the recording side with the recent introduction of writable CD, digital camera and digital camcorder.

As digital representation of audio and video signals requires large amount of data, it follows that the transmission of such representation requires high rate communication systems. Unfortunately, in wireless environments or even in a wireline infrastructure using existing low-quality cables (such as the telephone local loop), the characteristic of the transmission media makes high rate communication very unreliable. Orthogonal Frequency Division Multiplexing (OFDM) is a new technology, which enables reliable high rate transmission under these types of environment. Many advanced high rate digital communication systems use OFDM technology. On the wireline side, ADSL (asynchronous digital subscriber loop) uses OFDM to provide digital video on demand service and is currently used in HK Telecom's IMS services. On the wireless environment, DAB (digital audio broadcast) and DVB (digital video broadcast) are soon to replace the existing analog AM/FM broadcast.

Under the wireless multimedia project funded by HKTIIT, we have developed technologies in OFDM transmission and reception. Moreover, we have extended and enhanced the technology such that it can support wireless and mobile communications more efficiently and more reliably in a multi-user or multi-channel environment.
 

Smart Antenna For Wireless Personal Communication Systems

Communication over the wireless communication channel is one of the most difficult communication problems that engineers have faced. This is because the wireless channel suffers from echoing which causes much distortion and system performance degradation. To understand how difficult this is, recall the problems experienced when speaking in a room or hall that experiences significant echos.

Engineers have of course developed methods to overcome these problems but limits have been reached which constrain the speed of the communication and the number of mobile telephones that can be used together.

One hot area that promises to dissolve these limits is smart antennas. Smart antennas make use of multiple antennas in conjunction with computer algorithms to overcome limitations with conventional wireless communication systems. This is done by combining the signals from multiple antennas to reduce interference and strengthen the desired signal using algorithms which determine how to best adjust the phase and amplitude of the signals from each antenna before these signals are combined. Currently the focus of the great majority of research has been on smart antennas for basestations. Basestation systems are quite different from handsets because their greater size, power consumption and cost allow techniques, which are not feasible for handsets. We have developed intellectual property for implementing smart antenna techniques, which are suitable for low-complexity, low-power, compact handset implementation using both Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) wireless systems. We have demonstrated through system simulations that such techniques can substantially increase a wireless system's transmission quality and capacity. The implication of these techniques is that multi-media and internet browsing over mobile telephones and wireless communications devices may be much closer to reality than is commonly thought.