Wireless Communication: Contents

Wireless Communication

Recently the world has witnessed explosive growth in the use of wireless communications. The number of wireless services and subscribers has greatly expanded. Systems for mobile analog and digital cellular telephony, radio paging, and cordless telephony have become commonplace. Next generation systems will provide enhanced communication services, such as data, electronic mail, high resolution digital video or even full multimedia communication. The development of such systems appeared a technical challenge, with many issues still to be resolved.

The Adverse Wireless Channel

Every wireless system has to combat transmission and propagation effects that are nonexistent or different on a wired system. In the early days of wireless telegraphy, Marconi successfully demonstrated that wireless signals can cross the Atlantic and Pacific Oceans. Today, the role of radio has changed: we are not so much interested in covering large distances by radio, but more in the flexibility and comfort of (short-range) wireless access to the telecommunication infrastructure. Critical technical bottlenecks in a wireless link are the capacity of the radio channel, its unreliability due to adverse time-varying, multipath propagation and severe interference from other transmissions, in the same or a neighbouring cells.

Figure: Sample of "fading" signal envelope

Channel impairments such as fading and multipath dispersion are described in terms of the Doppler spread, the time constants of fading, fade durations, level crossing rates, ( Rayleigh , Rician and Nakagami) amplitude probability densities and the coherence bandwidth. Radio propagation depends on the environment, so these parameters essentially differ for outdoor micro and macro cellular systems, indoor systems, short-range vehicle to vehicle systems.

The developments in digital micro-electronics and signal processing provided methods to overcome the anomalies of the mobile channel, so these accelerated the growth of wireless communication. One of the fundamental concepts to tackle the channel problems, is diversity reception, which can substantially improve the link performance. Moreover advanced digital modulation methods, such as spread spectrum or MultiCarrier Modulation (MCM) appear suitable for wireless communication.

Orthogonal Frequency Division Multiplexing ( OFDM), a special form of MCM, will be used extensively in digital terrestrial broadcasting, e.g. in DAB and DTTB.

Meanwhile, many new developments in wireless office networks involve spread-spectrum transmission, coexisting with other services or in deregulated ( ISM) radio bands. Spreading methods include slow and fast frequency hopping, time hopping, direct sequence CDMA and multi-carrier CDMA. Spread spectrum signals appear relatively robust to multipath dispersion and can furthermore allow multiple users to share the same radio band. The rake receiver has been developed to optimally exploit the wideband character of the direct-sequence signal transmitted over a frequency-selective channel.

Managing Scarce Spectrum Resources

State-of-the-art signal processing can resolve most of the problems associated with the physical transmission over the unreliable wireless channel. Nowadays the main issues are the complexity of communication protocols that support full user mobility, including mobility across cells, networks or even across operators, the power consumption in the mobile or portable terminal, and the spectrum scarcity.

Due to spectrum scarcity, methods to efficiently share bandwidth among different users appeared of utmost importance. This implies the combined use of

frequency reuse, whether dynamic or fixed, but necessarily leading to cochannel interference, and
methods for multiple access (e.g. FDMA, TDMA, CDMA) and protocols for random access, such as ALOHA, CSMA, ISMA and the stack algorithm for collision resolution.

Cellular Frequency Reuse

Figure: Typical frequency reuse plan for cellular telephone network

In the planning of cellular networks, operators employ computer methods to estimate the coverage and outage probability networks. Capacity enhancements are obtained through cell sectorization, reuse partitioning and dynamic channel allocation and statistical voice multiplexing ( DSI, which is for instance used in PRMA). Relevant performance criteria, such as subjective voice quality (signal outages and speech clipping), call blocking probability and lost call rates are applied to cellular networks.

Multiple Access

In future wireless computer, multimedia networks or communication networks for Intelligent Transport Systems , the multiple access issue becomes substantially more important than it is for circuit-switched voice communication. The ALOHA , CSMA and ISMA protocols all allow multiple users to share radio communication resources. How these protocols perform differs substantially for guided (wired) and unguided (radio) channels; performance is highly dependent on the physical characteristics of the channel.

To attempt a realistic analysis of the performance of wireless radio networks, the assumptions that a data packet is always received successfully if no conflicting transmission (collision) occurs simultaneously on the same channel and that data packets involved in a collision are always lost, must be refined. Moreover, the performance seen by each participating terminal differs from the network average performance and is highly dependent on the terminal's location. The stability and delay depend on channel characteristics.

Specific solutions exist to efficiently resolve message collisions: these include the tree algorithm, the stack algorithm and dynamic frame length ALOHA. The combined use of CDMA and random access leads to interesting new system concepts.

What can be found on the CD ROM?

In this CD ROM journal, you will find many aspects of wireless communications covered. For instance, wireless systems and standards are covered, such as GSM, DECT, AMPS, Cellular CDMA, Iridium, wireless LANs, Digital Audio and Video Broadcasting . It also provides a brief historical setting as well a discussion of the potential of various system concepts. The journal gives you an in-depth discussion of the technical issues, concepts and solutions behind these system standards. This includes the fundamentals of the radio propagation channel, digital and analog transmission, multiple access schemes, cellular reuse, etc. Bibliographic references on many topics are given.

The material presented here has originally been used for the 290i graduate course at the University of California at Berkeley. In 1994 and 1995 it was offered by the U.C. Berkeley Continuing Education in Engineering, University Extension program. Later, several other authors contributed to the extension of this material.

The conversion of the course material into multimedia format is continuously under development. This is version 2, issued in 1996.
Questions, comments and suggestions for additional pages are welcome. Please use the email address linnartz@eecs.berkeley.edu. Please refer to the WWW pages of the publisher for more details about future publications and updated editions.

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