Wireless Communication Jean Paul Linnartz' Reference Website
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Cellular and cordless phones rapidly became mass-market consumer products. There were about 250 Million subscribers in the year 2000, and a market of 500 to 600 Million handhelds per year. Around that year many operators invested Billions of Euros on spectrum for Third Generation (3G) systems, such as UMTS. However, the insight that these could only be recouped over periods of rapid growth for ten years or more may have accelatered the malaise in the telecomm markets after 2001.

Products for enhanced communication services, such as data, electronic mail, high resolution digital video or even full multimedia communication entered the market. Services such as the GSM Short Message Service greatly extent the capabilities of pagers. I-mode is a successful text and multimedia service in Japan, and Europe is betteing on WAP: The Wireless Application Protocol. The projected growth of the number of Internet users to 500 Million worldwide indicates potential when wireless and computing technologies are merged. The (r)evolutionary development of such systems appeared is focussed towards larger capacity, better quality, more bandwidth, wider coverage, lower power consumption and more services. This development remains 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 substantially more hostile than for 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 using radio technology merely to cover large distances, but rather for its flexibility and comfort. Short-range wireless links provide access to the fixed 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 neighboring cells. Unless specific measures are taken, substantial fade margins are needed, in addition to the C/I or C/N protection ratio used in a stationary (non-varying) channel.

Sample of a "fading" signal envelope: amplitude in dB versus time or location of the antenna. Wave interference of multiple reflected waves, each with a different amplitude and phase, causes fluctuations of the received signal amplitude. Changing the antenna location or the carrier frequency also changes the signal amplitude.

Channel impairments, such as fading and multipath dispersion can be characterized by the Doppler spread, the time constants of fading, fade duration, level crossing rates, (Rayleigh , Rician and Nakagami) amplitude probability densities and the coherence bandwidth. Scatter plots summarize the most relevant aspects of channel behavior. The radio propagation characteristics depend on the environment, so the parameters essentially differ for outdoor micro and macro cellular systems, indoor systems, short-range vehicle to vehicle systems.

The developments in digital microelectronics and signal processing provided methods to overcome the anomalies of the mobile channel, thereby accelerating 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. While traditionally techniques such as maximum ratio combining have been developed to optimally receive signal in the presence of Gaussian noise only, current systems exploit adaptive antennas to cancel interference. 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 broadcast systems, e.g. in DAB and DTTB.

Meanwhile, many new developments in wireless office networks (e.g. based on HIPERLAN or IEEE 802.11) involve spread-spectrum transmission, and allow coexistence with other services. IEEE 802.11, Bluetooth and many proprietary solutions use 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. So, 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 multiple users appeared of utmost importance. This implies the combined use of

• frequency reuse, whether fixed or dynamic, but necessarily leading to cochannel interference, and
• multiple access (e.g. FDMA, TDMA, CDMA) or random access(e.g. ALOHA, CSMA, ISMA, stack algorithm)

Cellular Frequency Reuse

Figure: Theoretical frequency reuse plan for cellular telephone network. The coverage area is split into may cells, each using its own radio channel. Channels can be reused in cells that are separated far enough from each other to avoid mutual interference of their radio signals.

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

Multiple Access

In future wireless 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 highly depends on the physical propagation characteristics of the channel.

To attempt a realistic analysis of the performance of wireless radio networks, two common assumptions for wireline networks must be refined:

1. a data packet is always received successfully if no conflicting transmission (collision) occurs simultaneously on the same channel
2. data packets involved in a collision are always lost.

In wireless networks, fading, inter and intra cell interference and capture effects are present and significantly affect performance, such as the throughput, stability and delay for a wireless ALOHA-net. Moreover, the performance seen by each participating terminal differs from the network average performance and highly depends on the terminal's location. The 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.

The material presented on the first edition in 1996 mainly contained material from a graduate course. In 1994 and 1995 this course was offered by the U.C. Berkeley Continuing Education in Engineering, University Extension program. In the second edition released in 1997, about 20 other authors extended and elaborated this material. The third edition (1999) shows a continuation of this trend. Contributions from some 35 authors include more than 1200 html pages, The quizzes and design exercises have been introduced and tested during a company training for engineers and designers. 

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Wireless Communication Reference Website 
© Jean-Paul M.G. Linnartz (Editor-in-Chief), 1996-2010.