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Fundamentals of Wireless Communication. David Tse Wireless Foundations U.C. Berkeley USCD June 9-10, 2006. 1. Introduction. Course Objective. Past decade has seen a surge of research activities in the field of wireless communication. - PowerPoint PPT Presentation
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Fundamentals of Wireless Communication
David TseWireless Foundations
U.C. Berkeley
USCDJune 9-10, 2006
2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
1. Introduction
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Course Objective• Past decade has seen a surge of research
activities in the field of wireless communication.• Emerging from this research thrust are new
points of view on how to communicate effectively over wireless channels.
• The goal of this course is to study in a unified way the fundamentals as well as the new research developments.
• The concepts are illustrated using examples from several modern wireless systems (GSM, IS-95, CDMA 2000 1x EV-DO, Flash OFDM.)
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
System Implementation
Capacity limits and communication techniques
Channel modelling
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Course OutlinePart I: Basics
2. The Wireless Channel 3. Diversity
4. Multiple Access and Interference Management
5. Capacity of Wireless Channels
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Course Outline (2)
Part II: Modern Wireless Communication
6. Opportunistic Communication and Multiuser Diversity
7. MIMO : Spatial Multiplexing and Channel Modeling
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Assumed background:
• Basic signals and systems, linear algebra and proabability.
• Basic digital communications.
2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
2. The Wireless Channel
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Wireless Mulipath Channel
Channel varies at two spatial scales:large scale fadingsmall scale fading
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Large-scale fading• In free space, received power attenuates like 1/r2.
• With reflections and obstructions, can attenuate even more rapidly with distance. Detailed modelling complicated.
• Time constants associated with variations are very long as the mobile moves, many seconds or minutes.
• More important for cell site planning, less for communication system design.
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Small-scale multipath fading• Wireless communication typically happens at very high
carrier frequency. (eg. fc = 900 MHz or 1.9 GHz for cellular)
• Multipath fading due to constructive and destructive interference of the transmitted waves.
• Channel varies when mobile moves a distance of the order of the carrier wavelength. This is about 0.3 m for 900 Mhz cellular.
• For vehicular speeds, this translates to channel variation of the order of 100 Hz.
• Primary driver behind wireless communication system design.
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Game plan• We wish to understand how physical parameters such as
– carrier frequency– mobile speed– bandwidth– delay spread – angular spread
impact how a wireless channel behaves from the communication system point of view.
• We start with deterministic physical model and progress towards statistical models, which are more useful for design and performance evaluation.
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Physical Models• Wireless channels can be modeled as linear time-varying
systems:
where ai(t) and i(t) are the gain and delay of path i.
• The time-varying impulse response is:
• Consider first the special case when the channel is time-invariant:
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Passband to Baseband Conversion• Communication takes place at• Processing takes place at baseband
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Complex Baseband Equivalent Channel
• The frequency response of the system is shifted from the passband to the baseband.
• Each path is associated with a delay and a complex gain.
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Modulation and Sampling
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Multipath ResolutionSampled baseband-equivalent channel model:
where hl is the l th complex channel tap.
and the sum is over all paths that fall in the delay bin
System resolves the multipaths up to delays of 1/W .
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Sampling Interpretation• hl is the l th sample of the
low-pass version of the channel response hb(¢).
• Contribution of the i th path is sinc(W(-i)) sampled at =l/W.
• Contribution is significant only when i is close to l/W.
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Multipath ResolutionSampled baseband-equivalent channel model:
where hl is the l th complex channel tap.
and the sum is over all paths that fall in the delay bin
System resolves the multipaths up to delays of 1/W .
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Flat and Frequency-Selective Fading• Fading occurs when there is destructive interference of
the multipaths that contribute to a tap.
Delay spread
Coherence bandwidth
single tap, flat fading
multiple taps, frequency selective
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Effective channel depends on both physical environment and bandwidth!
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Time Variations: Two-path Examplev= 60 km/hr, fc = 900 MHz:
direct path has Doppler shift of -50 Hz
reflected path has shift of +50 Hz
Doppler spread = 100 Hz
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Time Variations: General
Doppler shift of the i th path
Doppler spread
Coherence time
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Doppler Spread
Doppler spread is proportional to:
• the carrier frequency fc;
• the angular spread of arriving paths.
where i is the angle the direction of motion makes with the i th path.
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Types of Channels
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Typical Channels are Underspread
• Coherence time Tc depends on carrier frequency and vehicular speed, of the order of milliseconds or more.
• Delay spread Td depends on distance to scatterers, of the order of nanoseconds (indoor) to microseconds (outdoor).
• Channel can be considered as time-invariant over a long time scale.
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Statistical Models• Design and performance analysis based on statistical
ensemble of channels rather than specific physical channel.
• Rayleigh flat fading model: many small scattered paths
Complex circular symmetric Gaussian .
Squared magnitude is exponentially distributed.• Rician model: 1 line-of-sight plus scattered paths
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Correlation over Time• Specified by autocorrelation
function and power spectral density of fading process.
• Example: Clarke’s (or Jake’s) model.
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Additive Gaussian Noise• Complete baseband-equivalent channel model:
• Special case: flat fading:
• Will use this throughout the course.
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2: The Wireless Channel
Fundamentals of Wireless Communication, Tse&Viswanath
Summary• We have understood how time and frequency
selectivity of wireless channels depend on key physical parameters.
• We have come up with statistical channel models that are useful for analysis and design.
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