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Fundamentals of Wireless Communication, Tse&Viswanath Fundamentals of Wireless Communication David Tse Wireless Foundations U.C. Berkeley USCD June 9-10, 2006

Fundamentals of Wireless Communication

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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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Page 1: Fundamentals of  Wireless Communication

2: The Wireless Channel

Fundamentals of Wireless Communication, Tse&Viswanath

Fundamentals of Wireless Communication

David TseWireless Foundations

U.C. Berkeley

USCDJune 9-10, 2006

Page 2: Fundamentals of  Wireless Communication

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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Fundamentals of Wireless Communication, Tse&Viswanath

System Implementation

Capacity limits and communication techniques

Channel modelling

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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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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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Fundamentals of Wireless Communication, Tse&Viswanath

Assumed background:

• Basic signals and systems, linear algebra and proabability.

• Basic digital communications.

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2: The Wireless Channel

Fundamentals of Wireless Communication, Tse&Viswanath

2. The Wireless Channel

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Wireless Mulipath Channel

Channel varies at two spatial scales:large scale fadingsmall scale fading

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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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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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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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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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Passband to Baseband Conversion• Communication takes place at• Processing takes place at baseband

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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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Modulation and Sampling

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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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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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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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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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Effective channel depends on both physical environment and bandwidth!

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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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Time Variations: General

Doppler shift of the i th path

Doppler spread

Coherence time

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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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Types of Channels

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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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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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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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Additive Gaussian Noise• Complete baseband-equivalent channel model:

• Special case: flat fading:

• Will use this throughout the course.

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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.