Fundamentals of Wireless Communication

Transcription

1 Fundamentals of Wireless Communication David Tse University of California, Berkeley Pramod Viswanath University of Illinois, Urbana-Champaign Fundamentals of Wireless Communication, Tse&Viswanath

2 1. Introduction Fundamentals of Wireless Communication, Tse&Viswanath

3 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 x EV-DO, Flarion's Flash OFDM, ArrayComm systems.) Fundamentals of Wireless Communication, Tse&Viswanath 2

4 System Implementation Capacity limits and communication techniques Channel modelling Fundamentals of Wireless Communication, Tse&Viswanath 3

5 Part I: Basics Course Outline 2. The Wireless Channel 3. Diversity 4. Multiple Access and Interference Management 5. Capacity of Wireless Channels Fundamentals of Wireless Communication, Tse&Viswanath 4

6 Course Outline (2) Part II: Modern Wireless Communication 6. Opportunistic Communication and Multiuser Diversity 7. MIMO I: Spatial Multiplexing and Channel Modeling 8. MIMO II: Capacity and Multiplexing Architectures 9. MIMO III: Diversity-Multiplexing Tradeoff Fundamentals of Wireless Communication, Tse&Viswanath 5

7 Assumed background: Basic signals and systems, linear algebra and proabability. Basic digital communications. Fundamentals of Wireless Communication, Tse&Viswanath 6

8 These slides only gives an overview of the ideas. Full details can be found in: Fundamentals of Wireless Communication, Tse&Viswanath 7

9 2. The Wireless Channel Fundamentals of Wireless Communication, Tse&Viswanath

10 Wireless Mulipath Channel Channel varies at two spatial scales: large scale fading small scale fading Fundamentals of Wireless Communication, Tse&Viswanath 9

11 Large-scale fading In free space, received power attenuates like 1/r 2. 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. Fundamentals of Wireless Communication, Tse&Viswanath 10

12 Small-scale multipath fading Wireless communication typically happens at very high carrier frequency. (eg. f c = 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. Fundamentals of Wireless Communication, Tse&Viswanath 11

13 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. Fundamentals of Wireless Communication, Tse&Viswanath 12

14 Physical Models Wireless channels can be modeled as linear timevarying systems: where a i (t) and τ 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 timeinvariant: Fundamentals of Wireless Communication, Tse&Viswanath 13

15 Passband to Baseband Conversion Communication takes place at Processing takes place at baseband Fundamentals of Wireless Communication, Tse&Viswanath 14

16 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. Fundamentals of Wireless Communication, Tse&Viswanath 15

17 Modulation and Sampling Fundamentals of Wireless Communication, Tse&Viswanath 16

18 Multipath Resolution Sampled baseband-equivalent channel model: where h l 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. Fundamentals of Wireless Communication, Tse&Viswanath 17

19 Sampling Interpretation h l is the l th sample of the low-pass version of the channel response h b ( ). i = 0 1 W Main contribution l = 0 Contribution of the i th i = 1 Main contribution l = 0 path is the projection of a ib δ(τ-τ i ) onto sinc(wτ-l). i = 2 Main contribution l = 1 i = 3 Main contribution l = 2 i = 4 Main contribution l = l Fundamentals of Wireless Communication, Tse&Viswanath 18

20 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 Fundamentals of Wireless Communication, Tse&Viswanath 19

21 Effective channel depends on both physical environment and bandwidth! Fundamentals of Wireless Communication, Tse&Viswanath 20

22 Time Variations Doppler shift of the i th path Doppler spread Coherence time Fundamentals of Wireless Communication, Tse&Viswanath 21

23 v= 60 km/hr, f c = 900 MHz: Two-path Example direct path has Doppler shift of -50 Hz reflected path has shift of +50 Hz Doppler spread = 100 Hz Fundamentals of Wireless Communication, Tse&Viswanath 22

24 Doppler Spread Doppler spread is proportional to: the carrier frequency f c ; the angular spread of arriving paths. where θ i is the angle the direction of motion makes with the i th path. Fundamentals of Wireless Communication, Tse&Viswanath 23

25 Fundamentals of Wireless Communication, Tse&Viswanath 24

26 Types of Channels Fundamentals of Wireless Communication, Tse&Viswanath 25

27 Typical Channels are Underspread Coherence time T c depends on carrier frequency and vehicular speed, of the order of milliseconds or more. Delay spread T d 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. Fundamentals of Wireless Communication, Tse&Viswanath 26

28 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 Fundamentals of Wireless Communication, Tse&Viswanath 27

29 Specified by autocorrelation function and power spectral density of fading process. Example: Clarke s (or Jake s) model. Correlation over Time Fundamentals of Wireless Communication, Tse&Viswanath 28

30 Additive Gaussian Noise Complete baseband-equivalent channel model: Special case: flat fading: Will use this throughout the course. Fundamentals of Wireless Communication, Tse&Viswanath 29

31 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. Fundamentals of Wireless Communication, Tse&Viswanath 30