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Presentation Transcript

ECE 5221 Personal Communication Systems

Prepared by:

Dr. Ivica Kostanic

Lecture 4: Estimation of coverage reliability

Spring 2011

Outline

- Macroscopic propagation modeling
- Edge reliability
- Area reliability
- Reudnik curves and fade margin calculations
- Examples

Important note: Slides present summary of the results. Detailed derivations are given in notes.

Macroscopic propagation modeling

Log distance path loss model

- More input descriptors – more accurate models
- As the models become more accurate, the standard deviation of the unexplained portion of path loss becomes smaller
- The unexplained portion still retains log normal character

More general models

- Macroscopic models predict median path loss at some distance d
- As one measures the actual path loss, its value will always be different than predicted
- The difference is a log normal random variable with zero mean and variance that depends on environment

Expected accuracy of propagation model

- Macroscopic propagation models – limited accuracy
- Accuracy depends:
- Input data accuracy
- Type of the environment
- Computational time
- Model limitations
- The accuracy is quantified through standard deviation of prediction error
- For a well tuned model, standard deviation of prediction error is 6-8dB
- Note: the error is relatively large
- GOAL: coverage design using imperfect tools

Comparison of measurements and predictions

Distribution of prediction error

Edge reliability

- RSLT – Coverage threshold that needs to be met by the network. The threshold determined from coverage objectives
- RSLT – contour provides 50% reliability (i.e. if one walks around the contour the threshold is met only 50% of locations)
- RSLP – contour that provides required reliability for meeting the threshold RSLT
- RSLP=RSLT + D, where D is the value that needs to be determined based on required edge reliability
- Mathematically:

Goal: determine RSLP contour that meets edge reliability requirements

Edge reliability - example

Assume that one needs to perform design for RSLT = -90dBm. The area is characterized with standard deviation of s=8dB. What contour RSLP provides 70% edge reliability.

Answer: RSLP = -85.2dBm, D=4.8dB

Following the same approach one obtains the table

Concept of area reliability

- Coverage is an areal phenomenon
- Design needs to guarantee specified area reliability
- One needs to find RSLP contour such that

Where Rais the area reliability.

Typical values for area reliability are 90-95%

Note: there is tradeoff between coverage reliability and cell count

Illustration of cell coverage area

Calculation of area reliability (result)

Area reliability

- Notes:
- Equation – to complicated for day to day use
- Gives the answer
- Need for easier way to calculate

Based on log-distance path model

Where

Reudnik curves

Edge reliability

Area reliability calculations – complicated

Edge reliability calculations – easy

Reudnik curves relate area and edge reliabilities

Area reliability

Properties of environments

Area reliability - examples

Example 1: Consider environment with s/n = 3. Determine reliability over the area bounded with a contour having edge reliability of 70%

Answer: 85%

Example 2: Consider the following design task

Design threshold: -95dBm

Area reliability: 90%

Path loss exponent: 3.84

Standard deviation of the modeling accuracy: 8dB

Determine:

- Edge reliability requirement Answer: 75%
- Required prediction contour Answer: -89.4dBm

Fade margin – calculations (direct method)

- Fade margin – difference between RSLP and RSLT
- Can be calculated directly from area reliability requirement, s and n
- Process:
- Calculate s/n
- Determinez-score (table lookup)
- Fade margin is calculated as z-score x s

Example

- Calculate the fade margin for the following scenario
- Area reliability requirement: 95%
- Model uncertainty: 8dB
- Slope: 35dB/dec

Answers:

s/n = 2.29

z-score: 1.10

FM = 1.10 x 8 = 8.8 dB

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