EE5401 Cellular Mobile Communications

EE5401 Cellular Mobile Communications

EE5401 Cellular Mobile Communications Dr. Chai Chin Choy (Email : [email protected]) Dr. Sun Sumei (Email : [email protected]) Institute for Infocomm Research (I2R) 1 Fusionopolis Way, #21-01 Connexis, Singapore 138632 Lecture notes can be downloaded from http://www1.i2r.a-star.edu.sg/~chaicc/

Institute for Infocomm Research

National University of Singapore

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Mainly focus on physical layer subjects. Knowledge on Digital Communications, Probability and Random Processes are required. Continuous Assessment – 30%


Final Examination – 70%


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Subject Outline Part 1 (Dr. Chai)

Introduction to Cellular Mobile Communications Radio Propagation : Large Scale Effects - Path loss prediction models - Shadowing

Multiple Access Techniques - Packet radio and random access - FDMA, TDMA, CDMA, SDMA - Orthogonal Frequency Division Mulitplexing (OFDM)



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Cellular System Concepts -

Cellular coverage and frequency reuse Channel assignment and control Cellular traffic System expansion techniques Brief Overview of System Standards: GSM, IS-95 (if time allows)



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Part 2 (Dr. Sun) •

Radio Propagation : Small Scale Effects -

•

Multipath models : Rayleigh, Rician Doppler effect, power spectra and signal correlation Coherence time and bandwidth Flat and selective fading channel Modulation Techniques

- Constant envelope and phase modulation - QPSK, π /4 QPSK, FSK, GMSK •

Equalization, Diversity and Coding Techniques - Linear and non-linear equalization - Selection, equal-gain and maximal ratio combining - Interleaving and convolutional coding



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References 1.

Theodore S Rappaport, Wireless Communications: Principles & Practice, Prentice-Hall, 2nd Edition (1st edition is fine).

2.

Jon W Mark, Weihua Zhuang, Wireless Communications and Networking, Prentice Hall.

3.

Simon R Saunders, Antennas and Propagation for Wireless Communication Systems, Wiley.

4.

William CY Lee, Mobile Communications Engineering, McGraw-Hill.

5.

JD Parsons, The Mobile Radio Propagation Channel, Wiley, 2nd Edition.

6.

Michel Daoud Yacoub, Foundations of Mobile Radio Engineering, CRC Press. (RBR: TK6570 Mob.Ya)



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7.

William C Jakes, Microwave Mobile Communications, IEEE Press.

8.

John Proakis, Digital Communications, 3rd Edition, McGraw-Hill Inc., New York



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Introduction

The target for mobile communications is to provide communications for anyone, from anywhere, at any time. A demanding task. Technological challenges include: 1. Time–varying, hostile communication channel.



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2. Location and tracking complexities due to mobility. 3. Efficient use of scarce resources such as frequency spectrum ⇒ cellular structure. The amount of interference generated is critical. 4. Power restrictions due to health issues.



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For carrier at 100MHz, at year 1940, the stability of oscillator at the base station is more than 100kHz, at year 2000 it is only 10Hz. This means less frequency guard band is needed.



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The exponential growth of mobile subscribers worldwide is due to the decreasing service charges and diminishing hardware costs. The key enabling technologies are: 1. RF technologies (such as improved frequency stability in electronics) 2. IC design (size) 3. Battery technology (weight and size) 4. Higher order modulation is made possible due to the use of more sophisticated advanced digital signal processing techniques.



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(including overhead on guard frequency band, roll-off factor etc.)



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5. Speech coding techniques – reduces the required bandwidth per channel.



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Cellular system

Example : Consider a system allocated total bandwidth of 12.5 MHz and each voice channel requires a 10kHz slot. We can only support 12.5MHz/10kHz = 1250 simultaneous conversations. If the penetration rate in Singapore is 10%, for a population of 3M+, this is equivalent to 300k users. What happen if 1% of the users making call at the same time? So what can we do? Answer: Channels need to be in someway reused or shared. -

Frequency bands are reused at different locations. With this, higher user capacity in the same frequency spectrum can be achieved.

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Each cell has a base station (BS), providing the radio interface to the mobile station (MS).



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- Technical challenge: interference issue, location tracking, etc., needs to be overcome.-



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-

A sophisticated technique called a handover enables a call to proceed uninterrupted across cell boundaries.



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-

All the BS’s are connected to a mobile switching centre (MSC) which is responsible for connection users to the public switched telephone network (PSTN).

-

Communication between the BS and MSs is defined by a standard common air interface that specifies 4 different physical channels: Forward (Downlink) voice/data channel: BS to MS Reverse (Uplink) voice/data channel: MS to BS Forward (Downlink) control channel: BS to MS Reverse (Uplink) control channel: MS to BS

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Control channels transmit and receive data messages that carry call initiation and service requests, and are monitored by mobiles when they do not have a call in progress. ~5% of total available channels.

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A MS contains a transceiver, an antenna and control circuitry. A BS consists of several transmitters and receivers.



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Mobile Radio Propagation : Large Scale Path Loss

The radio propagation channel exhibits many different forms of channel impairments, as a result of time-varying signal reflections, blockage and motion.



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Continuous measurements made along the radial direction



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Summary

Receiver Source

Transmit antenna

Path Loss

Shadowing Fast fading

Multiplicative noise



Receive antenna

AWGN

Additive noise

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Short-term and long-term fading

After removing the path loss component, we result in an instantaneous fading signal

r (t ) = a (t )e jϕ (t ) ,

The envelope of the signal is given as a (t ) = m(t )α (t ) m(t ) and α (t ) represent the long-term fading and short-term fading, respectively.

At a given time t, when MS is at physical spot A, y from BS, then a ( y ) = m( y )α ( y )

When no short-term fading, α (y) is a constant, long-term fading are the major factors.



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Measurements made along the tangential direction



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If severe short-term fading is present in the mobile radio environment, then

1 y+L mˆ ( y ) = m( y ) ∫ α ( x)dx , 2L y − L where m( y ) is the true local mean. A proper chosen of L between 40 λ and 200 λ willc 1 y+L make mˆ ( y ) → m( y ) or ∫ α ( x)dx → 1 . 2L y − L

The fast fading component

α ( y) =

a( y) or α ( y ) dB = a ( y ) dB − mˆ ( y ) dB mˆ ( y )

Single path fading – the amplitude follows some distributions, such as Rayleigh distribution, Rician distribution, etc. Multipath path fading – in each path, the amplitude follows some distributions. Intersymbol Interference (ISI) is also presence.



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Instantaneous received signal amplitude changes with time

Non-resolvable multipath resulting in (flat) fading

Resolvable multipath (Frequency selective fading)

Channel impulse response



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Large scale propagation model: predict the mean signal strength for an arbitrary transmitter-receiver (T-R) separation distance. This is useful in estimating the radio coverage area of a transmitter. -

Path loss : attenuation with distance. Shadowing (long-term fading) : due to the nature of the terrain, the average received signal is strong when the MS is at the high spot and weak at the low spot, even at the same distance from BS. This average signal is called local mean and is a RV. Its statistics follow the log-normal distribution.

Small scale propagation model : characterize the rapid fluctuations of the received signal strength over short travel distances around a few wavelength or short time duration (short-term fading). Propagation equation : all terms are in dB scale LdB = L path loss,dB + Lshadowing ,dB + Lshort −term fading ,dB

(what about if all quantities are in linear scale?)



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Notes on power levels : 1. dBw=10 log10 [ P (in W )] 2. dBm=10 log10 [ P (in mW)] ⎛P ⎞ 3. dB is a power ratio, ie. 10 log10 ⎜⎜ 1 ⎟⎟ ⎝ P2 ⎠

Reflection, diffraction and scattering are the three major causes which impact propagation in a mobile communication system.



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Reflection

Reflection coefficient of ground (a) vertical polarization (v) or E field in the plane of incidence. (b) horizontal polarization (h) or E field perpendicular to the incident plane



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E (x)

k (z)

H (y)



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Propagation over smooth plane : the received signal is the phasor sum of the direct wave and the reflected wave from the plane (2-ray model).



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Received power in dB First Fresnel Third Fresnel zone zone

− 40dB/decade Second Fresnel zone



Distance from BS

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The phase relationship Δφ between the reflected ground wave and the direct wave changes with distance and antenna height. Signal nulls appear if the components are in anti-phase. The first so-called Fresnel zone distance D f is a useful parameter in cellular design. Two ray models are only used to understand the path loss mechanism. In general, multiple reflection paths present, and have impact on the path loss, shadowing and short-term fading phenomenon. In open terrian, actual measured power is normally much higher, ie, n < 4 , or logdistance path loss model is generally given as Pr ⎛ 1 ⎞ ∝⎜ ⎟ Pt ⎝ d ⎠

n


, 3 < n < 4.


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Diffraction

Diffraction allows radio signals to propagate around the curved surface of the earth,



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beyond the horizon, and to propagate behind obstacles.

-

-

The phenomenon of diffraction can be explained by Huygen’s Principle. Each element of a wavefront (a surface of constant phase) at a point in time may be regarded as the centre of a secondary disturbance, which gives rise to spherical wavelets. The position of the wavefront at any later time is the envelope of all such wavelets.

Young’s double slit experiment: a series of interference fringes were viewed Institute for Infocomm Research


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Young’s single slit experiment



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Knife-edge object



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Estimating the signal attenuation caused by diffraction of radio waves over hills and buildings is essential in predicting the field strength in a given service area. It is mathematically difficult to make very precise estimates of the diffraction losses over complex and irregular terrian. Some cases have been derived, such as propagation over a knife-edge object. Multiple Knife-edge diffraction : For the presence of two knife edges, replace it by an equivalent knife edge. One way is as follow.



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Scattering

When encountering rough surfaces, reflected energy is spread out in all directions. It is therefore expected that the received signal is stronger than predicted from reflection and diffraction models alone.



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Log Distance Path Loss Models

Pt

Beyond this point, the relationship not necessary still valid

Po

P d0 Institute for Infocomm Research


d

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After considering all these effects, log-distance path loss model is given as n

⎛1⎞ Pr ∝ ⎜ ⎟ ⎝d ⎠ where n is the path loss exponent. For Free-space propagation model, n=2, and for tworay model n=4.

At a reference point d 0 with received power P0 , can show that n ⎛d ⎞ Pr ⎛ d 0 ⎞ = ⎜ ⎟ = ⎜⎜ ⎟⎟ P0 ⎝ d ⎠ ⎝ d0 ⎠

−n

The path loss from the reference point L path (d ) = P0 (in dB) - Pr (in dB) ⎛d ⎞ = 10n ⋅ log10 ⎜⎜ ⎟⎟ ⎝ d0 ⎠

Be careful if we want to extrapolate the curve for d < d 0 , which follows another rule.



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Pt

Po P1

⎛d ⎞ ⎜⎜ ⎟⎟ ⎝ d0 ⎠

n0

⎛d⎞ ⎜⎜ ⎟⎟ ⎝ d1 ⎠

n1

Pr

d0


d1


d

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The value of n depends on the amount of clutter in the environment. Usually Environment Free space

2

Urban area cellular radio

2.7 to 3.5

Shadowed urban cellular radio

n

3 to 5

In building line-of-sight

1.6 to 1.8

Obstructed in building

4 to 6

Obstructed in factories

2 to 3

Sometime different values are used for n depending on the distance from the transmitter.



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-

n does not directly reflect the strength of the received power, for example,



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Log-normal Shadowing

Two locations at the same distance from the transmitter can experience substantial differences in signal level compared to the expected average value. This phenomenon is caused by large buildings, foliage, etc that obstruct the propagation path and is known as shadowing. Experimental trails have shown that shadowing effects can be well modeled by a RV with a log-normal distribution. ⎛d ⎞ L path + shadowing (d ) = 10 ⋅ n ⋅ log10 ⎜⎜ ⎟⎟ + X σ ⎝ d0 ⎠

Log-normal distribution 1. X = e Y where Y is normally distributed.



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2. A Gaussian distribution on a log-scale, ie. If xs be the attenuation due to shadowing. Then X dB = 10 log10 xs , then

⎛ X 2 ⎞ dB ⎟ exp⎜ − f ( X dB ) = 2 ⎟ ⎜ 2π σ X dB ⎝ 2σ X dB ⎠ The standard deviation σ X dB is known as the shadow dB spread. 1



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Empirical Models

Many empirical models have been suggested in the literature. Okumura’s model has gained widespread acceptance. This model is based entirely on measurements. L path = L free + Amu ( f , d ) − G (ht ) − G (hr ) − G Area h G (ht ) = 20 log10 ( t ) 1000 m < ht < 10 m 200 h G (hr ) = 10 log10 ( r ) hr ≥ 3 m 3

G (hr ) = 20 log10 (

hr ) 3

10 m > hr > 3 m

where Amu is the attenuation relative to free space, ht is the base station antenna height, hr is the mobile antenna height, G(.) is the antenna height gain factor for the base or mobile station, and G Area is the gain due to the type of environment. Institute for Infocomm Research


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Other corrections may also be applied to Okumura’s model. Some of the important terrain such as slope of terrain, mixed land-sea parameters,… Hata’s model : Empirical formulation of Okumura’s model.

L = 69.55 + 26.16 log( f MHz ) − 13.82 log(ht ) − a (hr ) + [44.9 − 6.55 log(ht )]log(d km )

For a small/medium city: a(hr ) = [1.1log( f MHz ) − 0.7]hr − [1.56 log( f MHz ) − 0.8] For a large city, where f c < 300MHz : a (hr ) = 8.29[log(1.54hr )]2 − 1.1 For a large city, where f c > 300MHz : a (hr ) = 3.2[log(11.75hr )]2 − 4.97



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Summary

Receiver Source

Transmit antenna

Path Loss

Shadowing Fast fading

Multiplicative noise



Receive antenna

AWGN

Additive noise

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Other useful References “Antenna and Propagation for Wireless Communication Systems” by Simon R. Saunders – More channel models for pico, micro and macro-cell can be found in the book “Mobile Communications Engineering”, by William CY Lee – More description on the principle behind the channel characterization, and provide with the relevant references.



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EE5401 Cellular Mobile Communications

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