1. A Problem in LTE Communication
Rayyan Sayeed
HON 202
February 26th 2015

2. Background
A signal transmitted from a source to a receiver experiences perturbations caused by four factors:
Path Loss (Attenuation)
Slow Fading
Fast Fading
Noise (AWGN)
My problem involves using the behaviors of these four factors, and the nature of the technology present in LTE communication, to enhance the wireless user experience in a specific capacity.

3. Attenuation
Attenuation, or path loss, refers to the gradual loss of signal power that occurs during the travel of a signal from a source to a receiver.

4. More on Path Loss Inverse Square Law for Free Space
: Path Loss Exponent
It is within (2, 4) and has been observed to be as high as 6
Rain, atmospheric conditions, foliage, vegetation affect the exponent

5. Far-field
We will make the assumption that the distance between the transmitting and receiving antennas is much larger than the wavelength of the emitted wave.
Under this assumption, our model for path loss will be accurate.

6. HATA Model for Path Loss
An empirical path loss model
i.e. based on a large set of collected data
Applies to signal frequencies between 150 MHz and 1.5 GHz
The signal frequency of Verizon Wireless is 700 MHz, which falls in the range of accepted frequencies.

7. Path Loss =
Lp (dB) = log10(fo) + ( log10 hb)log10d –13.82 log10 hb - a (hmu)
fo = carrier frequency ( In MHz)!!
d = distance between tx and rx in KM
hb = base station antenna height (m)
hmu = mobile unit antenna height (m)
a(hmu) = correction factor for MU antenna height
Large cities:
a(hmu) = 3.2 [log(11.75 hmu)]2 – dB fo  300 MHz
a(hmu) = 8.29 [log(1.54 hmu)]2 – 1.1 dB MHz  fo

8. Slow Fading (Shadowing)
Pr/Pt
(dB)
log d
Very slow
Slow
-10
Received signal is “shadowed” by obstructions, in addition to path loss (a function of distance).
Mobile antenna is well below buildings in the vicinity.
Mean Signal Strength is given by path loss (blue straight line)
Slow fading accounts for the variation of the mean (red curve)

9. More on Slow Fading Models attenuation from obstructions
Random due to random number, and type, of obstructions
CLT works with sums of random variables
Power expressed in dB’s is the sum of independent powers
Received Signal power in dBs (a log system) is normally distributed
Therefore, slow fading is log-normal.

10. Rayleigh Random Variable
In words, a Rayleigh random variable is obtained by taking the square root of the sum of the squares of two independent, random, Gaussian variables.

11. Fast Fading Fast fading is also known as multipath or Rayleigh fading.
Occurs when a transmitted signal is reflected by multiple objects in its path to the receiver.
These objects are roughly the size of one wavelength in dimension.
At 700 Mhz, 1 wavelength is about 42 cm.
Thus, the signal arrives at the receiver from multiple angles (multiple paths, hence multipath fading).

12. Many reflections coming to antenna Reflected paths are independent
Path gains are independent
Angle of arrival is uniformly distributed
No direct Line of Sight (LOS)
Power is distributed in space
Correlated by spatial separation

13. Power varies according to space.
Analytically found to be Rayleigh in distribution.

15. Noise (AWGN) Noise is additive We will deal with Thermal Noise
Thermal Noise Floor: The lowest possible noise power that any radio component must have
In frequencies of interest: f > 30 MHz, Thermal Noise dominates over Galactic Noise, Man-made noise, and atmospheric noise
Pn = Thermal Noise Floor = kTB watts
k = Boltzmann's Constant = 1.38X10-23 joules/oK
T = temperature in Kelvin (room temperature = 290 oK)
B = bandwidth of signal (in our case, 5 Mhz)

16. Problem Statement ! App Server knows nothing of LTE Network State
Application
Server
Application
Client
LTE
Wired
Network
Radio
Link
Mobile
Base Station
Changes quality depending on what YouTube thinks it gets
Path Loss, Fading, Noise
& Handoff
During handoff, nature shows the UE an uncorrelated channel
There is an abrupt change in SINR and Throughput
Can we predict the new SINR before handoff so that YouTube will know that a handoff is coming and knows the new quality it should transmit ?
A multidisciplinary and challenging problem
PhysicsTechnology  Mathematical Modeling  Statistics !
SINR = Signal to Noise to Interference Ratio = Link Quality