Understanding Multipath Fading and OFDM in Wireless Communication
Hata Model for Urban Path Loss Prediction
The Hata Model is an empirical formula used to estimate the median path loss for radio signals in urban environments with quasi-smooth terrain. The formula considers factors such as frequency, base station antenna height, mobile antenna height, and distance:
Lp = 69.55 + 26.16 log fc – 13.82 log hb – a(hm) + [44.9 – 6.55 log hb] log d
where:
- fc is the frequency in MHz (150 MHz < f < 1500 MHz)
- hb is the base station antenna height in meters (30m < hb < 200m)
- hm is the mobile antenna height in meters (1m < hm < 10m)
- d is the distance in kilometers (1km < d < 20km)
The correction factor a(hm) accounts for the mobile antenna height and varies depending on the city size:
Correction Factor a(hm)
- Small/Medium City: a(hm) = (1.1 log fc – 0.7) hm – (1.56 log fc – 0.8)
- Large City:
- a(hm) = 8.29 (log 1.54 hm)^2 – 1.1, for fc <= 200 MHz
- a(hm) = 3.2 (log 11.75 hm)^2 – 4.97, for fc <= 400 MHz
Path Loss in Suburban and Open Areas
- Suburban: Lp = Lp(Urban) – [2 log({fc / 28}^2 – 5.4)]
- Open: Lp = Lp(Urban) – 4.78 (log fc)^2 + 18.33 log fc – 40.94
Multipath Fading and Channel Models
Multipath fading occurs when radio signals arrive at the receiver via multiple paths, causing interference and fluctuations in signal strength. Different fading models characterize various propagation environments:
Types of Fading Channels
- AWGN (Additive White Gaussian Noise): Idealized channel with only thermal noise, not suitable for mobile communication.
- Log-Normal Fading: Characterizes gradual changes in local mean power due to obstacles like trees and buildings.
- Rayleigh Fading: Occurs when there is no direct line-of-sight path between transmitter and receiver, resulting in multiple indirect paths.
- Rician Fading: Exists when there is a direct line-of-sight path along with multiple indirect paths, often found in indoor environments.
OFDM (Orthogonal Frequency Division Multiplexing)
OFDM is a multicarrier modulation technique that divides a high-speed data stream into multiple lower-rate sub-streams transmitted over orthogonal subcarriers. This offers several advantages:
Advantages of OFDM
- Robustness to frequency-selective fading
- Efficient spectrum utilization
- Resilience to multipath interference
- Flexible subcarrier allocation
- Compatibility with MIMO (Multiple-Input Multiple-Output) systems
Disadvantages of OFDM
- High peak-to-average power ratio
- Sensitivity to carrier frequency offset
- Complex synchronization requirements
- Susceptibility to impulse noise
- Inherent latency
Multi-Antenna Techniques
Multi-antenna techniques, such as MIMO, improve communication performance by exploiting spatial diversity and multiplexing:
Types of Multi-Antenna Systems
- SISO (Single-Input Single-Output): One transmitter and one receiver antenna.
- MISO (Multiple-Input Single-Output): Multiple transmitter antennas and one receiver antenna.
- SIMO (Single-Input Multiple-Output): One transmitter antenna and multiple receiver antennas.
- MIMO (Multiple-Input Multiple-Output): Multiple transmitter and receiver antennas, offering significant improvements in data rates and reliability.
Multicarrier Basics
Multi-Carrier Modulation (MCM) is a technique that splits data into multiple sub-streams transmitted on different frequencies (subcarriers). OFDM is a popular MCM scheme used in wireless and cellular communications, offering high bandwidth and efficient spectrum utilization.