Essay: Multipath propagation

1.3 Multipath propagation
Effects of multipath propagation in cellular network:
1. Multipath Propagation:
For wireless communications, the transmission medium is the radio channel between transmitter TX and receiver RX. The signal transfers from the TX to the RX via number of different propagation paths.
In some cases, a Line Of Sight (LOS) connection might exists between the TX and the RX. Furthermore, the signal can get from the TX to the RX by being reflected or diffracted by different Interacting Objects (IOs) in the environment such as houses, mountains, windows, walls, etc. The number of these possible propagation paths is very large.
Multipath propagation
2. Fading:
A simple RX cannot distinguish between the different multipath Components (MPCs). It just add them up so that they can interfere with each other. The interference between them can be constructive or destructive by depending on the phases of the MPCs(see Figure).
The phases, in turn, depends mostly on the run length of the MPC and also on the position of the Mobile Station (MS) and the IOs. For this reason, the interference and the amplitude of the total signal changes with time if either TX, RX or IOs is moving. This effect is named as the changing of the total signal amplitude due to interference of the different MPCs – is called small-scale fading.
Principle of small scale fading
3. Inter symbol Interference:
The runtimes for different MPCs are different. This can lead to different phases of MPCs, which lead to interference in narrowband systems.
In a system with large bandwidth and good resolution in the time domain, the major consequence is signal dispersion. The impulse response of the channel is not a single delta pulse but rather a sequence of pulses has a distinct arrival time in addition to having a different amplitude and phase.
ISI is essentially determined by the ratio between symbol duration and the duration of the impulse response of the channel. This also implies that ISI is not only more important for higher data rates but also for multiple access methods that lead to an increase in transmitted peak data rate.
Multipath components with different runtimes and its channel impulse response
1.4 Spectrum Limitations
Concept of Spread Spectrum
In a wideband spread-spectrum (SS) system, the transmitted signal is spread over a frequency band that is much larger, in fact, than the maxim um bandwidth required to transmit the information bearing (baseband) signal. An SS system takes a baseband signal with a bandwidth of only a few kilohertz (kHz) and spreads it over a band that may be many megahertz (MHz) wide. In SS systems, an advantage in signal-to-noise ratio (SNR) is achieved by the modulation and demodulation process.
The SS signal is generated from a data-modulated carrier. The data-modulated carrier is modulated a second time by using a wideband spreading signal. An SS signal has advantages in the areas of security, resistance to narrowband jamming, resistance to multipath fading and supporting multiple -access techniques.
The spreading modulation may be phase modulation or a rapid change of the carrier frequency or it may be a combination of these two schemes. When spectrum spreading is performed by phase modulation, we call the resultant signal a direct-sequence spread spectrum (DSSS) signal. When spectrum spreading is achieved by a rapid change of the carrier frequency, we refer to the resultant signal as a frequency-hop spread spectrum (FHSS) signal.
When both direct-sequence and frequency-hop techniques are employed, the resultant signal is called a hybrid DS FHSS signal. Another way to also generate an SS signal is the time-hop spread spectrum (THSS) signal. In this case, the transmission time is divided into intervals called frames. Each frame is further divided into time slots. During each frame, one and only one time slot is modulated with a message.
Spread spectrum techniques
The DSSS is the averaging technique to reduce interference whereas FHSS and THSS are the avoiding techniques to minimize interference. The spreading signal is selected to have properties to facilitate demodulation of the transmitted signal by the intended receiver and to make demodulation by an unintended receiver as difficult as possible. These same properties also make it possible for the intended receiver to differentiate between the communication signal and jamming.
If the bandwidth of the spreading signal is large relative to the data bandwidth, the spread-spectrum transmission bandwidth is dominated by the spreading signal and is independent of the data signal bandwidth.
1.5 Noise and Interference limited systems
Noise:
Noise is the unwanted electronic signals that harm the quality of something (such as a radio or television broadcast or a digital photograph).
Types of noise:
Thermal noise
Man-made noise
Receiver noise
Interference:
Co-channel communication signal
Noise limited system:
A system where PN is the major limiting factor for SINR.
Sometimes, it is difficult to tell between interference and noise. One difference between interference and noise lies in the fact that interference suffers from fading, while the noise power is typically constant (averaged over a short time interval). We can vary location, frequency and time to avoid interference, while it’s not possible for noise.
Interference limited system:
A system where is the major limiting factor for SINR. This is the case when the channel is reused at different locations.
User’s throughput is higher with higher SINR. However, we are more concerned with the system throughput, which is the sum of all users’ throughput. Clearly, if we raise the power of this user for higher SINR, other users’ SINR is lowered.
A cellular system is about channel reuse, it is designed to separate co-channel users away from each other, to mitigate the co-channel interference.
1. One user per channel in each cell (the channel could be shared in time), a user will not be interfered by other users of the same cell.
2. Reuse the channel in neighboring cells, while avoiding generating too much interference among cells. One way is not to use the same channel among direct neighbor cells. It’s a simple and effective solution.