In this topic we give a high-level description of the wireless communications system and the technical challenges faced in network. Most notably, they are as follows:
- multipath propagation
- spectrum limitations;
- energy limitations;
- user mobility;
- noise limited systems;
- interference limited systems.
1. MULTIPATH PROPAGATION
For wireless communications, the transmission medium is the radio channel between transmitter TX and receiver RX. The signal can get from the TX to the RX via a number of different propagation paths. In some cases, a Line Of Sight (LOS) connection might exist between TX and RX, or the signal can get from the TX to the RX by being reflected at or diffracted by different Interacting Objects (IOs) in the environment. The number of these possible propagation paths is very large. Each of the paths has a distinct amplitude, delay (runtime of the signal), direction of departure from the TX, and direction of arrival;most importantly, the components have different phase shifts with respect to each other.
2. Spectrum Limitations
The spectrum available for wireless communications services is limited and regulated by international agreements. For this reason, the spectrum has to be used in a highly efficient manner. Two approaches are used: regulated spectrum usage, where a single network operator has control over the usage of the spectrum, and unregulated spectrum,where each user can transmit without additional control, as long as he complies with certain restrictions on the emission power and bandwidth.
3. Limited Energy
Truly wireless communications requires not only that the information is sent over the air (not via cables) but also that the MS is powered by one-way or rechargeable batteries. Otherwise, a MS would be tied to the “wire” of the power supply, batteries in turn impose restrictions on the power consumption of the devices. The requirement for small energy consumption results in several technical imperatives:
• The power amplifiers in the transmitter have high efficiency
• Signal processing must be done in an energy-saving manner.
• Maximum transmission power should be used only when required.
• For cellular phones, and even more so for sensor networks, an energy-efficient “standby” or “sleep” mode has to be defined.
4. User Mobility
Mobility is an inherent feature of most wireless systems has its important consequences for system design. Fading has already been discussed above. A second important effect is particular to mobile users in cellular systems: the system has to know at any time which cell a user is in:
• If there is an incoming call for a certain MS (user), the network has to know in which cell the
user is located.
• If an MS moves across a cell boundary, a different BS becomes the serving BS;in other words,
the MS is handed over from one BS to another. Such a handover has to be performed without
interrupting the call; as a matter of fact, it should not be noticeable at all to the user.
5. Noise-Limited Systems
Wireless systems are required to provide a certain minimum transmission quality.This transmission quality in turn requires a minimumSignal-to-Noise Ratio(SNR) at the receiver (RX). Consider now a situation where only a single BS transmits, and a Mobile Station (MS) receives; thus, the performance of the system is determined only by the strength of the (useful) signal and the noise. As the MS moves further away from the BS, the received signal power decreases, and at a certain distance, the SNR does not achieve the required threshold for reliable communications. Therefore, the range of the system is noise limited; equivalently, we can call it signal power limited. Depending on the interpretation, it is too much noise or too little signal power that leads to bad link quality.
6. Interference-Limited Systems
Consider now the case that the interference is so strong that it completely dominates the performance, so that the noise can be neglected. Let a BS cover an area (cell) that is approximately described by a circle with radius R and center at the location of the BS. Furthermore, there is an interfering TX at distance Dfrom the “desired” BS, which operates at the same frequency, and with the same transmit power. How large does D have to be in order to guarantee satisfactory transmission quality 90% of the time, assuming that the MS is at the cell boundary (worst case)? The computations follow the link budget computations of the previous section. As a first approximation, we treat the interference as Gaussian. This allows us to treat the interference as equivalent noise, and the minimum SIR,SIRmin, takes on the same values as SNRminin the noise-limited case.
SOME POSSIBLE QUESTIONS AND ANSWERS
1. Mention 2 different effects of multipath propagation.
Ans Multipath propagation leads to Fading and ISI (Inter Symbol Interference).
Fading: A simple RX cannot distinguish between the different Multi Path Components(MPCs); it just adds them up, so that they interfere with each other. Interference between them can be constructive or destructive, depending on the phases of the MPCs. The phases, in turn, depend mostly on the run length of the MPC, and thus 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 – namely, the changing of the total signal amplitude due to interference of the different MPCs – is called fading.
ISI: In a system with large bandwidth, and thus good resolution in the time domain, the major consequence is signal dispersion: in other words, the impulse response of the channel is not a single delta pulse but rather a sequence of pulses (corresponding to different MPCs), each of which has a distinct arrival time in addition to having a different amplitude and phase. This signal dispersion leads to Inter Symbol Interference (ISI) at the RX.
2. How to overcome spectrum Limitation in a spectraly limited region.
Ans. Proper Frequency splitting and reuse can increase the re-usablity of a frequency spectrum.