Wireless Communications: Power, Channel and System Parameters

Definition of Key Concepts: Power Density, Power Spectral Density, Energy

Q1.A.   Briefly define the following:

(i)Peak power

(ii)Average power. Can this be used with non-periodical signals?

(iii)Power density 

(iv)Power spectral density

(v)Energy. Estate in what situations energy is used instead of

power and why.

Using an equation explain how the power density decreases with the distance from a transmitter and state the impact of this in terms of the signal to noise ratio at the mobile receiver.

B.Write down an expression for the channel capacity (bits/s/km2) in terms of the three key system parameters and discuss their limitations, and drawbacks and suggest what actions need taken.

C.A mobile service provider offers wireless services with the following specifications:

The carrier frequency = 3 GHz

Total allocated bandwidth = 170 MHz

Two 40 kHz simplex channels to provide a full duplex voice channel

The total coverage area = 200 km2

Cell radius = 0.5 km.

Calculate:  

(i)Total number of channels

(ii)The cell area

(iii)The total number of cells.

(iv)The control channel, which is 2% of the total number of channels.

Q2.A.There are several mitigation techniques that could be adopted in wireless systems to combat the multipath fading channel. Name four of them and state which technique is the most widely used in wireless communications and why.

B.Draw a schematic block diagram of single input multiple output wireless system. Using mathematical symbols label all modules shown in the diagram including the transmitter, channel, and receiver. 

Using the vector model:

(i)Dreive an expression for the combined signal at the receiver.

Q3 A.In wireless mobile communications there are many factors that contribute to the propagating signal experiencing fading effect, which will result in performance degradation. State five of them and their impact on the link performance.

B.In wireless communication systems, there are two types of fading. Briefly describe what they are and use time waveforms to illustrate them. Note, the illustration must be properly and accurately labelled.

C.In fast fading channel environments, the received radio frequency signal amplitude can be represented as the sum of delayed components from different directions. Write expressions for:

(i)The received signal in terms of the phasor notation and decompose it into its in-phase and quadrature components. 

(ii)The envelope of the the received signal. State what would be the probability density function of the envelop assuming that all received components have approximately the same power and all are due to scattering.

Q4.A.The 4th generation (4G) wireless technology is being widely used in today’s mobile communications, and are being replaced by the  5th generation (5G) wireless technologies. Compare and contrast these two technologies in terms of the following key system parameters:

Supporting multimedia (voice, video, and data)

Data traffic

Peak data rates

Latency

Connectivity density

Available frequency spectrum (i.e., bandwidth).

Name three disadvantages of 4G wireless systems. 

B.In 4G and 5G wireless technologies the most widely adopted modulation technique is the Orthogonal Frequency Division Multiplexing (OFDM). 

(i)Outline the four key features of OFDM 

(ii)Using frequency spectrums compare OFDM with the standard Frequency Division Multiplexing (FDM)

(iii) Comment on the total bandwidth usage of OFDM and FDM.

Q5 A.Briefly describe the following:

(i)Why the picocell is not the best candidate for a high-capacity indoor application.

(ii)The coherence bandwidth Bc and write an expression for it in terms of the multipath time (or the channel delay spread).

B.In wireless communications the transmitted signal may arrive at the receiver via different paths, thus multipath propagation. Write expressions for:

(i)The time invariant multi-path channel impulse response

(ii)The channel transfer function.

And define all the terms in both equations.

C.Consider a 7-cell cluster cellular mobile system where base station antennas are located at the centre of each cell. The operating carrier frequency for cell 1 is 1.5 GHz and the antenna has an equivalent isotropic radiation power (ERIP) of 250. In cell 1, the mobile unit with a gain of 3 dB is located right at the edge of the cell. 

Calculate:

(i)The effective aperture area Ae of the receiving antenna

(ii)The frequency re-use factor.