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EEEN60162 Wireless Communication and Mobile Networks
Answer all questions.
Electronic calculators may be used in accordance with the University regulations.
Question 1
A mobile user of an LTE system is standing in between a macro base station (BS) and a micro BS. The macro and micro BS are set up at the height of 25 m and 5 m above ground respectively, and the antenna gains are 13 dBi and 10 dBi respectively. The maximum transmission power for the macro and micro BS is 23 dBW and 20 dBm respectively, and the carrier frequencies are 800 MHz and 2.6 GHz respectively. The height of the user is 1.6 m, and the user equipment (UE) has an antenna gain of 0 dBi. The user is 900 m away from the macro BS, and 200 m away from the micro BS. The minimum received signal power is -103 dBm. The handoff threshold for this system is set to be 3 dB. (Speed of light = 3108 m/s)
(a) The radio channel from the macro BS to the UE has a path loss exponent of 3.5. Free space propagation can be used for the first 1 m. Calculate the received signal power (in dBm) at the UE. [6 marks]
(b) From the micro BS to the user, the channel can be modelled using the plane earth model. Calculate the received signal power (in dBm) at the UE. [4 marks]
(c) The UE is currently connected to the macro BS.
(i) Explain whether a handoff request be initiated.
(ii) Calculate the path loss exponent from the macro BS such that the handoff request from part c(i) is reversed. [2, 4 marks]
(d) Considering the “Ping-pong” effect:
(i) Explain this phenomenon and discuss how it can be avoided.
(ii) Explain whether this effect will occur when a handoff takes place in the case described in part (c). [2, 2 marks]
Question 2
The downlink LTE OFDMA system is being evaluated using the ITU Vehicular A channel model with the following parameters:
|
Tap |
1 | 2 | 3 | 4 | 5 | 6 |
|
Excess delay (ns) |
0
|
310
|
710
|
1090
|
1730
|
2510 |
|
Average power (dB) |
0
|
-1
|
-9
|
-10
|
-15
|
-20 |
(a) Calculate the mean excess delay and RMS delay spread (in µs) for this channel. [6 marks]
(b) In LTE, each resource block (RB) contains 12 subcarriers, with each subcarrier having 15 kHz. Considering the 50% coherence bandwidth, explain whether or not this design is appropriate for this channel. [4 marks]
(c) The LTE standard specifies the time slot for an RB as 0.5 ms, which contains 7 symbols. The system schedules transmissions based on 2 consecutive slots (known as the RB-pair).
(i) In order to have slow fading within an RB-pair and using 50% coherence time, calculate the maximum vehicular speed (in km/hr) of a UE that uses the 800 MHz spectrum. (Speed of light = 3108 m/s)
(ii) Based on the answer in part (i), explain what happens if 2.6 GHz is used for high vehicular speed users and why operators should use 800 MHz spectrum for these users. [4, 3 marks]
(d) Explain why LTE schedules the transmissions per an RB-pair, but not per individual subcarriers and symbols. [3 marks]
Total [20 marks]
Question 3
(a) Consider an eigen-beamforming MIMO system with 4 transmit antennas and 4 receive antennas.
(i) Draw the block diagram of the system.
(ii) Describe the water-filling algorithm that achieves optimal power allocation for this system. [3, 4 marks]
(b) Name three common diversity types suitable for terrestrial mobile networks, and give an example for each one. [3 marks]
(c) For each of the following statements, state if it is True or False, and provide explanations to support your answers.
(i) The uncorrelated scattering assumption states that the autocorrelation of the same arrival path at different time is zero.
(ii) Selection combining performs better than equal gain combining when one branch has a much larger SNR than the others.
(iii) The Doppler spread equals to two times the Doppler frequency because the users can be moving towards and away from the base station
(iv) In Load Control for CDMA, handover to another radio access technology is the last resort to reduce the load in a cell.
(v) Maximum rate scheduling approach is an unfair scheduling approach. [2, 2, 2, 2, 2 marks]
Total [20 marks]
Question 4
(a) Design an OFDM system with 2.2 MHz bandwidth using 64 QAM. The coherence bandwidth of the channel is 20 kHz, and the coherence time is 1 ms. The last arrival path is 2 μs.
(i) Determine the number of subcarriers.
(ii) Considering cyclic prefix, calculate the maximum frame duration.
(iii) If 30 overhead symbols are needed, calculate the data rate of your system in bps. [3, 3, 4 marks]
(b) Consider a 2.2 MHz bandwidth CDMA system with a 100 kbps user and a 50 kbps user.
(i) What is the processing gain for each user?
(ii) If the signal to noise ratio (SNR) for both users are the same and SNR is 12 dB, what is the pre-detection signal to noise plus interference ratio (SNIR) for each user?
(iii) Assuming that there is a jammer transmitting at 44 dB over the noise level, what is the SNIR for each user now? [2, 4, 4 marks]
Total [20 marks]
Question 5
(a) Consider a WCDMA network having 33 voice users. The data rate for each voice user is 11.2 kbps, and the chip rate is 3.84 Mcps. The target SINR is 8 dB, and the voice activity factor is 0.55. The outer cell interference factor is 0.66, and the orthogonality factor is 0.75. Calculate the load factor in the network for the existing users. [5 marks]
(b) A network uses strict fractional frequency reuse (FFR) with an inter-site distance (ISD) of 540 m and serves users within 90 m of the BS by the inner cell frequencies. The Tx power for all inner cells is 19 dBm. If the path loss (PL) exponent is 3.6, calculate the SIR in dB for inner cell users. [5 marks]
(c) The same network described in part (b) now uses soft FFR (SFR) with cluster size 3. The Tx power for the major band is set at 9 times larger than TX power of the minor band.
(i) Using the simplified model, calculate the SIR in dB experienced by a user at the edge of the cell.
(ii) If the minimum SIR of an inner cell user is 10dB, calculate the max inner cell radius. [3, 7 marks]
Total [20 marks]