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# EEET5102 Mobile Communications And Wireless Access

## Question:

Consider a multipath channel with the following normalised power delay profile
τ (µs) 0 0.1 0.2 0.5 2
P dB 0 -10 0 -10 -30
1. Compute the (90% correlation) coherent bandwidth of the channel. Assume a bandwidth of 20MHz is available. Design the OFDM transmission scheme for the system, such that the subcarrier spacing satisfies ?f =Bc

Assume that one reference pilot for channel estimation is placed every 5 subcarriers. Determine
(a) The FFT size and the number of active subcarriers.
(b) The OFDM symbol duration, including a suitable choice of cyclic prefix period.
(c) The sampling rate of the system.
(d) The achievable bit rate if the 16-QAM input constellation is used.

3. State one advantage and one disadvantage of choosing a larger subscarrier spacing ?f.
4. Consider a SISO LTE system with frequency division duplex and a bandwidth of 1.4MHz.Extended CP is employed. The channel condition is such that 16-QAM can be used on all subcarriers. Compute the bit rate, taking into account spectral efficiency loss due to pilot signals.
Hint: this question requires you to do some research on the LTE radio access standard, for e.g. http: // www. teletopix. org/ 4g-lte/
lte-frame-structure-and-resource-block-architecture/

Assignment 2:

Question 2: CDMA
Data for an uplink CDMA transmission is stored in the file CDMA data.mat. You can import the data into MATLAB by load ’CDMA data.mat’. The data include • channel (3 × 10) is the channel impulse response for each user.• U (3 × 100) is the spreading codes for each user.• rxsig: the received signal at the base station.Each user transmits 4 binary symbol. The data for user k is dk = [1, dk,1, dk,2, dk,3],
where the first bit is the pilot (known at the receiver) and dk,j ∈ {−1, 1}. The spreading code
U(k, :) is used for each data symbol of user k.

The channel model is summarised as txsigk ⊗ channel(k, :) + noise.where txsigk is the transmit signal from user i, and ⊗ denotes convolution.
1. Use rxsig to estimate the bits transmitted by user 1. At this stage, recover data from
one of its paths is sufficient. Show in your report the codes used for detection.
2. Assume that each user is active 30% of the time, and directional antenna provides an angular gain of 2.5. Determine the number of users that the system can simultaneously support if the signal-to-interference ratio requirement is 6dB.
The following questions are optional. You can attempt to get some bonus marks.
3. Show how you make use of multipath to improve reliability of user 1.

4. In fact, each user has an additional channel path compared to that reported in channel.Determine the delay and gain of the additional paths of user 1 using the pilot symbol and the received signal rxsig. Hint: You can work out the missing delay by correlating the pilot transmit signal with rxsig. Then estimate the missing channel gain.
Question 3: Multi-antenna system
Consider a single-input multiple output system defined as y = HP x + n,where H is the channel matrix, x ∈ {−1, 1} is the transmit signal, and n = [n1, n2] is the noise vector, where ni is complex Gaussian noise with zero mean variance.

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