EEEM061: Advanced 5GWireless Technologies

Semester 2 2021/2

A1.

(a)         In relation to the transmission of wireless systems:

(i)          In less than 100 words explain what the “water-filling” power allocation is, and what its main practical requirement is in order to apply it. [10 %]

(ii)         In less than 100 words, and without using equations, explain how the water- filling power allocation can be applied to single-user 5G MIMO systems. [10 %]

(iii)        In less than 100 words, give another example than single-user 5G MIMO, that water-filling power allocation can be applied. Justify your answer. [10 %]

(b)        Assume a 5G  Multi-Input, Single-Output system, with two transmit and one receive antennas, where ℎi (i  = 1,2) is the  (complex) transmission channel from transmit antenna i to the receiver. Also assume complex additive white Gaussian  Noise  of variance σ 2 .

(i)          Derive the capacity or the system in the case where we transmit the same signal from  both  antennas with a total transmit signal power P = 1, and when ℎ1 = 1 + j, ℎ2 = 2 + j and σ 2 =  1. [20 %]

(ii)         For the transmission method that maximises the capacity, of part (i) derive the “effective” transmission channel that the receiver experiences. Explain all steps of your derivation. [20 %]

(iii)        If BPSK symbols (e.g., +1, -1) are transmitted with equal probability and by using the method of part (ii), and if the received signal at the receive antenna is y = 3 + j, calculate the Log-Likelihood-Ratio of the detected bit, give the value of the detected bit, and comment on the reliability of the detected bit (i.e., if it is high or not). Assume the transmission parameters of part (i). Also assume that the bit 0 is mapped onto symbol +1 and that the bit 1 is mapped onto BPSK symbol -1. Justify your answer. [30 %]

A2.

(a)         In relation to OFDM systems:

(i)          Given the bandwidth W and time duration T, and in less than 100 words

explain 1) whether it is possible to have infinite number of subcarriers and why; 2) what the maximum number of subcarriers is. [10 %]

(ii)         In less than 100 words describe the orthogonality condition in OFDM

systems in the frequency- domain and in the time-domain, respectively. [10 %]

(iii)        In less than 100 words explain how inter-symbol interference (ISI) and inter- carrier interference (ICI) can be avoided in OFDM systems. [10 %]

(iv)       Assume a baseline OFDM system employing QPSK modulation and a rate ½ channel code. Propose an OFDM system that can achieve 267% higher data rate than the baseline system. In less than 100 words explain which parts of the system need to be changed and the price to pay for the increased data rate. [10 %]

(v)        Given the OFDM signal model

Yk = XkHk +Vk ; k = 0,1, … N -1

Zk = WkYk

where Yk is the kth sample of the DFT output; Xk is the symbol carried by the kth subcarrier;

Hk is the channel frequency response at the kth subcarrier; Vk is the noise at the kth subcarrier;

Wk is the equalizer coefficient for the kth subcarrier; Zk is the equalizer output.

Explain why the MMSE equalizer can avoid the noise enhancement  problem. Your interpretation needs to be supported by mathematic analysis. [10 %]

(vi)       Calculate the Peak-to-Average-Power-Ratio (PAPR) value of the signal

s(t) =sin(2πft)+cos(2πft); 0

[10 %]

(b)        In relation to the Sparse-Code Multiple Access (SCMA) system with 6 users sharing 4 subcarriers as shown in Fig. 1.

(i)          Draw the factor graph of such an SCMA system. [10 %]

Figure 1

(ii)        Convert the factor graph into a signature matrix. [10 %]

(iii)       Calculate the value of the overloading factor dv (effective spreading) and dc (where dc is the number of symbols that are allowed to interfere to each other at each subcarrier). In less than 100 words explain the practical significance of each parameter. [10 %]

(iv)        Illustrate how to construct the four codewords to form the codebook for the 3rd user (UE3). [10 %]

A3.

(a)         In relation to 5G wireless systems:

(i)         A generic expression of the array factor can be expressed as

In less than 100 words, propose a technique to suppress sidelobes. [10 %]

(ii)         In less than 100 words explain three major benefits of non-orthogonal

multiple access (NOMA) in comparison to orthogonal multiple access (OMA). [10 %]

(iii)        In less than 100 words explain the purpose of network densification and discuss the two contradictory effects when cell density becomes higher. [10 %]

(iv)        In less than 150 words explain the main requirements for the mMTC and

URLLC services, respectively, and how to use different numerologies to

support them. Imagine the problems we will encounter if these two services are provided by 4G-LTE networks. [15 %]

(v)        Given the capacity formula C = W ∑j ∑kl0g2 [1 + SINRj,k ],  suggest a

practical solution that can fulfil the double summation in the above formula. [10 %]

(b)        Consider an indoor mm-wave communication scenario where the transmitter is mounted on the wall. The penetration loss of the wall is PeL=40dB. Convert the penetration loss into linear scale and interpret the result. [10 %]

(c)        Figure 2 shows an indoor mm-Wave communication system where the operating frequency f is 30GHz. Suppose the distance between the transmitter (source) and the user (destination) ddirect is 4 meters, and there is a blockage between them. For reliable transmissions, the channel gain has to be greater than -80dB (this threshold in linear scale is  Gthresold  =10-8).

(i)         Calculate the channel gain of direct link between the transmitter and the

user, and evaluate the possibility for reliable transmissions by relying on the direct link.

Hint:   the  channel  gain of the direct  link  between the transmitter and the  user is modelled as: where the penetration loss PeLdB  is -35dB in decibel scale or PeL= 10一3.5  in linear scale. [15 %]

(ii)        Suppose that the distance between the transmitter and the wall is dg  =  6

meters, the angle θi=30o; the distance between the wall and the user is dh = 3 meters, the  angle  θs=15o .  To   combat the blockage  effect,  we  design  a reconfigurable intelligent surface (RIS) that can be mounted on the wall. As a rule of thumb, the size of each element on the RIS is where λ is the wavelength.  Determine  the  minimum number of elements Nmin that is required to achieve reliable communication.

Hints: The total effective channel gain of the end-to-end channel through RIS is expressed as GRIS   = PL × Rc,  where PL and Rc are path loss and reflection coefficient. In linear scale, they are defined respectively as

[20 %]