ELEC6252W1
SEMESTER 2 EXAMINATIONS 2022 - 2023
FUTURE WIRELESS TECHNIQUES
Section A
Question A1.
(a) In a cooperative system shown in Figure 1, when the source node S, relay node R and destination node D are all half-duplex nodes, the achievable spectral efficiency is
(1)
when amplify-and-forward cooperation is employed. In (1), γ is the signal-to-noise ratio (SNR) measured at the destination node D, E[·] represents the expectation with respect to the involved channels, and the factor 1/2 is because of the shortcoming that two time-slots are required to deliver one symbol from source node S to destination D.
(i) Suggest an alternative cooperative system to improve the spectral efficiency of the system by avoiding the above-mentioned shortcoming;
(ii) Describe in detail the operations of signal transmission in your suggested system. [5 marks]
(b) Assume that two base-stations (BSs) can conduct cooperation based on data exchange only. State three types of BS cooperative processing that the BSs may operate. [5 marks]
(c) Consider a Non-Orthogonal Multiple-Access (NOMA) downlink, where a BS broadcasts x1 and x2 , which satisfy E[xk(2)] = 1 for k = 1, 2, to users 1 and 2 using power P1 and P2 , respectively. At some time, the signals received respectively by users 1 and 2 can be expressed as
(1)
(2)
where h1 and h2 represent the channel gains from the BS to users 1 and 2, respectively, and n1 and n2 are Gaussian noise distributed with zero mean and a variance of σ 2.
• Assume that j h1 j 2 < j h2 j 2 , and correspondingly the power assigned by BS to users 1 and 2 satisfies P1 > P2. Derive the sum rate achieved by this NOMA downlink.
• Describe the detection (decoding) procedures carried out, respectively, by users 1 and 2 for achieving the above sum rate. [5 marks]
(d) Provide two application examples to show the benefit of employing full-duplex instead of half-duplex. You may use drawings to support your explanation. [5 marks]
(e) Consider a multiple-input multiple-output (MIMO) system employing M transmit and N receive antennas. Draw and annotate the MIMO system model and write the received signal equation and explain the different terms used. [5 marks]
(f) Explain the concept of massive MIMO and comment on the motivation for using massive MIMO from a channel capacity perspective and also from a transmission and detection perspective. [5 marks]
(g) Discuss the concept of pilot contamination and antenna correlation and analyse their effect on the performance of massive MIMO. [5 marks]
(h) Explain the reasons for using beamforming for communications at millimetre wave frequencies and also the reasons for the need to use hybrid beamforming for millimetre wave communications. [5 marks]
Section B
Question B1.
(a) There is a sparse-spread code-division multiple-access (CDMA) system, which has the input-output relationships of
(4)
Draw the factor graph of this sparse-spread CDMA system for operating the message-passing algorithm, in order to detect the data symbols x1 , x2 , . . . , x8. [5 marks]
(b) Fig. 2 shows a two-hop communication link, where d1 and d2 represent the distances, h1 and h2 represent the fast fading gains, and P1 and P2 represent the transmit power, of the first and second hops, respectively. Assume that signals transmitted over either hops experience propagation path-loss with a path-loss exponent Q, and noise added at relay R and destination D obeys the Gaussian distribution with zero mean and variance σ 2. Furthermore, relay R is assumed to be operated in half-duplex mode, and it also has no buffer for storing the data received from node S.
Assuming that the decode-and-forward (DF) relaying scheme is employed by relay R, derive an expression for the spectral-efficiency achieved by this two-hop link. [5 marks]
(c) Fig. 3 illustrates a network having two pairs of distributed nodes, (S1 , D1 ) and (S2 , D2 ), where two destination nodes D1 and D2 are close to each other. In this network, node S1 needs to send a symbol x1 to D1 , while node S2 needs to send a symbol x2 to D2.
Assume that nodes S1 and S2 can cooperate with each other by exchanging their data to be sent to their destinations, respectively, and that the channels h11 , h12 are only known to D1 , while the channels h21 , h22 are only known to D2.
(i) Based on Alamouti’s space-time code, design a cooperative transmission scheme for S1 and S2 to send x1 and x2 , respectively, to D1 and D2. Explain in detail the transmission steps. [4 marks]
(ii) Assuming the maximal ratio combining (MRC) assisted decoding scheme, derive the expressions for the decision variable obtained by D1 or D2. [4 marks]
(d) Figure 4 shows a three-hop communication link for node S to send information to node D with the help of two relay nodes R1 and R2. As shown in the figure, signals sent by node S can be received by relay R1 with the signal-to-noise ratio (SNR) of 吖01 and by relay R2 with the SNR of 吖02 ; signals sent by relay R1 can be received by relay R2 with the SNR of 吖12, and by node D with the SNR of 吖13 ; signals sent by relay R2 can be received by node D with the SNR of 吖23.
Assume that all nodes are operated in half-duplex, and that both the relay nodes R1 and R2 use the amplify-and-forward (AF) relaying protocol. Furthermore, assume that relay node R2 uses the maximal ratio combining (MRC) scheme to combine the signals received from nodes S and R1, and that node D also uses the MRC scheme to combine the signals received from nodes R1 and R2. Based on the above settings and assumptions,
(i) provide a formula for the SNR achieved by node D for detecting a symbol sent by node S; [4 marks]
(ii) provide a formula for the spectral-efficiency achieved by this three-hop communication link. [2 marks]
(e) In MultiCell Cooperation/Processing (MCCP), two Base-Stations (BSs) may cooperate based on exchanging both Channel State Information (CSI) and Data (CSID-MCCP mode), exchanging CSI only (CSI-MCCP mode) or exchanging data only (D-MCCP mode). For each of the three MCCP modes, provide an example to explain the principle of the corresponding BS cooperative processing. [6 marks]
Question B2.
(a) Fig. 5 is a cooperative network, which uses a direct-link (S → D) and a relay-link (S → R → D) to send information from source node S to destination node D. The distance from node S to node R is d1 , that from node R to node D is d2, and that from node S to node D is d. Transmited signals experience both the propagation pathloss with a pathloss exponent of α, and the small-scale fading with the fading gains shown in the figure. Assume that the transmit power of node S is P1 and
that of relay R is P2 , and all nodes are operated in half-duplex mode. Noise power is σ 2. Furthermore, assume that hSR is known to node R, and hD , hRD are known to node D.
Based on the above settings/assumptions and assuming amplify-and-forward (AF) relaying at node R, derive an expression for the signal-to-noise ratio (SNR) achieved by node D for detecting the symbol x sent by node S. [6 marks]
(b) Consider a non-orthogonal multiple-access (NOMA) system, where two users send their information to a base-station (BS). The signal received by the BS can be expressed in the form of
where x1 and x2 are the information sent respectively by users 1 and 2, which satisfy E[xk(2)] = 1 for k = 1, 2, P1 and P2 represent the transmit power of users 1 and 2, while h1 and h2 represent the channel gains, respectively, from users 1 and 2 to the BS. Finally, n is Gaussian noise distributed with zero mean and a variance of N0.
(i) Assume that j h1 j 2 P1 ≥ j h2 j 2 P2 , derive the sum rate achieved by users 1 and 2.
(ii) Describe the BS’s detection (decoding) procedure for achieving the above sum rate. [6 marks]
(c) Assume that a single-antenna BS broadcasts x1 , x2 , . . . , xK , satisfying E[x
2
k
] = 1, via Gaussian channels to users 1, 2, . . . , K using power P1 , P2, . . . , PK , respectively. The channel gains from the BS to users 1, 2, . . . , K are h1 , h2, . . . , hK , respectively.
(i) Assume that the transmit power of the BS satisfies P1 ≥ P2 ≥ . . . ≥ PK , which models that user 1 is the user furthest from BS, then user 2, and finally, user K is the one closest to BS. Describe the optimum detection scheme of user k , k = 1, 2, . . . , K, to achieve the sum rate of the NOMA system. [5 marks]
(ii) In addition to the assumption in (c)(i), further assume that the noise variance is N0. Derive an expression for the sum rate achieved by the K users. [3 marks]
(d) The biggest challenge to implement full-duplex in practice is the self-interference cancellation (SIC), which may be implemented in propagation domain, analog-circuit domain and digital domain.
(i) State two SIC techniques operated in the propagation domain, and discuss respectively their operational principles, advantages and the challenges they may face in practice. [5 marks]
(ii) State two SIC techniques operated in the analog-circuit domain, and discuss their operational principles, advantages and the challenges they may face in practice. [5 marks]
Section C
Question C1.
(a) Consider a single-user millimetre wave (mmWave) multiple input multiple output (MIMO) system that employs hybrid analog-digital beamforming, where the transmitter is equipped with Nt antennas and the receiver with Nr antennas. The transmitter is assumed to have NR(t)F radio frequency (RF) chains, while the receiver employs NR(r)F RF chains, where the number of RF chains is assumed to satisfy (NR(t)F ≤ Nt ) and (NR(r)F ≤ Nr ). The transmitter and receiver communicate via Ns data streams, where Ns ≤ min(NR(t)F , NR(r)F ).
Draw the block diagrams of the sub-array connected hybrid beamforming architectures and briefly explain the processing stages. [10 marks]
(b) Consider a multiple-input multiple-output (MIMO) system, where a base station (BS) equipped with NT = 4 antennas is communicating with a user equipment having NR = 4 antennas. The BS has NRF = 2 radio frequency (RF) chains.
(i) Design a transmission scheme that would result in throughput of 6 bits per channel use. You should decide on the modulation scheme used and the processing carried out at the transmitter.
(ii) Write the mathematical representation of the transmitted signal and the received signal, highlighting the dimensions of any vectors or matrices used.
(iii) Design a detection scheme to decode your received signal. [20 marks]
Question C2.
(a) (i) Explain the concept of preprocessing aided spatial modulation.
(ii) Write the mathematical representation of the transmitted signal and the received signal, highlighting the dimensions of any vectors or matrices used.
(iii) Explain how the signal can be detected at the receiver side. [14 marks]
(b) Consider a single-user millimetre wave (mmWave) multiple input multiple output (MIMO) system that employs hybrid analog-digital beamforming, where the transmitter is equipped with Nt antennas and the receiver with Nr antennas. The transmitter is assumed to have NR(t)F radio frequency (RF) chains, while the receiver employs NR(r)F RF chains, where the number of RF chains is assumed to satisfy (NR(t)F ≤ Nt ) and (NR(r)F ≤ Nr ). The transmitter and receiver communicate via Ns data streams, where Ns ≤ min(NR(t)F , NR(r)F ).
(i) The mmWave channel matrix H (t) of size CNr ×Nt at time instant t is given by:
Explain your understanding of the mmWave channel model and what the equation above represents.
(ii) Draw the block diagrams of the fully-connected hybrid beamforming architectures and briefly explain the processing stages. [16 marks]