ELEC6252W1
SEMESTER 2 EXAMINATIONS 2023 - 2024
FUTURE WIRELESS TECHNIQUES
Section A
Question A1.
(a) Assume an extended Wyner’s system model, as shown in Fig. 1, where adjacent base-stations (BSs) conduct cooperation under ideal data exchange.
FIGURE 1: Extended Wyner’s system model for multicell SDMA systems.
(i) Assume optimum and minimum mean-square error (MMSE) multiuser detection (MUD). For each of these two MUDs, suggest a multicell cooperation/processing (MCCP) scheme and describe in detail its operations.
(ii) Analyse the pros and cons of the proposed MCCP schemes. [5 marks]
(b) Fig. 2 illustrates a network having two pairs of distributed nodes, (S1 , D1 ) and (S2 , D2 ). 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 S1 and S2 can exchange their data symbols. Propose a cooperative transmission scheme for S1 and S2 to send x1 and x2 , respectively, to D1 and D2. State in detail the transmission steps, and explain the benefit obtained from the proposed cooperation scheme. [5 marks]
FIGURE 2:
(c) There is a sparse-spread code-division multiple-access (CDMA) system, which has the input-output relationships of
(1)
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]
(d) Provide two application examples to explain the benefits and challenges of employing full-duplex instead of half-duplex. [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 benefits and challenges of using multiple-input multiple-output (MIMO) systems. Also, explicitly highlight the different MIMO gains. [5 marks]
(g) Figure 3 shows the oxygen, water vapour and rain attenuation versus frequency. Explain your observations on the figure and describe how this affects the transceiver design in millimeter wave frequencies.
FIGURE 3: Attenuation curves of O2 , H2 O and rain at sea level. The term ρ refers to the density of H2 O in grams per meter3 .
[5 marks]
(h) Explain the Vertical Bell Labs Space Time (V-BLAST) multiple-input multiple-output (MIMO) transmission process and elaborate with mathematical equations the transmission model, and then describe one V-BLAST detection technique using mathematical equations.
Additionally, explain the main characteristics of your chosen detection technique as compared to the other detection methods. [5 marks]
Section B
Question B1.
(a) Explain the operational principles of the amplify-and-forward (AF), decode-and-forward (DF) and compress-and-forward (CF) relaying protocols. [3 marks]
(b) Consider that nodes S1 (having data x1) and S2 (having data x2) use a two-way relaying network, as shown in Fig. 4, to exchange x1 and x2 between S1 and S2.
FIGURE 4: A two-way relaying network.
Assume that all nodes are operated in half-duplex mode. Based on the network coding principles, design a two-way relaying scheme for S1 and S2 to exchange x1 and x2. Explain the operations in details. [6 marks]
(c) 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]
(d) 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. Transmitted signals experience both the propagation pathloss with a pathloss exponent of α, and the small-scale fading with the fading gains
FIGURE 5: A cooperative network with direct transmission.
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.
(i) By assuming the amplify-and-forward (AF) relaying at node R, derive the spectral-efficiency achieved by the cooperative network. [4 marks]
(ii) By assuming the decode-and-forward (DF) relaying at node R, derive the spectral-efficiency achieved by the cooperative network. [3 marks]
(e) Figure 6 represents a two-hop communication network, where node S (having one antenna) sends data to node D (having one antenna) with the aid of a relay node R, which employs L antennas for receiving and transmission. Assume that all nodes are operated in half-duplex mode, the transmit power of node S is P1 , the transmit power of the relay is P2, the distance between node S and the relay is d1 , and the distance between the relay and node D is d2. Assume that signals transmitted by node S and the relay experience the propagation path-loss with a path-loss exponent α, and the small-scale fading with the fading gains as shown in the figure. Furthermore, assume that the channel knowledge, i.e., {hij }, is only employed by the relay.
FIGURE 6
(i) Consider a relay processing scheme in the principles of either AF or DF, describe in detail the operations carried out by the relay. [4 marks]
(ii) Under the relay processing scheme considered in (e)(i), derive an expression for the spectral-efficiency achieved by the two-hop communication network. [4 marks]
Question B2.
(a) Assume a downlink multicarrier system, where the base-station (BS) of a cell uses M subcarriers to support K = 2M users randomly distributed in the cell. Based on the principle of non-orthogonal multiple-access (NOMA), design a transmission scheme for the BS to simultaneously transmit information to the 2M users. [5 marks]
(b) Consider a 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.
(i) 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.
(ii) Describe the detection (decoding) procedures carried out, respectively, by users 1 and 2 for achieving the above sum rate. [5 marks]
(c) Assume that users 1, 2, . . . , K simultaneously send x1 , x2 , . . . , xK , satisfying E[x
2
k
] = 1, to a BS (with one antenna) using power P1 , P2 , . . . , PK via Gaussian channels. The channel gains from users 1, 2, . . . , K to the BS are given by h1 , h2, . . . , hK , respectively.
(i) Assuming that j h1 j 2 P1 ≥ j h2 j 2 P2 ≥ . . . ≥ jhK j 2 PK , describe the optimum detection scheme of the BS to achieve the sum rate of the NOMA system. [4 marks]
(ii) In addition to the assumption in (c)(i), further assume that the noise variance is σ 2. Derive an expression for the sum rate achieved by the K users. [4 marks]
(d) Frequency-division duplex (FDD) and time-division duplex (TDD) are two well-known half-duplex schemes implemented in practical mobile communications systems. Describe the operational principles of FDD and TDD. Aid your description using illustrations whenever needed. [3 marks]
(e) 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. [6 marks]
(ii) State one SIC technique operated in the analog-circuit domain, and discuss its operational principles, advantages and the challenges it may face in practice. [3 marks]
Section C
Question C1.
(a) Consider a multiple-input multiple-output (MIMO) system employing M transmit and N receive antennas. Let x = [x1 , x2 , · · · , xm] denote the signal transmitted from the M antennas and y = [y1 , y2 , · · · , yN ] denote the received signal vector. H represents the channel matrix between the transmitter and receiver of size N × M.
When the receiver employs perfect channel knowledge, while the transmitter only knows the MIMO channel’s distribution and the transmitted signal vector x is independent of the channel matrix H, and when considering the case that M is fixed and N → ∞, then the ergodic MIMO capacity can be evaluated as:
(4)
Explain your understanding of the concept of capacity and elaborate on your observations on the derived capacity in Equation (4). [7 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 ).
Draw the block diagrams of the Fully-connected hybrid beamforming architectures and briefly explain the processing stages. [8 marks]
(c) 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 9 bits per channel use. You should decide on the modulation scheme used and the processing carried out at the transmitter. [7 marks]
(ii) Write the mathematical representation of the transmitted signal and the received signal, highlighting the dimensions of any vectors or matrices used. [5 marks]
(iii) Design a detection scheme to decode your received signal. [3 marks]
Question C2.
(a) Consider a multiple-input multiple-output (MIMO) system, where a base station (BS) equipped with N = 8 antennas is communicating with one user equipment having 2 antennas. Also, consider a scenario where QPSK is used as the modulation scheme.
(i) Design a transmission scheme using the above system configurations for attaining a rate of 4 bits per channel use, while also attaining a diversity order of 2 using Alamouti’s space-time block code. Explain in details your transmission scheme and write the mathematical representation of the transmitted signal as well as the received signal. [8 marks]
(ii) Describe and explain in details with mathematical equations one detection technique that can be employed to detect the received signal. [6 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. [8 marks]
(ii) Draw the block diagrams of the sub-array connected hybrid beamforming architectures and briefly explain the processing stages. [8 marks]