ARC Future Fellow at The University of Melbourne Sessional Lecturer at Monash University
October 3, 2022
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 1 / 27
Copyright By PowCoder代写 加微信 powcoder
ECE5884 Wireless Communications Week 10: Multiple-user Systems and Cellular Systems
Course outline
This week: Ref. Ch. 14 and Ch. 15 of [Goldsmith, 2005]
● Week 1: Overview of Wireless Communications
● Week 2: Wireless Channel (Path Loss and Shadowing)
● Week 3: Wireless Channel Models
● Week 4: Capacity of Wireless Channels
● Week 5: Digital Modulation and Detection
● Week 6 : Performance Analysis
● Week 7: Equalization
● Week 8: Multicarrier Modulation (OFDM)
● Week 9: Multiple-Antenna Systems: Diversity Techniques
● Week 10: Multiple-User/Multiple-Antenna Systems (MIMO)
● Week 11: Guest Lecture “Multi-antenna systems: From theory to standardization in 5G-NR (New radio)”
● Week 12: Selected 4G/5G/6G Topics
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 2 / 27
Diversity for SIMO
● Diversity: is to send the same data over independent fading paths/links.
● SC, MRC, EGC or Threshold Combining
● Single-user pair with multiple antennas: simultaneous Tx is
possible,no channel sharing.
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 3 / 27
Multiple-user system
● A “multiuser” channel is any channel that must be shared among multiple users.
1 Downlink channel (broadcast channel or forward channel): one Tx sending to many Rxs
2 Uplink channel (multiple access channel or reverse channel): many Txs sending to one Rx
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 4 / 27
Downlink (DL) channel
● The signals transmitted to all users originate from the downlink Tx.
s(t) = ∑ sk (t); where sk (t) is the kth user signal (1)
● Examples: all radio and TV broadcasting, the transmission link from a satellite to multiple ground stations, and the transmission link from a base station to the mobile terminals in a cellular system.
● The total signaling dimensions and power of the transmitted signal must be divided among the different users.
● Synchronization of the different users is relatively easy.
● Both signal and interference are distorted by the same channel.
yk(t)= ∑hksk(t)+n(t);
∣hk ∣2Psk SINRk= 2 K
∣hk∣ ∑j=1,j≠k Psj +No ECE5884 Wireless Communications @ Monash Uni.
October 3, 2022
Uplink (UL) channel
● Many Txs sending signals to one Rx.
● Examples: laptop wireless LAN cards to a wireless LAN access point, from ground stations to a satellite, and from mobile terminals to a base station in cellular systems.
● Each signal must be within the total system bandwidth B.
● Each user may have an individual power constraint Pk associated with
its transmitted signal sk (t ).
● Synchronization is required.
● The uplink signals from different users are distorted by different channels.
y (t ) = ∑ hk sk (t ) + n(t ); sk (t ) is the k th user Tx signal (4)
SINRk= K 2 (5)
∑j=1,j≠k∣hj∣Psj +No
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022
Interference
Most communication systems are bi-directional; and thus consist of both uplinks and downlinks – Interference
1 Duplexing (UL and DL inf.): bi-directional systems must separate the UL and DL channels into orthogonal signaling dimensions:
● Time: time-division duplexing (TDD)
● Frequency: frequency-division duplexing (FDD)
2 Multiple access (User inf.): dedicated channels are allocated to users –
to create orthogonal channels (but not possible always!).
∣hk ∣2 Ps k
k ∣hk ∣2 ∑Kj=1,j≠k Psj +No
● time-division multiple access (TDMA)
● frequency-division multiple access (FDMA)
● code-division multiple access (CDMA)
● space division multiple access (SDMA) – directional ant., beamforming
● DL:γ = k ;
∑Kj=1,j≠k ∣hj ∣2Psj +No
C = log2(1 + γk ) bits/sec/Hz
ECE5884 Wireless Communications @ Monash Uni.
October 3, 2022
Frequency-Division Multiple Access (FDMA)
● Divided along the frequency axis into non-overlapping channels, and each user is assign a different frequency channel.
● The channels often have guard bands between them to avoid interference, etc.
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 8 / 27
Time-Division Multiple Access (TDMA)
● Divided along the time axis into non-overlapping channels, and each user is assigned a different cyclically repeating time-slot.
● Occupy the entire system bandwidth.
● A major difficulty of TDMA, at least for uplink channels, is the
requirement for synchronization among the different users.
● Used in the GSM.
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 9 / 27
Code-Division Multiple Access (CDMA)
● The information signals of different users are modulated by orthogonal or non-orthogonal spreading codes.
● The resulting spread signals simultaneously occupy the same time and bandwidth.
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 10 / 27
Space-Division Multiple Access (SDMA)
● uses direction (angle) as another dimension in signal space, which can be channelized and assigned to different users -directional antennas/sectorized antenna arrays.
● TDMA or FDMA is used to channelize users within a sector. ● Hybrid Techniques!
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 11 / 27
TDMA/FDMA/CDMA
https://www.ccs.neu.edu/home/futrelle/teaching/com1204sp2001/uwash- intro/final.html
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 12 / 27
Cellular Systems
● A type of infrastructure-based network that make efficient use of spectrum by reusing it at spatially separated locations.
● Exploit the power falloff with distance of signal propagation in order to reuse the same channel at spatially separated locations.
● A given spatial area (a city) is divided into non-overlapping cells.
Figure 1: Cellular system.
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 13 / 27
“Cell”ular Structure
● The hexagon is an ideal choice for representing cellular coverage areas, because it closely approximates a circle and offers a wide range of tessellating reuse cluster sizes.
● Typical cells: Femto-, Pico-, Micro-, Macro-cells
● Clustersize:N=i2+ij+j2;i≥j≥0⇒N=1,3,4,7,9,⋯
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 14 / 27
“Cell”ular Structure
● Clustersize:N=i2+ij+j2;i≥j≥0⇒N=1,3,4,7,9,⋯
● i= no. of cells (center to center) along any chain of hexagon ● j= no. of cells in 60 degree counter-clockwise of i
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 15 / 27
Cellular System Fundamentals
● Every cell is then assigned a channel set, and these sets can be reused at spatially separated locations – frequency reuse or channel reuse, e.g., Channel set Cn where n = 1, 2, 3.
● Cells that are assigned the same channel set, called co-channel cells, must be spaced far enough apart (to minimize interference).
● Handed off: When a mobile moves between two cells.
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 16 / 27
Co-channels in cellular systems
● Use the same frequency channels.
● Co-channel reuse factor: the ratio of the co-channel reuse distance (D) between cells using the same set of carrier frequencies and the radius of the cells (R)
● Co-channel interference: signals from cells that share same channel(s) cause co-channel interference.
● See figure for the First Tier interfering cells.
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022
Interference in Cellular Systems
1 Intercell or co-channel interference: interference from outside the cell (e.g., due to reuse).
2 Intracell interference: interference from within a cell (e.g., non-orthogonal channelization).
3 Signal-to-interference-plus-noise power ratio (SINR): Pr
SINR = P + N (11) I0
where Pr is the received desired signal power, and PI is the received power associated with both intracell and intercell interference.
4 Interference limited system: the interference power is much larger than the noise power – Signal-to-interference-power ratio (SIR) or carrier-to-interference-power ratio (CIR)
CIR=SIR= P (12)
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022
Co-channel interference (CCI)
● In a fully equipped hexagonal-shaped system, the number of interfering channels in the first tier is always 6.
Figure 2: The first tier interfering cells.
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 19 / 27
DL worst case CCI
1 DL: The BS to the mobile user.
2 The worst case: the mobile user is far away from the desired BS (and
more closer to interfering BSs)
● Assume that the BS antennas are all the same height and all BSs
transmit with the same power.
● Ignore the effects of shadowing and multipath fading, and assume that
the propagation path-loss is described by the inverse law or the simplified model.
C Pr Pt K ( R ) =6=α
Base station
Mobile user
Pr (d ) = Pi (d ) = Pt K ( dr ) (13)
Carrier-to-interference ratio (C/I): dr α
I ∑i=1Pi ∑6 PtK(dr) i=1 D
∑i=1 Di ECE5884 Wireless Communications @ Monash Uni.
October 3, 2022
UL worst case CCI
1 UL: The mobile user to the BS.
2 The worst case: the mobile user is far away from the desired BS; and
interfering mobile users are closer to the desired BS.
Base station
Mobile user
=6 dα ∑ PtK(r) i=1 Di
= 6D−α where D = Di ∀i (16)
3 You may use this for network design:
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022
Cell Sectoring
1 Cell sectoring reduces the number of co-channel base stations ● 120o sectoring: 2 or 3 co-channel cells cause interference
● 60o sectoring: 1 or 2 co-channel cells cause interference
2 Example: For N = 7, 120o cell sectoring yields an approximately 6 dB C/I improvement over Omni-cells.
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 22 / 27
Cell Splitting
1 Depending on traffic patterns, the smaller cells may be activated/deactivated in order to efficiently use cell resources.
● subdividing a congested cell into smaller cells (Femto, Pico).
● Each has its own base station, and use reduced transmission power. ● Channel borrowing
Large cell (low density)
Small cell (high density) Smaller cell (higher density)
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 23 / 27
Signal Strength
● Signal strength contours indicating actual cell tiling. This happens because of
● presence of obstacles
● signal attenuation in the atmosphere, etc…
Cell i -60
Cell j -60
Signal strength (in dBm)
ECE5884 Wireless Communications @ Monash Uni.
October 3, 2022
● Handoff – when crossing a cell boundary while continuing the call.
● By looking at the variation of signal strength from BSs, it is possible to
decide on the optimum area where handoff can take place.
● When to initiate handoff, e.g., the mean signal (over some
predetermined time) is below some threshold.
Signal strength due to BSi
Signal strength due to BSj
BSi MS X1 X3
BSj X5 Xth X4 X2
1 Hard handoff – A user on the edge of a cell is either assigned to one cell or the other but not both.
2 Soft handoff – A user on the edge of a cell can receive or transmit
signals to two or more BSs to improve reception.
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 25 / 27
References
A. Goldsmith, Wireless Communications, Cambridge University Press, USA, 2005.
ECE5884 Wireless Communications @ Monash Uni. October 3, 2022 26 / 27
Thank You! See you again
程序代写 CS代考 加微信: powcoder QQ: 1823890830 Email: powcoder@163.com