LTE – Channel Structure
0
LTE – DL Logical Channels
•
• • •
• •
•
PCCH = Paging Control Channel
• Transfers paging and system information changes notifications
• Used for paging when the user’s location is unknown to the network
BCCH = Broadcast control Channel
• Used for broadcasting system control information CCCH = Common Control Channel
• For transmitting control information network and Ues DCCH = Dedicated Control Channel
• A point-to-point channel that transmits dedicated control information between a UE and the network
• Used by UEs having an RRC connection DTCH = Dedicated Traffic Channel
• A point-to-point channel dedicated to one UE, for the transfer of user information • Can exist in both UL and DL
MCCH = Mutlicast Control Channel
• A point-to-point DL channel used for transmitting Multmedia Broadcast Multicast Service (MBMS) control info from the network to the UE for one or several MTCHs
• Used by UEs that receive MBMS MTCH = Multicast Traffic Channel
• A point-to-point DL channel for transmitting traffic data from the network to the UE • Channel is only used by UEs that receive MBMS
1
LTE – DL Transport Channels
•
• •
•
PCH = Paging Channel
• Supports UE discontinuous reception (DRX) to enable UE power saving
• Broadcasts in the entire coverage are of the cell
• Mapped to physical resources which can be used dynamically also for traffic/other control chans
BCH = Broadcast Channel
• Fixed and pre-defined transport format • Broadcast in the entire coverage area
MCH = Multicast Channel
• Broadcasts in the entire coverage area of the cell
• Supports MBSFN combining of MBMS transmission on multiple cells
• Supports semi-static resource allocation, e.g. with a time frame of long cyclic prefix
DL-SCH = Downlink Shared Channel
• Supports Hybrid ARQ
• Supports dynamic link adaption by varying modulation, coding, and transmit power
• Supports cell broadcast, beamforming, and dynamic and semi-static resource allocation • Supports UE DRX
• Supports MBMS transmission
2
LTE – DL Physical Channels
• •
PDSCH = Physical Downlink Shared Channel
• Carries the DL-SCH and PCH
• QPSK, 16-QAM, and 64-QAM modulation
PDCCH = Physical Downlink Control Channel
• Informs UE about the resource allocation of PCH and DL-SCH, and Hybrid ARQ information related to DL-SCH
• Carries the uplink scheduling grant • QPSK modulation
PHICH = Physical Hybrid ARQ Indicator Channel
• Carries hybrid ARQ ACK/NAKs in response to uplink transmissions • QPSK modulation
•
3
LTE – DL Physical Channel
•
PBCH = Physical Broadcast Channel
• Coded BCH transport block is mapped to four sub-frames within a 40ms interval (no explicit signaling to indicate 40ms timing)
• Each sub-frame is designed to be decoded from a single reception, assuming good channel conditions
• QPSK modulation
PMCH = Physical Multicast Channel
• Carries MCH
• QPSK, 16-QAM, and 64-QAM modulation
•
4
LTE – Channel Structure
5
LTE – UL Logical Channels
• • •
CCCH = Common Control Channel
• Channel for transmitting control info between UE and the network • Used for UEs having no RRC connection to the network
DCCH = Dedicated Control Channel
• P2P bi-directional channel that transmits dedicated control info between a UE and network • Used by UEs having an RRC connection
DTCH = Dedicated Traffic Channel
• A P2P channel dedicated to one UE to transfer user info • A DTCH can exist in both UL and DL
6
LTE – UL Transport Channels
•
•
RACH = Random Access Channel
• Used during initial access to the network and as part of synchronization, RRC re-establishment, timing advance in RRC_CONNECTED state
• Transmissions are vulnerable to loss due to collisions
UL-SCH = Uplink Shared Channel
• Optional support for beamforming
• Supports dynamic link adaptation by varying the Tx power and mod and coding • Supports Hybrid ARQ
• Supports dynamic and semi-static resource allocation
7
LTE – UL Physical Channels
•
• •
PRACH = Physical Random Access Channel
• Carries random access preambles used for initiation of random access procedure
• Random access preambles are generalized from Zadoff-Chu sequences with zero correlation
zones and generated from one or several root Zadoff-Chu sequences
PUSCH = Physical Uplink Shared Channel
• Carries the UL-SCH
• QPSK, 16-QAM, 64-QAM modulation
PUCCH = Packet Uplink Control Channel
• Carries Hybrid ARQ ACK/NAKs in response to DL transmissions • Carries Scheduling Request (SR)
• Carrie Channel Quality Indicator (CQI) reports
• BPSK and QPSK modulation
8
Cell Geometry
d d
RC
R
9
Cell Geometry
10
Cell Geometry
v
(y)
(j=3)
(i=1)
(j=2)
𝑢𝑢,𝑣𝑣 = 2𝑅𝑅𝑅𝑅,2𝑅𝑅𝑅𝑅 = (𝑅𝑅 3𝑅𝑅,𝑅𝑅 3𝑅𝑅) 𝑥𝑥 = 𝑢𝑢 𝑢𝑢 𝑢𝑢 𝑢𝑢 𝜋𝜋6 = 12 𝑢𝑢 3 𝐶𝐶 𝐶𝐶
(i=3)
Nonorthogonal coordinate system:
(i=2) (j=1)
u
y=𝑢𝑢𝑢𝑢𝑅𝑅𝑢𝑢 𝜋𝜋6 +𝑣𝑣=12𝑢𝑢+𝑣𝑣
d
30
(x)
11
Cell Geometry – Rings
6 2√7 2√7 46 2√7 2√7 42√3 22√34 6
2√3 2 0 2 2√3 2√7
4 2 42√7 6 2√342√3 6
6
12
Cell Geometry – Superhexagons
Number of cells in a cluster = cluster size
Only certain cluster sizes can be used to cover the service area without gaps
Constraint of hexagonal geometry each cluster is surrounded by six similar clusters with the same orientation
Each cluster has a total area equivalent to what is called a superhexagon
The number of cells in a cluster that satisfies the constraint is the ratio of the area in the superhexagon to that in one hexagon
We know:
1
𝑎𝑎𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 =6�2 𝑏𝑏𝑎𝑎𝑢𝑢𝑏𝑏�h𝑏𝑏𝑅𝑅𝑒𝑒h𝑒𝑒 =3𝑅𝑅𝐶𝐶𝑅𝑅=2 3𝑅𝑅2
𝑑𝑑𝑅𝑅,𝑅𝑅 =2𝑅𝑅 𝑅𝑅2+𝑅𝑅2+𝑅𝑅𝑅𝑅
𝑅𝑅′ = 31𝑅𝑅𝐶𝐶 = 2𝑅𝑅 3
Since d(i,j) is the distance between the centers of two
′
𝑅𝑅 = 2𝑑𝑑 𝑅𝑅,𝑅𝑅 =𝑅𝑅 𝑅𝑅 +𝑅𝑅 +𝑅𝑅𝑅𝑅
clusters of cells:
22
13
Cell Geometry – cell clusters
If cluster size is K, then:
𝐾𝐾 = 𝐴𝐴𝐴𝐴𝑏𝑏𝑎𝑎 𝑢𝑢𝑜𝑜 𝑢𝑢𝑢𝑢𝑠𝑠𝑏𝑏𝐴𝐴h𝑏𝑏𝑥𝑥𝑎𝑎𝑒𝑒𝑢𝑢𝑢𝑢 = 2 3 (𝑅𝑅′)2 𝐴𝐴𝐴𝐴𝑏𝑏𝑎𝑎 𝑢𝑢𝑜𝑜 𝑢𝑢𝑢𝑢𝑏𝑏 h𝑏𝑏𝑥𝑥𝑎𝑎𝑒𝑒𝑢𝑢𝑢𝑢 𝑎𝑎𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐
𝐾𝐾= 𝑅𝑅𝑅2=𝑅𝑅2+𝑅𝑅2+𝑅𝑅𝑅𝑅 𝑅𝑅
Cluster ‘shape’ is further constrained by Dijkstra’s algorithm to maximize compactness.
K=7
K=3
K=12
FDMA – Co-Channel Interference
System performance is dependent upon controlling the cell size, frequency spacing, and various physical parameters associated with the base stations and mobile stations.
Cluster size for frequency reuse in FDMA is a tradeoff between co-channel interference and spectral efficiency:
• •
Large K give a larger reuse distance and lower interference power
Dividing available frequency channels by K reduces the trunking efficiency for each cell and the number of subscribers that may be supported per FDMA channel
MS1 BS1 f0
f0
𝑑𝑑 𝑅𝑅 , 𝑅𝑅 = 2 𝐶𝐶𝑅𝑅 𝑅𝑅 2 + 𝑅𝑅 2 + 𝑅𝑅 𝑅𝑅 𝑑𝑑𝑅𝑅,𝑅𝑅=𝑅𝑅 3𝑅𝑅2+𝑅𝑅2+𝑅𝑅𝑅𝑅
Interference
BS2
D = reuse distance
f0 MS2
FDMA – Co-Channel Interference and System Capacity
reuse ratio, Q
𝑄𝑄=𝐷𝐷= 3𝐾𝐾 𝐶𝐶
• Given RC = radius of cell, D = di𝑅𝑅stance between co-channel cell centers, co-channel
D
𝜃𝜃(K)
FDMA – Co-Channel Interference and System Capacity
𝑄𝑄=𝐷𝐷= 3𝐾𝐾
• Given RC = radius of cell, D = distance between co-channel cell centers, co-channel reuse ratio 𝑅𝑅𝐶𝐶
• Larger Q implies better transmission quality due to reduced interference but also implies lower capacity per cell
• Let i be the number of co-channel interfering cells
0
• Signal to interference ratio for a receiver is:
𝑆𝑆𝑆𝑆
𝐼𝐼 = ∑𝑖𝑖0 𝐼𝐼𝑖𝑖 𝑖𝑖=1
• Assuming a log-distance path loss (exponent: n) and interference from the first layer of equidistant interfering cells: 𝐷𝐷� 𝑛𝑛
𝑆𝑆= 𝑆𝑆 = 𝑆𝑆0 = 𝑑𝑑0−𝑛𝑛 = 𝑅𝑅𝐶𝐶−𝑛𝑛 = 𝑅𝑅𝐶𝐶 = 3𝐾𝐾𝑛𝑛 𝐼𝐼 ∑ 𝑖𝑖 0 𝐼𝐼 𝑖𝑖 ∑ 𝑖𝑖 0 𝑆𝑆 ( 𝑑𝑑𝑑𝑑 ) − 𝑛𝑛 ∑ 𝑖𝑖 0 𝑑𝑑 − 𝑛𝑛 ∑ 𝑖𝑖 0 𝐷𝐷 − 𝑛𝑛 𝑅𝑅 0 𝑅𝑅 0
𝑖𝑖=1 𝑖𝑖=1 0 0 𝑖𝑖=1
𝑖𝑖=1
FDMA – Example of System Capacity Calculation
• Assume system can use all frequencies
• System bandwidth = 50MHz
• System uses FDDbandwidth = 25MHz
• Carriers space at 200KHz
• Ncarrier= Bsys/Bcarrier
• Ncarrier= 125
• System capacity depends upon reuse factors and cell size
Frequency Reuse Factor Calculation
• Let signal to interference ratio of 18dB or more be acceptable (typical voice req)
• Assume the nearest 6 co-channel equidistant cells interfere
• Assume path loss exponent is 4
𝑆𝑆 3𝐾𝐾
4
𝐼𝐼 ≥18𝑑𝑑𝑑𝑑=63.1= 6
• Frequency reuse factor, K >= 6.5 = 7
Cell Capacity
• Ncarrier= 125 (25e6 / 20e3)
• Ncell= Ncarrier / K
• K=7,Ncell=17(17.8)
• Let there be 8 channels per carrier (as in TDMA)
• Thus, 136 channels/cell (Acell)
• Each cell has a capacity of 136 simultaneous voice calls
• IfK=3
• Ncell = 41
• 8 channels per carrier
• 328 channels/cell
System Capacity
• Network size = Z square miles
• Cell size = C square miles
• Cells/network = Z/C • Channels/network, Anet
• Anet = Acell * Z/C
• Z=1,000,C=10,K=7,Anet =13,600
• Z=1,000,C=10,K=3,Anet =32,800
• Z=1,000,C=25,K=7,Anet =5,440
• System capacity has a linear, inverse relationship with cell size and frequency reuse pattern under ideal conditions
Capacity and Blocking
• Cellular systems rely on trunking to accommodate a large number of users with a limited number of channels
• Trunking exploits statistical multiplexing of large number of users (calls)
• System is engineered with enough channels to handle the peak hour ‘offered load’ ata
given maximum blocking rate
• Typically, blocking for new calls is maintained at 2% or les
• To calculate blocking, we utilize queueing theory
Performance: Blocking
λλλλ
012 N
μ 2μ 3μ ρn
pN=pB= N! ∑N ρn
Nμ
1
2 μ
μ
n=0 n!
An .
.
N!n N μ
λ
pN = pB =
A is offered load in Erlangs: λ/μ
∑N An!
n=0 Models input (call rate) of λ, N trunks, holding time of μ-1
Cell Capacity Planning
• Based on spectrum allocation and frequency reuse patterns, calculate the number of channel available per cell
• Based on user density, calling and holding patterns, calculate load per cell in Erlangs
• Use Erlang B formula for calculate blocking given the load and number of channels
LTE – Frequency Reuse Patterns
• Yellow = full system bandwidth
• Band edge color = frequency reuse to avoid inter-cell interference
• Problem of co-channel interference a the cell boundaries is resolved by dedicating a small segment of the available spectrum for cell edges
• In Soft Frequency Reuse (SFR) the cell area is divided into a central region where all BW is available and a cell edge where only a small portion of BW is available
• Compensate for loss in Shannon Channel capacity with high power carriers
• Use (bits/sec/Hz) rather than number of voice channels to determine capacity