LTE physical layer-Brief Description

1. LTE Physical
layer
Frame Structure, Resource Block

2. OFDMA
Orthogonal Frequency Division Multiple Access:
In LTE OFDMA Divide MHz bandwidth into
Subcarriers and Each subcarriers having 15KHz
bandwidth
Ex-1MHz is Having 1000KHz
5
MHZ
Subcarrier-
15KHz

3. Frame
Structure
1
R
B
=
1
8
0
K
H
Z
0.5 MS
Subframe-
1 MS
1 Frame
Time
F
R
E
Q
U
E
N
C
Y
 1 Resource block is having 12 Subcarriers
and Each Subcarriers having 15KHz
bandwidth.
 1 Resource block is having 180 KHz.
bandwidth.
12 Sub*15KHz = 180 KHz.
 1 Frame is Having 10 Subframe.
 1 Subframe having 2 slots/ 1 Subframe=1
Ms.
 Each slot is 0.5 Ms.
 Each Slots having 7 OFDMA Symbols.
 Each OFDMA Symbols is 71.2 Micro/Sec.

4. Resource Element:-
RE is the Smallest Unit made by
1 RE = 1OFDM Symbols/1 Subcarriers.
Total 84 Resource element in LTE.
1 Slot-0.5 MS
71.2 71.2 71.2 71.2 71.2 71.2 71.2
OFDMA Symbols
1 Subcarrier
RE RE

5. How to calculate resource block in Channel
Bandwidth:-
For 1.4 MHz.
1.4 MHz= 1400 KHz
Formula:- Guardband =B.W-(Nref. DeltaF-DeltaF)/2
= 1400/180=7 Nref
=Delta=180 KHz
=1400-(7*(180)-180)/2
=320/2
=160 Guard band
=1400-320
=1080/180=6 RB.
For 3 MHz.
3 MHz= 3000 KHz
Formula:- Guardband =B.W-(Nref. DeltaF-DeltaF)/2
= 3000/180=16 Nref
=Delta=180 KHz
=3000-(16*(180)-180)/2
=300/2
=150 Guard band
=3000-300
=2700/180=15 RB.
1080
160 160 150 150
2700

6.  There are two types of LTE frame structure:
Type 1: FDD mode frame Structure.
Type 2: TDD mode frame structure.
Type-1:FDD Frame Structure
As LTE FDD is full duplex system, means both the
downlink and uplink transmission happens at the
same time at different frequencies.

7. Type-2:TDD Frame Structure:
In TDD, the transmission is divided into time domain, means
at one moment of time either downlink subframe is
transmitted or uplink.
As one can see in above image, one frame is divided into 10
subframes (1ms each), and that subframe can be either
downlink, uplink or special subframe.

8. Now the question comes, who decides the sequence
of these subframes. That has been defined by 3GPP
with the name TDD Frame Configurations.
There are fixed patterns of these configurations and
network operator can choose out of these defined
patters. There are total 7 TDD configurations as
shown below: