LTE Physical Layer 2 radio frames are supported:
1. Type 1: FDD : Frequency Division Duplex
2. Type 2: TDD : Time Division Duplex
For multiple access, LTE uses:
For down link: OFDMA: Orthogonal Frequency Division Multiple Access
For up link: SC-FDMA: Single Carrier Frequency Division Multiple Access
1. Type 1: FDD : Frequency Division Duplex
1. It is valid for both half duplex, and full duplex FDD modes
2. Type 1 radio frame has a duration 10 ms and consists of equally sized 20 slots each of 0.5 ms.
3. A subframe comprises two slots, thus one radio frame has 10 sub‐frames.
There are six time units: frame, half-frame, subframe, slot, symbol, and the basic time unit (Ts):
Time Unit Value
Frame 10 ms
Half-frame 5 ms
Subframe 1 ms
Slot 0.5 ms
Symbol (0.5 ms) / 7 for normal CP, (0.5 ms) / 6 for extended CP
Ts 1/(15000 * 2048) sec » 32.6 ns
A resource block (RB) is the smallest unit of resources that can be allocated to a user. It is 180Khz (12 carriers) in the frequency domain and 0.5 ms in time domain.
So minimum resource allocation to a user in LTE is 180Khz or 1 ms.
TTI is 1ms.
Below is the pictorial representation of FDD
Here the down link and uplink will be done in 2 different frequencies.
In full-duplex FDD, uplink and downlink frames are separated by frequency and are transmitted continuously and synchronously.
In half-duplex FDD, UE cannot receive while transmitting.
LTE Bandwidths
LTE standard bandwidths are 1.4, 3, 5, 10, 15, and 20 MHz.
Type 1 Frame Structure
2. Type 2: TDD : Time Division Duplex
1. Half of the subframes are available for downlink, and the other half are available for uplink transmission.
2. In each 10 ms interval, where downlink and uplink transmission are separated in the frequency domain.
3. The uplink and downlink subframes are transmitted on the same frequency and are multiplexed in the time domain.
4. There are seven possible configurations given in the standard.
5. Depending upon the traffic, carrier/operator can choose any of the configurations
Special subframes
Special subframes are used for switching from downlink to uplink and contain three sections: DwPTS, GP, and UpPTS.
DwPTS is Downlink Pilot Time Slot.
UpPTS is Uplink Pilot Time Slot.
GP is guard period between DwPTS and UpPTS.
Note:
1. Subframes 0 and 5 and DwPTS in TDD frames are always allocated to downlink transmissions.
2. Subframe 1 is always configured to be a special subframe.
3. Subframe 6 can also be configured to be a special subframe.
Below is the pictorial representation of TDD
Here the down link and uplink will be done in 2 different time slots.
UPLINK / DOWNLINK SUBFRAME CONFIGURATIONS FOR LTE TDD (TD-LTE)
OFDMA:
Number of OFDM symbols in each slot depends on type of Cyclic Prefix(CP) used.
– For Normal CP – 7 OFDM symbols per slot
– For Extended CP – 6 OFDM symbols per slot
what is Cyclic Prefix? (CP)
The cyclic prefix is a guard interval to protect the OFDM signals from intersymbol interference.
It is the copy of last part of OFDM that is perpended to transmitted symbol and removed at the receiver before demodulation.
To make OFDM insensitive to time dispersion, CP is inserted.
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