Lte Slot Symbol
An FDD LTE radio frame is 10 milliseconds in length, and is divided into 10 subframes of 1 millisecond, which are in turn divided into two slots of 0.5 milliseconds. Following these calculations, each radio frame is 20 slots in length, with each slot being 7 symbols in length, with each symbol being 0.071428 ms in length. In general configuration and LTE deployments for urban areas, the Normal CP is used so the number of symbols per slot would be 7. The symbol time (Ts) for each LTE symbol is 66.67 us (much shorter than the symbol time for 10 MHz WiMAX symbol). Adding Cyclic Prefix to Ts makes it around 71.34 us.
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
Lte Slot Symbol Image
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. That is 12 carriers in the frequency domain and 0.5 ms (or 7 CP) in time domain.
So minimum resource allocation to a user in LTE is 180Khz or 1 slot(0.5ms).
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:
Lte Subframe Symbol
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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LTE_DL_DemuxSlot (Downlink Slot De-multiplexer)
Description:
Lte Slot Symbol Meaning
Downlink Slot De-multiplexer for LTE DL receiver
Library: LTE, MultiplexParameters
Name
Description
Default
Type
Bandwidth
bandwidth: BW 1.4 MHz, BW 3 MHz, BW 5 MHz, BW 10 MHz, BW 15 MHz, BW 20 MHz
BW 5 MHz
enum
NumRxAnts
number of Rx Antennas: Rx1, Rx2, Rx4
Rx1
enum
OversamplingOption
Ratio 1, Ratio 2, Ratio 4, Ratio 8: Ratio 1, Ratio 2, Ratio 4, Ratio 8
Ratio 2
enum
CyclicPrefix
type of cyclic prefix: Normal, Extended
Normal
enum
Sym_StartPos
start position (without oversampling) to get the OFDM symbol for FFT operation for long CP and short CP symbols respectively, compared to the start position of the OFDM body after CP
{-3,-3}
int array
Pin Inputs
Pin
Name
Description
Signal Type
1
FrameData
input downlink frame symbol
multiple complex
Pin Outputs
Pin
Name
Description
Signal Type
2
OFDMSig
OFDM signal
multiple complex
Notes/Equations
- This model is used to demultiplex 3GPP LTE downlink slot.
- Each 10 ms radio frame consists of 20 slots of length, numbered from 0 to 19. Each 0.5 ms slot consists of 7 OFDM symbols for normal cyclic prefix or 6 OFDM symbols for extended cyclic prefix.
The cyclic prefix of each OFDM symbol is removed in this model. For normal cyclic prefix, the first OFDM symbol within a slot has different cyclic prefix lengths from other 6 OFDM symbols. For extended cyclic prefix, the 6 OFDM symbols have same cyclic prefix lengths. - The start of each output OFDM symbol is illustrated in the following figure.
- Sym_StartPos is an array of two elements, indicating the start position (without oversampling) of the first output OFDM symbol and other output OFDM symbols in each slot respectively. Here the ‘start position’ is compared to the first sample of the input OFDM symbol body excluding the cyclic prefix. Hence the value is negative indicating the start position locates in the CP duration of the input symbol as shown in the figure.
References
- 3GPP TS 36.211 v8.4.0, ‘Physical Channels and Modulation’, Sept 2008.
- 3GPP TS 36.104 v8.3.0 ‘Base Station (BS) radio transmission and reception’, Sept 2008.
- TR 25.814 ‘Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA),’, V7.0.0, June 2006.
