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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. 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:

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.

List Of Tutorials available in this website:

PSS and SSS (LTE Synchronization)

After power-up, the UE tries to obtain time and frequency synchronization with the system. For this, UE needs to detect signals to find out where the frame begins and ends. LTE define two type of synchronization signals:

• Primary synchronization signal (PSS)
• Secondary synchronization signal (SSS)

UE uses the Synchronization Signals to:

• Achieve radio frame, subframe, slot and symbol synchronization in the time domain
• Identify the center of the channel bandwidth in the frequency domain
• Deduce the Physical layer Cell Identity (PCI)

Primary Synchronization Signals:

Primary synchronization signals consist of one of three 62-symbol Zadoff-chu sequences in a cell.

The Primary Synchronization Signal (PSS) is broadcast twice during every radio frame and both transmissions are identical.

In the case of FDD: the PSS is broadcast using the central 62 subcarriers belonging to the last symbol of time slots 0 and 10 «

In the case of TDD: the PSS is broadcast using the central 62 subcarriers belonging to the third symbol of time slot 2 (subframe 1) and the third symbol of time slot 12 (subframe 6)

PSS helps to achieve subframe, slot and symbol synchronization in the time domain, identify the center of the channel bandwidth in the frequency domain and deduce a pointer towards 1 of 3 Physical layer Cell Identities (PCI).

The PSS cannot be used to achieve radio frame synchronization because both transmissions within the radio frame are identical and equally spaced in time.

Secondary Synchronization Signals:

After the primary sync signal acquisition, UE tries to detect the secondary synchronization signal. Secondary synchronization signals are one of 168 codes which are 62-bit sequences. The Secondary Synchronization Signal (SSS) is broadcast twice within every radio frame. The two transmissions of the SSS are different so the UE can detect which is the first and which is the second

In the case of FDD: the SSS is broadcast using the central 62 subcarriers belonging to the second to last symbol of time slots 0 and 10 «

In the case of TDD: the SSS is broadcast using the central 62 subcarriers belonging to the last symbol of time slot 1 (subframe 0) and the last symbol of time slot 11 (subframe 5)

SSS helps to achieve radio frame synchronization and deduce a pointer towards 1 of 168 Physical layer Cell Identity (PCI) groups.

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Below diagram shows how PSS and SSS helps finding the Physical Layer Cell ID.

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