Radio Frame Structure in LTE

LTE uses Orthogonal Frequency Division Multiple Access (OFDMA) for downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) for uplink. The radio frame structure is the backbone of this system, defining how time and frequency resources are allocated for user data transmission, control signaling, and other essential functions.

The LTE frame structure is designed to provide flexibility in various deployment scenarios, including different bandwidths and spectrum allocations, to support mobile broadband services effectively.

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Before understanding the frame structure, let's define some key terminology used in LTE:

• Resource Block (RB): The smallest unit of resource allocation in LTE, consisting of 12 subcarriers in the frequency domain and one slot in the time domain. It is used as the primary unit for scheduling and allocation of resources.
• Resource Element (RE): The smallest unit in the LTE time-frequency grid, representing one subcarrier in frequency and one OFDM symbol in time.
• Subframe: A 1 ms duration composed of two slots. Each subframe is used for data transmission, and its configuration depends on the LTE mode (FDD or TDD).
• Frame: A 10 ms duration consisting of 10 subframes. It is the basic unit of time in LTE, and frames are transmitted continuously.

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Frame Structure Types in LTE

There are two main frame structure types in LTE:

Type 1: Frequency Division Duplex (FDD)

1. In FDD, uplink and downlink transmissions occur simultaneously but on different frequency bands.
2. The LTE frame is divided into 10 subframes, each with a duration of 1 ms. Each subframe contains two slots of 0.5 ms each.

Type 2: Time Division Duplex (TDD)

1. In TDD, uplink and downlink transmissions share the same frequency band but occur at different times.
2. The frame structure consists of 10 subframes of 1 ms each, with certain subframes designated for uplink, downlink, or special signalling.

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Frame Structure in LTE

Frame and Subframe

An LTE radio frame lasts for 10 ms and contains 10 subframes, each of 1 ms. Each subframe consists of two slots of 0.5 ms duration. This configuration provides the temporal granularity needed for high-speed data transfer and precise control over resource allocation.

Each slot consists of 7 OFDM symbols when the normal cyclic prefix (CP) is used or 6 symbols when the extended CP is used.

Resource Blocks (RBs) and Resource Elements (REs)

A Resource Block (RB) in LTE represents the smallest unit for resource scheduling. It spans 12 subcarriers in the frequency domain and one slot (0.5 ms) in the time domain.

Given a bandwidth of 10 MHz:

• Subcarrier spacing is 15 kHz.
• Number of subcarriers per RB = 12.
• Total bandwidth in one RB = 12 subcarriers * 15 kHz = 180 kHz.

Example Calculation:

Assuming a normal CP and 10 MHz bandwidth:

• Total RBs in one slot = 50 RBs.
• Total REs in one RB = 84 REs.
• Total REs per slot = 50 RBs * 84 REs = 4200 REs.

These REs are used for data transmission, signaling, reference signals, and control information.

Calculating Resource Allocation

Consider a scenario where we have a 10 MHz LTE system:

• Bandwidth = 10 MHz
• Subcarrier spacing = 15 kHz
• Total subcarriers = 600 (within 10 MHz)
• Total RBs = 50

Let’s assume we are allocating resources for a user. If each RB carries 84 REs:

• If we allocate 10 RBs, the total REs for the user in one slot = 10 RBs * 84 REs = 840 REs.
• Over one subframe (2 slots), the user gets 840 * 2 = 1680 REs.

This allocation would be adjusted based on modulation and coding schemes (MCS), providing different data rates depending on channel conditions.

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LTE TDD Frame Structure Type 2

Overview Frame Structure Type 2

Frame structure type 2 is used specifically for Time Division Duplex (TDD) in LTE. It consists of the following key features:

Uplink-Downlink Configuration

The uplink-downlink configuration in a cell dictates whether a given subframe is designated for uplink, downlink, or special transmissions. This configuration is crucial for the operation of TDD because it determines the allocation of time for uplink and downlink communications within the radio frame.

• The uplink-downlink configuration can vary between frames and is determined based on the network’s requirements and conditions.
• Subframes are classified as follows:

• D (Downlink): Reserved for downlink transmissions.
• U (Uplink): Reserved for uplink transmissions.
• S (Special Subframe): Contains DwPTS (Downlink Pilot Time Slot), GP (Guard Period), and UpPTS (Uplink Pilot Time Slot) for transitioning between downlink and uplink.

Special Subframes and Switch-Point Periodicity

• The special subframe, denoted as "S," is designed to handle the switch between downlink and uplink periods. It contains three distinct parts:

• DwPTS: Used for downlink pilot signals and synchronization.
• GP: A guard period to prevent interference between uplink and downlink transmissions.
• UpPTS: Reserved for uplink pilot signals and scheduling requests.

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LTE supports two types of switch-point periodicities:

• 5 ms Switch-Point Periodicity: The special subframe appears in both half-frames, providing flexibility in scheduling.
• 10 ms Switch-Point Periodicity: The special subframe appears only in the first half-frame, which may be used in scenarios where uplink and downlink periods need to be longer.

Uplink-Downlink Configurations (Table 4.2-2)

The table provides different uplink-downlink configurations, showing how subframes in a radio frame are allocated as downlink (D), uplink (U), or special (S) subframes. The switch-point periodicity and the combination of these subframes allow the network to balance between uplink and downlink transmissions based on traffic demand.

• Configuration 0: All subframes are for downlink except subframes 2, 3, 4, and 7, which are uplink.
• Configuration 1: Downlink dominates, with only subframe 2 and 7 as uplink.
• Configuration 2: Similar to configuration 1 but includes a special subframe.
• Configuration 3 and 4: Extend the switch-point periodicity to 10 ms, providing fewer transitions between uplink and downlink.
• Configuration 5 and 6: Utilize different combinations of uplink and downlink subframes for varied scheduling options.

The radio frame structure in LTE is intricately designed to optimize spectral efficiency, flexibility, and performance. Understanding the breakdown of frames, subframes, RBs, and REs helps grasp the technical operations of LTE networks. Calculations based on these structures demonstrate the efficiency and resource allocation capabilities of LTE, which supports high data rates and robust connectivity.

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