Overview of the physical layer of LTE and explanation of basic concepts

Source: Internet
Author: User

There is types of frame structure in the LTE standard, type 1 and type 2. Type 1 uses Frequency Division duplexing (Uplink and downlink separated by Frequency), and TDD uses time division Duplexin G (Uplink and downlink separated in time). This overview covers both LTE FDD Type 1 signals and LTE TDD type 2 signals described in the standard documents listed in The About Opts BHD and bhe:lte modulation analysis topic.

This overview isn't a exhaustive description of the physical layer, but is intended to provide you with a useful Backgro und when you configure the 89600 VSA LTE demodulator to make measurements.

For a more in depth explanation of LTE, see the 3GPP Long Term Evolution:system overview, Product Development, and Test C Hallenges Application Note. Keysight have also released a book called LTE and the Evolution to 4G wireless:design and measurement challenges which con Tains detailed information on many of the aspects of LTE. Terminology

First, an introduction to some of the terms used in describing an LTE Frame. There is six time units:frame, Half-frame, subframe, slot, symbol, and the basic Time Unit (Ts), as shown in the Followi ng table.

Time Unit


Frame Ten MS
Half-frame 5 ms
Subframe 1 ms
Slots 0.5 ms

(0.5 ms)/7 for normal CP

(0.5 ms)/6 for extended CP

Ts 1/(15000 * 2048) sec»32.6 NS

Below is a illustration of an FDD frame.

A resource block (RB) is the smallest unit of resources so can be allocated to a user. The resource block is a wide in frequency and 1 slots long in time. In frequency, resource blocks is either x khz subcarriers or x 7.5 kHz subcarriers wide The number of subcarrier S used per resource block for most channels and signals are subcarriers.

The 89600 VSA LTE demodulator currently only supports resource blocks that is subcarriers wide.

Frequency units can is expressed in number of subcarriers or resource blocks.  For instance, a 5 MHz downlink signal could be described as + resource blocks wide or 301 subcarriers wide (DC subcarrier Is isn't included in a resource block).

The underlying data carrier for a LTE frame is the resource element (RE). The resource element, which is 1 subcarrier x 1 symbol, was the smallest discrete part of the frame and contains a single C Omplex value representing data from a physical channel or signal. bandwidths

The bandwidths defined by the standard is 1.4, 3, 5, ten, and-MHz. The table below shows how many subcarriers and resource blocks there is in each bandwidth for uplink and downlink.

Frequency measures 5 MHz 301 /td> tr>


Resource Blocks

Subcarriers (downlink)

Subcarriers (uplink)

1.4 MHz 6 up
3 M Hz 181
ten MHz - 601 -
901 100
MHz 1201 the

For downlink signals, the DC subcarrier are not transmitted, but are counted in the number of subcarriers. For uplink, the DC subcarrier does not exist because the entire spectrum are shifted down in frequency by half the Subcarri Er spacing and is symmetric about DC. FDD Frame Type 1

In FDD mode, uplink and downlink frames is both 10ms long and is separated either in frequency or in time.

For Full-duplex FDD, uplink and downlink frames is separated by frequency and is transmitted continuously and Synchronou Sly.

For Half-duplex FDD, the only difference are a UE cannot receive while transmitting.

The base station can specify a time offset (in PDCCH) to being applied to the uplink frame relative to the downlink frame. TDD Frame Type 2

In TDD mode, the uplink and downlink subframes is transmitted on the same frequency and is multiplexed in the time Domai N. The locations of the uplink, downlink, and special subframes is determined by the Uplink-downlink configuration. There is seven possible configurations given in the standard. The following is an illustration of a TDD frame with Uplink-downlink configuration set to 2 and special subframe Configura tion set to 6.

Special Subframes

Special subframes is used for switching from downlink to uplink and contain three sections:dwpts, GP, and uppts.

Dwpts is the downlink Pilot time Slot. Dwpts contains P-SS. Pdsch can also be transmitted during dwpts when Dwpts was configured to be longer than a slot.

Uppts is the uplink Pilot time Slot. Uppts can contain PRACH and SRS, but cannot contain or PUCCH or PUSCH.

GP is a guard period between dwpts and Uppts. PRACH Format 4 begins in the guard period. Otherwise, nothing else is transmitted during the guard period.

The lengths of these three sections is determined by the special Subframe configuration index (specified by the Dw/gp/up Len parameter). There is 9 possible configurations. UL/DL Configuration

The Uplink-downlink configuration (specified by Ul/dl Config) of a TDD frame determines the locations of uplink, downlink, and special subframes. There is 7 possible uplink-downlink configurations.

Subframes 0 and 5 and dwpts in TDD frames is always allocated to downlink transmissions.

Uppts and the subframe after a special subframe is always allocated to uplink transmissions.

Subframe 1 is always configured to be a special Subframe. Subframe 6 can also is configured to be a special Subframe. Uplink

Uplink user transmissions consist of uplink user data (PUSCH), random-access requests (PRACH), user control channels (PUCC H), and sounding reference signals (SRS).

FDD and TDD uplink transmissions have the same physical channels and signals. The only difference are that TDD frames include a special subframe, part of which can being used for SRS and PRACH Uplink Tran Smissions.

The following illustration shows part of a LTE uplink frame and contains an allocation for each type of uplink channel. The illustration is applicable to both TDD and FDD.

User 1 has a PUSCH allocation of [RB, Slots 4-5], and user 2 have a PUCCH allocation of [Subframe 2, PUCCH index 0]. User 3 have been given an SRS allocation of Subcarrier 94 to 135 in Subframe 2, and User 4 are transmitting in a PRACH alloc ation.

A user cannot transmit both PUCCH and PUSCH data in the same slot. TDD

In TDD mode, uplink and downlink transmissions occupy the same frequency spectrum but is separated in time. Uplink users transmit during subframes configured for uplink. In addition to uplink subframes, UE's can transmit random-access requests (PRACH) and SRS during the uppts section of the Special subframe. FDD

The uplink FDD frame is the same length as the downlink frame and contains only uplink user transmissions. modulation

For uplink data signals (PUSCH), the LTE standard uses single Carrier-frequency division multiple Access (SC-FDMA) modul ation, which have a lower peak-to-average power ratio, meaning lower cost amplifiers and less power usage.

In SC-FDMA, the user data was modulated onto a single carrier modulation format (QPSK, 16QAM, or 64QAM), and the time Domai n symbols is transformed to the frequency domain by an FFT. Then the frequency domain points is mapped onto the subcarriers assigned to the user in the OFDM symbol. Finally, an IFFT was performed on the entire OFDM symbol and resulting time data is transmitted. The following image illustrates this process.

The value M in the M-point FFT in the illustration above are the width (in subcarriers) of the uplink allocation assigned T o the user. Typically a UE is not allocated the entire spectrum, therefore M was less than or equal to N.

For symbols that contain demodulation reference signals, PRACH, PUCCH, or SRS, the uplink transmitter places the Modulatio n symbols directly onto the OFDM Subcarriers, performs the IFFT, and transmits the data in a manner similar to downlink of Dma. Synchronization

Uplink signals does not have a dedicated sync signal. In a real world environment, the uplink signals would is synchronized using the downlink signal. However, to an analysis of uplink separate from downlink when using the 89600 VSA LTE demodulator, uplink frames can is Synchronized using PUCCH dm-rs, PUSCH dm-rs, PRACH, or SRS. Reference Signals

There is types of uplink reference signals, the Demodulation reference Signal (DMRs) and the sounding reference Signa L (SRS). Reference signals is used for channel estimation or equalization. DMRs

The demodulation Reference Signal (DMRs) is used by the base station to equalize and demodulate the UE ' s transmissions.

The PUSCH demodulation reference signal is a zadoff-chu sequence, which results in constellation points on a circle center Ed about the origin.

The PUCCH demodulation reference signal, however, is a reference sequence transmitted on a rotated QPSK constellation. The amount of rotation is determined by cyclic shift (a) as defined in the.

Each uplink user transmits a demodulation Reference Signal during certain symbols in each resource block allocated to the User. DMRs is transmitted on all subcarriers allocated to the user during the symbols listed in Table in 3GPP TS 36. 211. SRS

The sounding reference signal (SRS) is a transmitted separately from PUCCH and PUSCH. SRS can be transmitted on any number of subcarriers in the last symbol in a uplink subframe whether or not the subcarrier S is assigned to another channel. The exception is this PRACH transmissions and PUCCH Format 1 and 2/2a/2b transmissions take precedence over SRS Transmissi Ons.

SRS is transmitted by a UE to give the base station an idea of the channel characteristics for that UE. The base station can use the information to assign good uplink allocations for the UE to transmit on. Physical Channels PUCCH

There is only one control channel transmitted by uplink users-the Primary Uplink control channel (PUCCH)-which contains in Formation including channel quality info, acknowledgements, and scheduling requests.

PUCCH is assigned by subframe the instead of by slot. The location of the PUCCH in the first slot alternates in the pattern:

Resource Block 0, N-1, 1, N-2, 2, N-3, ...

Where N is the frequency width of the frame in units of RB.

The location of the PUCCH resource blocks in the second slots is the one, the mirrors the location of the resource block in The first slot. For example, with a 5 MHz uplink LTE Signal, a pucch allocation of (subframe 1, PUCCH index 0) means that PUCCH is Transmitt Ed on (slot 2, RB0) and (slot 3, RB24).

The PUCCH index (set by the first RB parameter) determines which resource block in the first and second slots of the SUBFR AME is used for PUCCH.

PUCCH ' s modulation format is determined by PUCCH type and can be QPSK, BPSK, on/off keying, or a combination of QPSK and B Psk. See the PUCCH Frame Summary topic for a table of PUCCH types and their corresponding modulation formats. PUSCH

The Primary uplink Shared Channel (PUSCH) is used by uplink users to transmit data to the base station. All subcarriers not allocated for PRACH, PUCCH, or SRS is available for assignment to users. PUSCH data is modulated using SC-FDMA.

PUSCH data is transmitted using QPSK, 16QAM, or 64QAM as the single carrier modulation type before spreading (FFT, Subcarr IER mapping, then IFFT). PRACH

The physical Random Access Channel (PRACH) is used by a uplink user to initiate contact with a base station. The base station broadcasts some basic cell information, including where random-access requests can be transmitted. A UE then makes a PRACH transmission asking for PUSCH allocations, and the base station uses the downlink control channel (PDCCH) to reply where the UE can transmit PUSCH.

PRACH consists of a cyclic prefix followed by the PRACH sequence. There is five PRACH preamble formats with various CP length and sequence lengths. Format 4 is short enough to be transmitted during the GP and uppts sections

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