lte interface physical structure

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LTE Radio Interface - Physical Layer

• LTE Interface Physical Structure

LTE Radio Interface - Physical Layer • LTE Interface Physical Structure


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Table of Contents:

1 Introduction ........................................................................................................ 4 2 FDD Frame Structure ........................................................................................ 5 3 Physical Resource Blocks in Downlink .............................................................. 6 4 Physical Resource Blocks in Uplink ................................................................... 8 5 Physical Channels in Downlink .......................................................................... 9 6 Physical Channels in Uplink............................................................................. 13 7 Exercise 1 ........................................................................................................ 16 8 Adaptive Resource Allocation .......................................................................... 17 9 Exercise 2 ........................................................................................................ 18 10 Hybrid ARQ Operation ..................................................................................... 19 11 MIMO Transmission (1) ................................................................................... 20 12 MIMO Transmission (2) ................................................................................... 21 13 UE Categories ................................................................................................. 22 14 Exercise 3 ........................................................................................................ 23



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1 Introduction

Let us next turn our attention to the physical structure of the LTE radio interface.

LTE supports both frequency division duplex (FDD) and time division duplex (TDD) modes of operation.

In FDD, the uplink and downlink signals in a cell are carried in different frequency bands. In TDD, separate time slots are reserved for carrying uplink and downlink traffic.

Regardless of the duplex mode, LTE offers a variety of channel bandwidths between 1.4 and 20 MHz.

A small channel bandwidth is beneficial for a mobile operator short on spectrum. On the other hand, a large channel bandwidth is required if large peak data rates are to be supported. The possibility of using different bandwiths in different transmission scenarios is one of the major benefits of LTE.

Each channel bandwidth offers a certain number of subcarriers, or - expressed in another way - a certain number of resource blocks. The concept of resource block and the significance of using such a basic resource unit in LTE will be explained later in this course. From the table you can easily see that a resource block always occupies 12 subcarriers.

The subcarrier spacing is 15 kHz. In downlink, also 7.5 kHz subcarrier spacing is possible

.



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2 FDD Frame Structure

The frame structure for frequency division duplex (FDD) operation is straightforward.

Across the signal bandwidth (that is, over all the subcarriers), the downlink and uplink information is carried within a continuous sequence of radio frames of 10 millisecond duration.

Each frame is divided into ten subframes, also called transmission time intervals (TTI), which, in turn, consist of two time slots.

In the downlink, each slot normally contains seven OFDM symbols, as shown in the figure, where each symbol starts with a short cyclic prefix. However, in a radio environment with a large delay spread, it may be necessary to use an extended cyclic prefix. In this case, the slot can carry only six OFDM symbols. The cyclic prefix length values are shown in the figure.

Note that the non-cyclic-prefix part (that is, the information-carrying part) of the symbol always has the same length of 66.66 microseconds.

In the uplink, each slot correspondingly contains seven SC-FDMA symbols starting with a short cyclic prefix, or alternatively six SC-FDMA symbols starting with an extended cyclic prefix.

Note that in the case of time division duplex (TDD) operation, the frame structure is different. However, OFDM or SC-FDMA symbols are carried in slots of 0.5 millisecond duration exactly as in the case of FDD.



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3 Physical Resource Blocks in Downlink

In both the downlink and uplink direction, data is allocated to users in terms of resource blocks (RBs). A resource block consists of 12 consecutive subcarriers in the frequency domain, that are reserved for the duration of one 0.5 millisecond time slot.

Depending on the required data rate and the scheduling decision done in the eNodeB, each UE may or may not be assigned resource blocks during each transmission time interval of 1 ms. The smallest resource unit a scheduler can assign to a user is a scheduling block which consists of two consecutive resource blocks. In downlink, the resource blocks may be located adjacently in the frequency domain, or in a distributed fashion for added frequency diversity.

In downlink, resource blocks can carry several types of channels - to be introduced later - and must also carry certain reference and synchronisation signals.

In each resource block, subcarriers zero and six in the first OFDM symbol, and subcarriers three and nine in the third OFDM symbol, counting backwards from the timeslot, carry the downlink reference signal. This signal, consisting of a known pseudorandom sequence, is required for channel estimation in the UEs. Note that in the case of MIMO transmission, additional reference signals must be embedded into the resource blocks.

Before a UE can start communicating with the base station, it must lock on to a primary and secondary synchronisation signal, located in the downlink OFDMA signal as shown in the figure. Regardless of the channel bandwidth, 72 central subcarriers are reserved for these synchronisation signals during specific OFDM symbols in the radio frame.



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4 Physical Resource Blocks in Uplink

Also in uplink, a resource block consists of 12 consecutive subcarriers in the frequency domain, reserved during one 0.5 millisecond time slot. Unlike in downlink, however, resource blocks allocated to a certain UE during a certain transmission time interval and carrying user data must be located adjacently in the frequency domain.

In uplink, resource blocks can carry several types of channels - to be introduced later - and carry two types of reference signals.

The demodulation reference signal is transmitted in the fourth SC-FDMA symbol in all resource blocks allocated to the Physical Uplink Shared Channel (PUSCH) carrying the user data. This signal is needed for channel estimation which in turn is essential for coherent demodulation of the uplink signal in the eNodeB.

Note that when the subframe contains the Physical Uplink Control Channel (PUCCH), the demodulation reference signal is embedded in a different way.

The sounding reference signal provides uplink channel quality information as a basis for scheduling decisions in the base station. This signal is distributed in the last SC-FDMA symbol of such subframes that carry neither PUSCH nor PUCCH data.



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5 Physical Channels in Downlink

Let us next examine the different types of physical channels in the downlink. First, there will be a short presentation. At the end of the animation you can use your mouse pointer to return to the channels for more details.

During cell search, the UE must decode important information carried over the Physical Broadcast Channel (PBCH), before it can start communicating with the eNodeB.

The user data in the downlink is carried over the Physical Downlink Shared Channel (PDSCH). Unlike in uplink, resource blocks allocated to a certain UE during a time slot need not necessarily be located adjacently in the frequency domain.

The Physical Downlink Control Channel (PDCCH) contains important downlink and uplink scheduling information for all mobile terminals in the cell. Using the PDCCH, each UE is able to identify the resource blocks allocated to it on the PDSCH during each subframe. The information also includes the selected modulation scheme and coding rate in downlink, and downlink hybrid ARQ related information.

Note that the PDCCH also informs each UE which resource blocks it can use for transmission on the physical shared channel in uplink, as well as the uplink modulation scheme and coding rate to be used in each subframe.

There are three more physical channels in the downlink. Use your mouse pointer to take a closer look.



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6 Physical Channels in Uplink

In the uplink direction, the user data is carried over the Physical Uplink Shared Channel (PUSCH). Resource blocks allocated to a certain UE on the PUSCH must always be located adjacently in the frequency domain. However, frequency hopping can be applied to achieve frequency diversity.

The Physical Uplink Control Channel (PUCCH) carries various uplink control information in those subframes where there is no Physical Uplink Shared Channel. Note that when the UE transmits data on the shared channel, the control information is embedded with the user data and the PUCCH is not needed. In fact, there is relatively little control information to be sent in uplink, since the eNodeB is responsible for scheduling uplink resources and the scheduling information is sent over the Physical Downlink Control Channel (PDCCH) to the UE.

PUCCH resource blocks are located at both edges of the uplink bandwidth, and inter-slot hopping is applied as shown in the figure.

Every time the UE wishes to initiate communication with the network, a procedure called random access has to be performed. A so-called random access preamble is sent to the eNodeB over the Physical Random Access Channel (PRACH). The response from the network is sent over the PDSCH.



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7 Exercise 1



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8 Adaptive Resource Allocation

In the LTE radio interface, the eNodeB dynamically schedules both downlink and uplink radio resources on a subframe-to-subframe (that is, TTI-to-TTI) basis.

When allocating radio resources, the eNodeB takes into account the following:

downlink radio conditions, measured by the UEs and conveyed to the eNodeB in channel quality indication (CQI) reports

uplink radio conditions, measured directly by the eNodeB

quality-of service (QoS) requirements, for instance a large or small delay is allowed depending on the service

possibly the traffic conditions.

Scheduling means that the eNodeB decides how many (and which) resource blocks are allocated to each UE during each subframe in uplink and downlink.

Furthermore, the eNodeB decides - again on a subframe-to-subframe basis - which modulation scheme, coding rate, and power level should be applied in uplink and downlink.

Adaptive modulation and coding has already been implemented in High-Speed Packet Access (HSPA) systems. However, the possibility of adaptively allocating signal blocks in the frequency domain is new in LTE.

The small example illustrates how two UEs experience different channel conditions in the downlink during a specific subframe. The eNodeB allocates the resource blocks taking these channel conditions into account.



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9 Exercise 2



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10 Hybrid ARQ Operation

LTE employs the Hybrid Automatic Repeat reQuest (HARQ) fast retransmission scheme both in downlink and in uplink. Up to eight HARQ processes can be active both in downlink and in uplink at the same time.

The HARQ scheme provides error correction by “soft-combining” the information received in successive retransmissions until the packet is received correctly. This process is known as incremental redundancy.

HARQ uses a stop-and-wait protocol. After transmitting a data block, the transmitting entity waits until it receives an acknowledgment (ACK) or negative acknowledgement (NACK) before transmitting the next data block or retransmitting the error-containing data block.

Downlink HARQ processes are asynchronous in time. Retransmissions are possible in any order without fixed timing. As a result, HARQ-related information such as the HARQ process identifier must be sent over the PDCCH in parallel with the data sent over the PDSCH.

In contrast, uplink HARQ processes are synchronous in time. If the data of a certain HARQ process is sent in subframe n, the ACK or NACK is sent back over the Physical Hybrid ARQ Indicator Channel in subframe n+4. The data block is then retransmitted - or the next data block is sent - in subframe n+8.



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11 MIMO Transmission (1)

The LTE radio interface initially supports 2x2 (and later 4x4) Multiple Input Multiple Output (MIMO) transmission in the downlink. 2x2 MIMO employs two transmit antennas at the eNodeB side and two receive antennas at the mobile terminal side of the transmission link.

There are actually two kinds of MIMO techniques:

Multistream transmission (also known as spatial multiplexing) MIMO

Diversity (or space-time coding) MIMO.

In the multistream transmission case, each transmit antenna transmits a different data stream. Although the data streams are transmitted simultaneously at the same frequency, the receiver can nevertheless detect the different streams received via different antennas - hence the name spatial multiplexing. This technique significantly increases the peak data rate over the radio link. For instance, 4x4 MIMO effectively increases the peak data rate by a factor of four. However, spatial multiplexing requires high signal-to-noise-plus-interference ratio (SNIR) radio conditions in order to be effective.

In the diversity MIMO case, each transmit antenna transmits the same data stream. This type of MIMO application does not increase the peak data rate over the radio link, but is beneficial in low SNIR conditions.

In the special case of beamforming, the same data stream is transmitted, but with a different phase shift in each transmit antenna, thus effectively sending the transmit signal in a certain direction.



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12 MIMO Transmission (2)

The basic MIMO-related processing in the eNodeB transmitter in the case of a 2x2 MIMO system using spatial multiplexing is shown in the figure.

Two encoded data streams are first scrambled and modulated, using for instance 64QAM. As part of the MIMO processing, the modulated data streams are mapped on to two transmission layers. The layered signals are then precoded, resulting in the signals to be sent over the two antenna ports. The resource element mapping and OFDM signal generation is performed separately in the two antenna port branches.

In summary, the MIMO processing takes place after the coding and modulation, but before the OFDM signal generation.



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13 UE Categories

Five UE categories have been defined for LTE terminal equipment, according to 3GPP Release 8 Technical Specification 36.306.

The UE category specifies the peak data rate in the downlink and uplink.

Furthermore, 2x2 MIMO should be supported in the downlink in categories two, three and four, and 4x4 MIMO in category five.

Regarding the modulation, all UE categories must be able to handle 64QAM in the downlink and 16QAM in the uplink. One exception is UE category five, which must support 64QAM also in the uplink.

Finally, all UE categories must be able to handle bandwidths up to 20 MHz.



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14 Exercise 3

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