LTE Physical-Layer Overview

Peter Wang

January, 16, 2011

OUTLINE

• LTE requirements & features

• OFDMA Frame and Resource Block Structure

• Protocol Architecture

• Physical Channel Structure and Procedure

• UE Measurements

• RSRP, RSRQ, & RSRP Ês/Iot

• Conclusion

• Appendix

LTE Requirements

• Peak bit (not data) rate

– 100 Mbps DL/ 50 Mbps UL within 20 MHz bandwidth (i.e., SISO)

• Up to 200 active users in a cell (5 MHz)

• Less than 5 ms user-plane latency condition (i.e., single user with single data stream)

• Mobility

– Optimized for 0 ~ 15 km/h

– 15 ~ 120 km/h supported with high performance

– Supported up to 350 km/h or even up to 500 km/h

• Enhanced multimedia broadcast multicast service (E-MBMS)

• Spectrum flexibility: 1.25 ~ 20 MHz

• Enhanced support for end-to-end QoS & QoE

LTE Enabling Technologies

• OFDM (Orthogonal Frequency Division Multiplexing) for Down Link

• Frequency domain equalization

• SC-FDMA (Single Carrier FDMA) for Up Link

• Utilizes single carrier modulation and orthogonal frequency Multiplexing using DFT-spreading in the transmitter and frequency domain equalization in the receiver

• A salient advantage of SC-FDMA over OFDM/OFDMA is low PAPR.

• Efficient transmitter and improved cell-edge performance

• MIMO (Multi-Input Multi-Output)

• e.g., Open loop, Close loop, Diversity, Spatial multiplexing

• Multicarrier channel-dependent resource scheduling

• Fractional frequency reuse

• Active interference avoidance and coordination

LTE Key Features

• Multiple access scheme

• DL: OFDMA with CP (Cyclic Prefix)

• UL: Single Carrier FDMA (SC-FDMA) with CP

• Adaptive modulation and coding

• DL/UL modulations: QPSK, 16QAM, and 64QAM

• Convolutional code and Rel-6 turbo code

• Advanced MIMO spatial multiplexing techniques

• (2 or 4)x(2 or 4) downlink and uplink supported

• Multi-user MIMO also supported

• Support for both FDD and TDD

• H-ARQ, mobility support, rate control, security, and etc...

LTE Standard Specifications

Specification index

Description of contents

TS 36.1xx Equipment requirements: Terminals, base stations, and repeaters

TS 36.2xx Physical layer

TS 36.3xx Layers 2 and 3: Medium access control, radio link control, and radio resource control

TS 36.4xx Infrastructure communications (UTRAN = UTRA Network) including base stations and mobile management entities

TS 36.5xx Conformance testing

OFDM (1/3)

Figure 1. Frequency-time representation of an OFDM signal

OFDM (2/3)

Figure 2. OFDM useful symbol generation using an IFFT

OFDM essential concept: Subcarrier spacing ( f) = 1/Tu

OFDM (3/3)

• High spectrum efficiency

• Inter-OFDMsymbol-interference caused by Multipath Delay Spread

• Inter-carrier-interference caused by Doppler Frequency Spread

• High Peak to Average Power Ratio (PAPR) caused by multiple frequency harmonics

• UL SC-FDMA reduces PAPR, but of more significance - particularly for the amplifier – is the Cubic Metric (CM)

OFDMA FDD Frame Structure (Type 1)

Frame structure type 1

OFDMA FDD Frame Structure (Type 2)

Frame structure type 2

OFDMA Resource Block Structure

OFDMA time-freq multiplexing

Protocol Architecture

PSS and SSS frame and slot structure in time domain in the FDD case

Physical Channel Structure

• DL

– PBCH: Transmit Broadcast channel

– PCFICH: Indicate PDCCH symbol

– PDCCH: Assign PDSCH/PUSCH

– PHICH: Indicate HARQ-ACK for UL

– PDSCH: Transmit Data

– PMCH: Transmit Multicast channel

– Synchronization Signal: UE synchronization

• UL

– PUCCH: Transmit ACK/NACK, CQI, SR

– PUSCH: Transmit Data

– PRACH: Transmit Random Access Preamble

– SRS: For UL CQI measurement

Physical Channel Procedure (1/2)

MIB: Master Information Blocks SIB: System Information Blocks

Physical Channel Procedure (2/2)

MIB: Master Information Blocks SIB: System Information Blocks

Cell Search

• Cell search: UE acquires time and frequency synchronization with a cell and detects the cell ID

• Based on BCH (Broadcast Channel) signal and hierarchical SCH (Synchronization Channel) signals.

• P-SCH (Primary-SCH) and S-SCH (Secondary-SCH) are transmitted twice per radio frame (10 ms) for FDD

• Cell search procedure

• 5 ms timing identified using P-SCH

• Radio timing and group ID found from S-SCH

• Full cell ID found from DL RS

• Decode BCH

UE Measurements (1/4)

• In cellular networks, when a mobile moves from cell to cell and performs cell selection/reselection and handover, it has to measure the signal strength/quality of the neighbor cells.

• In UMTS, a UE measures Carrier RSSI, CPICH RSCP, and CPICH Ec/No on preamble.

• In LTE network, a UE measures two parameters on reference signal: RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality).

UE Measurements (2/4)

Definition Reference signal received power (RSRP), is defined as the linear average over the power contributions (in [W]) of the resource elements that carry cell-specific reference signals within the considered measurement frequency bandwidth.For RSRP determination the cell-specific reference signals R0 according TS 36.211 [3] shall be used. If the UE can reliably detect that R1 is available it may use R1 in addition to R0 to determine RSRP.

The reference point for the RSRP shall be the antenna connector of the UE.

If receiver diversity is in use by the UE, the reported value shall not be lower than the corresponding RSRP of any of the individual diversity branches. diversity branches.

Applicable for RRC_IDLE intra-frequency,RRC_IDLE inter-frequency,RRC_CONNECTED intra-frequency,RRC_CONNECTED inter-frequency

3GPP TS 36.214 V9.2.0

UE Measurements (3/4)

Definition Reference Signal Received Quality (RSRQ) is defined as the ratio N×RSRP/(E-UTRA carrier RSSI), where N is the number of RB’s of the E-UTRA carrier RSSI measurement bandwidth. The measurements in the numerator and denominator shall be made over the same set of resource blocks.

E-UTRA Carrier Received Signal Strength Indicator (RSSI), comprises the linear average of the total received power (in [W]) observed only in OFDM symbolscontaining reference symbols for antenna port 0, in the measurement bandwidth, over N number of resource blocks by the UE from all sources, including co-channel serving and non-serving cells, adjacent channel interference, thermal noise etc.

The reference point for the RSRQ shall be the antenna connector of the UE.

If receiver diversity is in use by the UE, the reported value shall not be lower than the corresponding RSRQ of any of the individual diversity branches.

Applicable for RRC_IDLE intra-frequency,RRC_IDLE inter-frequency,RRC_CONNECTED intra-frequency,RRC_CONNECTED inter-frequency

UE Measurements (4/4)

• For example, assume that only reference signals are transmitted in a resource block, and that data and noise and interference are not considered. In this case RSRQ is equal to (1/2) or -3 dB. If reference signals and subcarriers carrying data are equally powered, the ratio corresponds to (1/12) or -10.79 dB.

• RSRQ is not suitable for LTE measurement. We use RSRP and Ês/Iot measurement defined in TS 36.133 to determine the intra frequency cell delectability.

• An intra frequency cell is considered to be detectable if: RSRP|dBm > -124 dBm for Bands 1, 4, 6, 10, 11, 18, 19, 21, 33, 34, 35, 36, 37, 38, 39, 40 and RSRP Ês/Iot -4 dB,…).

Ês: Received energy per RE (power normalized to the subcarrier spacing) during the useful part of the symbol, i.e. excluding the cyclic prefix, at the UE antenna connector.

Iot: The received power spectral density of the total noise and interference for a certain RE (power integrated over the RE and normalized to the subcarrier spacing) as measured at the UE antenna connector.

CPICH RSCP: Received Signal Code Power, the received power on one code measured on the Primary CPICH.UMTS FDD carrier RSSI: The received wide band power, including thermal noise and noise generated in the receiver, within the bandwidth defined by the receiver pulse shaping filter.

CPICH_Ec/No: The received energy per chip divided by the power density in the band. If receiver diversity is not in use by the UE, the CPICH Ec/No is identical to CPICH RSCP/UTRA Carrier RSSI.

Reference Signal with 6 frequency-shift predefined pattern

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Conclusions

• LTE Requirements and Key Features

• OFDMA Frame and Resource Block Structures

• Physical Channel Structure and Procedure

• UE measurements

• RSRP & RSRQ

Reference

[1] 3GPP LTE http://www.3gpp.org/ftp/Specs/html-info/36-series.htm.

[2] 3GPP TR 25.892; Feasibility Study for Orthogonal Frequency Division Multiplexing (OFDM) for UTRAN enhancement (Release 6)

[3] S. Sesia, et.al. “LTE-The UMTS Long Term Evolution- from Theory to Practice”, John Wiley & Sons Ltd. (Good book on PHY layer concept)

[4] H. Holma, et.al. “LTE for UMTS OFDMA and SC-FDMA Based Radio Access”, John Wiley & Sons Ltd. (Good book on System Architecture concept)

[5] H.G. Myung, Technical Overview of 3GPP LTE. http://hgmyung.googlepages.com/scfdma.pdf

[6] P. Wang, et. Al. “RF Pattern Matching Performance in LTE”, Polaris Wireless internal report, April 17, 2010.

Appendix

LTE bit rate calculation

• From the 3gpp specification:-1 Radio Frame = 10 Sub-frame-1 Sub-frame = 2 Time-slots-1 Time-slot = 0.5 ms (i.e 1 Sub-frame = 1 ms)-1 Time-slot = 7 Modulation Symbols (when normal CP length is used)-1 Modulation Symbols = 6 bits; if 64 QAM is used as modulation scheme

Radio resource is manage in LTE as resource grid....-1 Resource Block (RB) = 12 Sub-carriers

Assume 20 MHz channel bandwidth (100 RBs), normal CP

Therefore, number of bits in a sub-frame

= 100RBs x 12 sub-carriers x 2 slots x 7 modulation symbols x 6 bits

= 100800 bits

Hence, data rate = 100800 bits / 1 ms = 100.8 Mbps

* If 4x4 MIMO is used, then the peak data rate would be 4 x 100.8 Mbps = 403 Mbps.

* If 3/4 coding is used to protect the data, we still get 0.75 x 403 Mbps = 302 Mbps as data rate.

3G LTE specification overview (1/2)

WCDMA(UMTS)

HSPAHSDPA / HSUPA

HSPA+ LTE

Max downlink speedbps

384 k 14 M 28 M 100M

Max uplink speedbps

128 k 5.7 M 11 M 50 M

Latencyround trip timeapprox

150 ms 100 ms 50ms (max) ~10 ms

3GPP releases Rel 99/4 Rel 5 / 6 Rel 7 Rel 8

Approx years of initial roll out

2003 / 4 2005 / 6 HSDPA2007 / 8 HSUPA

2008 / 9 2009 / 10

Access methodology

CDMA CDMA CDMA OFDMA / SC-FDMA

LTE can be seen for provide a further evolution of functionality, increased speeds

and general improved performance.

3G LTE specification overview (2/2)Parameter Details

Peak downlink speed64QAM(Mbps)

100 (SISO), 172 (2x2 MIMO), 326 (4x4 MIMO)

Peak uplink speeds(Mbps)

50 (QPSK), 57 (16QAM), 86 (64QAM)

Data type All packet switched data (voice and data). No circuit switched.

Channel bandwidths(MHz)

1.4, 3, 5, 10, 15, 20

Duplex schemes FDD and TDD

Mobility 0 - 15 km/h (optimised),15 - 120 km/h (high performance)

Latency Idle to active less than 100msSmall packets ~10 ms

Spectral efficiency Downlink: 3 - 4 times Rel 6 HSDPAUplink: 2 -3 x Rel 6 HSUPA

Access schemes OFDMA (Downlink)SC-FDMA (Uplink)

Modulation types supported QPSK, 16QAM, 64QAM (Uplink and downlink)

OFDM offers distinct advantages compared to the CDMA technology

When compared to the CDMA technology upon which UMTS is based, OFDM offers a number of distinct advantages:

• OFDM can easily be scaled up to wide channels that are more resistant to fading.

• OFDM channel equalizers are much simpler to implement than are CDMA equalizers, as the OFDM signal is represented in the frequency domain rather than the time domain.

• OFDM can be made completely resistant to multi-path delay spread. This is possible because the long symbols used for OFDM can be separated by a guard interval known as the cyclic prefix (CP). The CP is a copy of the end of a symbol inserted at the beginning. By sampling the received signal at the optimum time, the receiver can remove the time domain interference between adjacent symbols caused by multi-path delay spread in the radio channel.

• OFDM is better suited to MIMO. The frequency domain representation of the signal enables easy precoding to match the signal to the frequency and phase characteristics of the multi-path radio channel.

OFDM does have some disadvantages

• The subcarriers are closely spaced making OFDM sensitive to frequency errors and phase noise. For the same reason, OFDM is also sensitive to Doppler shift, which causes interference between the subcarriers (ICI).

• Pure OFDM also creates high peak-to-average signals, and that is why a modification of the technology called SC-FDMA is used in the uplink. SC-FDMA is discussed later.

• It is known that OFDM will be more difficult to operate than CDMA at the edge of cells. CDMA uses scrambling codes to provide protection from inter-cell interference at the cell edge whereas OFDM has no such feature. Therefore, some form of frequency planning at the cell edges will be required.

• LTE system information is one of the key aspects of the air interface. It consists of the Master Information Block (MIB) and a number of System Information Blocks (SIBs). The MIB is broadcast on the Physical Broadcast Channel (PBCH), while SIBs are sent on the Physical Downlink Shared Channel (PDSCH) through Radio Resource Control (RRC) messages. SIB1 is carried by "SystemInformationBlockType 1" message. SIB2 and other SIBs are carried by "SystemInformation (SI)" message. An SI message can contain one or several SIBs.

• 1. The MIB is the first thing a UE looks for after it achieves downlink synchronization. The MIB carries the most essential information that is needed for the UE to acquire other information from the cell. It includes:

• The downlink channel bandwidth

• The PHICH configuration. The Physical Hybrid ARQ Indicator Channel carries the HARQ ACKs and NACKs for uplink transmissions

• The SFN (System Frame Number) which helps with synchronization and acts as a timing reference

• The eNB transmit antenna configuration specifying the number of transmit antennas at eNB such as 1, 2, or 4, which is carried by CRC mask for PBCH

• 2. SIB1 is carried in a SystemInformationBlockType1 message. It includes information related to UE cell access and defines the schedules of other SIBs, such as:

• The PLMN Identities of the network

• The tracking area code (TAC) and cell ID

• The cell barring status, to indicate if a UE may camp on the cell or not

• q-RxLevMin, which indicates the minimum required Rx Level in the cell to fulfill the cell selection criteria

• The transmissions times and periodicities of other SIBs

LTE system information (1/3)

LTE system information (2/3)

• 3. SIB2 contains radio resource configuration information common for all UEs, including:

• The uplink carrier frequency and the uplink channel bandwidth (in terms of the number of Resource Blocks, for example n25, n50)

• The Random Access Channel (RACH) configuration, which helps a UE start the random access procedure, such as preamble information, transmit time in terms of frame and subframe number (prach-ConfigInfo), and powerRampingParameters which indicates the initial Tx power and ramping step.

• The paging configuration, such as the paging cycle

• The uplink power control configuration, such as P0-NominalPUSCH/PUCCH

• The Sounding Reference Signal configuration

• The Physical Uplink Control Channel (PUCCH) configuration to support the transmission of ACK/NACK, scheduling requests, and CQI reports

• The Physical Uplink Shared Channel (PUSCH) configuration, such as hopping

LTE system information (3/3)

• 4. SIB3 contains information common for intra-frequency, inter-frequency, and/or inter-RAT cell reselection. This information does not necessarily apply to all scenarios; please refer to 3GPP TS 36.304 for the details. The basic parameters include:

• s-IntraSearch: the threshold for starting intra-frequency measurement. When s-ServingCell(i.e., cell selection criterion for serving cell) is higher than s-IntraSearch, the UE may choose not to perform measurement in order to save battery life.

• s-NonIntraSearch: the threshold for starting inter-frequency and IRAT measurements• q-RxLevMin: the minimum required Rx level in the cell• Cell reselection priority: the absolute frequency priority for E-UTRAN or UTRAN or GERAN or

CDMA2000 HRPD or CDMA2000 1xRTT• q-Hyst: the hysteresis value used for calculating the cell-ranking criteria for the serving cell,

based on RSRP. • t-ReselectionEUTRA: the cell reselection timer value for EUTRA. t-ReselectionEUTRA and q-

Hyst can be configured to trigger cell reselection sooner or later.• 5. SIB4 contains the intra-frequency neighboring cell information for Intra-LTE intra-

frequency cell reselection, such as neighbor cell list, black cell list, and Physical Cell Identities (PCIs) for Closed Subscriber Group (CSG). CSG can be used to support Home eNBs.

• 6. SIB5 contains the neighbor cell related information for Intra-LTE inter-frequency cell-reselection, such as neighbor cell list, carrier frequency, cell reselection priority, threshold used by the UE when reselecting a higher/lower priority frequency than the current serving frequency, etc.

• (Note that 3GPP states that LTE neighbor cell search is feasible without providing an explicit neighbor list. Since the UE can do blind detection of neighbor cells in LTE, the broadcast of LTE neighbor cells is optional.)

‘RSRQ reporting range’ and ‘RSRQ relationship to Es/Iot’

• “R4-081419_RSRQ_reporting_rang” gives an idea on how to calculate RSRQ and how to define RSRQ reporting range.

• “R4-103007_Relay RSRQ Reporting Range” gives a relationship between RSRQ and Ês/Iot .

• After a simple manipulation from this contribution,

• RSRQ(dB) = 10*log10[Ês/(12*(Ês+Iot)]. The unit is in dB.

– (where RSRQ =[(N*RSRP)/RSSI], RSSI=12*N*(Ês+Iot) and RSRP=Ês. The unit is in Watts.)