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  • LTE System Overview

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  • LTE System Overview

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    l 3GPP set up LTE study item for feasibility in December, 2004, and set up LTE work item for standards in September, 2006,

    l The first version of LTE commercial protocols were released in 3GPP R8 at the end of 2009, in which 36.XXX are the main protocols.

    l Requirements of LTE

    p Peak data rate:100 Mbps DL/ 50 Mbps UL within 20 MHz bandwidth

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

    p Less than 5 ms user-plane latency

    l Mobility

    p Optimized for 0 ~ 15 km/h

    p Supported for high performance for 15 ~ 120 km/h

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

    l Coverage

    p Performance should be met for 5 km cells with slight degradation for 30 km cells. Up to 100 km cells not precluded

    l Enhanced multimedia broadcast multicast service (E-MBMS)

    l Spectrum flexibility:1.25 ~ 20 MHz

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    l Market opportunities:

    p Laptops, PDAs and Fixed Broadband Access

    p Premium VOD/MOD Services

    p Multimedia Upload and Exchange Services

    p Consumer Electronics

    p Business Applications for Vertical Markets

    l Increased data throughput & spectral efficiency

    p DL target: average user throughput per MHz to be 3-4 times greater than HSDPA Rel.6 (instantaneous downlink peak data-rate 100Mb/s within 20 MHz downlink allocation 5bps/Hz)

    p UL target: average user throughput per MHz to be 2-3 times greater than HSUPA Rel.6 (instantaneous uplink peak data-rate 50Mb/s within 20 MHz downlink allocation 2.5bps/Hz)

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    l Simplified/Flat architecture:

    p Most of the RNC functionalities moved to the eNodeB

    p UMTS RNC removed

    p eNodeB connected directly to the Evolved Packet Core (EPC)

    p The E-UTRAN only transfers PS service in which the voice is transferred by VOIP.

    p S1 interface connects eNB and EPC (Evolved Packet Core) and functions like Iu-PS.

    p X2 interface conntets eNB and other eNB and functions like Iur.

    l eNB

    p All radio-related functions.

    l MME

    p Manage/store UE control plane context

    p UE authentication.

    p Mobility management.

    l SGW

    p Manage/store UE context

    p Packet routing/forwarding

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    l The smallest amount of resource that can be allocated in the uplink or downlink is called a resource block (RB). An RB is 180 kHz wide and lasts for one 0.5 ms timeslot. For standard LTE, an RB comprises 12 subcarriers at a 15 kHz spacing,

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    l OFDMA is a variant of orthogonal frequency division multiplexing (OFDM). OFDM makes use of a large number of closely spaced orthogonal subcarriers that are transmitted in parallel. Each subcarrier is modulated with a conventional modulation scheme (such as QPSK, 16QAM, or 64QAM) at a low symbol rate. The combination of hundreds or thousands of subcarriers enables data rates similar to conventional single-carrier modulation schemes in the same bandwidth.

    l The diagram in the slide illustrates the key features of an OFDM signal in frequency and time. In the frequency domain, multiple adjacent tones or subcarriers are each independently modulated with data. Then in the time domain, guard intervals are inserted between each of the symbols to prevent inter-symbol interference at the receiver caused by multi-path delay spread in the radio channel.

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  • LTE System Overview

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    l With standard OFDM, very narrow UE-specific transmissions can suffer from narrowband fading and interference. That is why for the downlink 3GPP chose OFDMA, which incorporates elements of time division multiple access (TDMA). OFDMA allows subsets of the subcarriers to be allocated dynamically among the different users on the channel, as shown in Figure 7. The result is a more robust system with increased capacity. This is due to the trunking efficiency of multiplexing low rate users and the ability to schedule users by frequency, which provides resistance to multi-path fading.

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  • LTE System Overview

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    l The key technologies of IFFT and FFT are separately used in the transmitter and the receiver, IFFT modulates the parallel data to different sub-carriers, and implements the transmission from frequency domain to time domain, and FFT has the inverse procedure

    l Adding Cyclic Prefix is to avoid the inter-carrier interference, and insure the orthogonality of the different sub-carriers

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    l With standard OFDM, very narrow UE-specific transmissions can suffer from narrowband fading and interference. That is why for the downlink 3GPP chose OFDMA, which incorporates elements of time division multiple access (TDMA). OFDMA allows subsets of the subcarriers to be allocated dynamically among the different users on the channel, as shown in Figure 7. The result is a more robust system with increased capacity. This is due to the trunking efficiency of multiplexing low rate users and the ability to schedule users by frequency, which provides resistance to multi-path fading.

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  • LTE System Overview

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    l A graphical comparison of OFDMA and SC-FDMA as shown in the slide is helpful in understanding the differences between these two modulation schemes. For clarity this example uses only four (M) subcarriers over two symbol periods with the payload data represented by quadrature phase shift keying (QPSK) modulation. As described earlier, real LTE signals are allocated in units of 12 adjacent subcarriers.

    l Visually, the OFDMA signal is clearly multi-carrier with one data symbol per subcarrier, but the SC-FDMA signal appears to be more like a single-carrier (hence the SC in the SC-FDMA name) with each data symbol being represented by one wide signal. Note that OFDMA and SC-FDMA symbol lengths are the same at 66.7 s; however, the SC-FDMA symbol contains M sub-symbols that represent the modulating data. It is the parallel transmission of multiple symbols that creates the undesirable high PAR of OFDMA. By transmitting the M data symbols in series at M times the rate, the SC-FDMA occupied bandwidth is the same as multi-carrier OFDMA. But, crucially, the PAR is the same as that used for the original data symbols. Adding together many narrow-band QPSK waveforms in OFDMA will always create higher peaks than would be seen in the wider-bandwidth, single-carrier QPSK waveform of SC-FDMA. As the number of subcarriers M increases, the PAR of OFDMA with random modulating data approaches Gaussian noise statistics but, regardless of the value of M, the SC-FDMA PAR remains the same as that used for the original data symbols.

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  • LTE System Overview

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    l In wireless communications, MIMO refers to a wireless channel with multiple inputs and multiple outputs.

    l In a MIMO system, there are N*M signal paths from the transmit antennas and the receive antennas, and the signals on these paths are not identical.

    l MIMO uses space multiplexing to increase the data rate.

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    l Here 2*2 MIMO is used as a example.

    l With MIMO system, the multiplexing gain is obtained with independent data streams on different antennas. For example 2*2 MIMO can double the peak data rate if compared with non-MIMO system.

    l Whether MIMO is applicable is related to channel condition. Only when the channel conditions are good, two parallel data streams can be carried in different transmitters. This is dual-stream case. Otherwise only one data stream is carried even though two transmitters are used. This is single-stream case. Receiver will feedback channel information to transmitter. Transmitter will decide whether space multiplexing can be used.

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    l The baseline configuration of the UE has one transmitter. This configuration was chosen to save cost and battery power, and with this configuration the system can support MU-MIMOthat is, two different UE transmitting in the same frequency and time to the eNB. This configuration has the potential to double uplink capacity (in ideal conditions) without incurring extra cost to the UE.

    l An optional configuration of the UE is a second transmit antenna, which allows the possibility of uplink Tx diversity and SU-MIMO (single-user MIMO). The latter offers the possibility of increased data rates depending on the channel conditions.

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  • LTE System Overview

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    l Transmit power control can be seen as one type of link adaptation and used in WCDMA and CDMA system, that is the adjustment of transmission parameters, in this case the transmit power, to adapt to differences and variations in the instantaneous channel conditions to maintain the received Eb/N0 at a desired level. This results in a basically constant data rate, regardless of the channel variations.

    l Actually, even in case of typical constant-rate services such as voice and video, (short-term) variations in the data rate are often not an issue, as long as the average data rate remains constant, assuming averaging over some relatively short time interval. In such cases, that is when a constant data rate is not required, an alternative to transmit power control is link adaptation by means of dynamic rate control.

    l Instead, with rate control in LTE system, the data rate is dynamically adjusted to compensate for the varying channel conditions. In situations with advantageous channel conditions, the data rate is increased and vice versa.

    l Rate control in principle implies that the power amplifier is always transmitting at full power and therefore efficiently utilized.

    l In case of advantageous radio-link conditions, the Eb/N0 at the receiver is high and the main limitation of the data rate is the bandwidth of the radio link. Hence, in such situations higher-order modulation, for example 16QAM or 64QAM, together with a high code rate is appropriate. Similarly, in case of poor radio-link conditions, QPSK and low-rate coding is used.

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    l The eNB hosts the following functions:

    p Functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling);

    p IP header compression and encryption of user data stream;

    p Selection of an MME at UE attachment when no routing to an MME can be determined from the information provided by the UE;

    p Routing of User Plane data towards Serving Gateway;

    p Scheduling and transmission of paging messages (originated from the MME);

    p Scheduling and transmission of broadcast information (originated from the MME or O&M);

    p Measurement and measurement reporting configuration for mobility and scheduling;

    l The MME hosts the following functions (see 3GPP TS 23.401 [17]):

    p NAS signalling;

    p NAS signalling security;

    p AS Security control;

    p Inter CN node signalling for mobility between 3GPP access networks;

    p Idle mode UE Reachability (including control and execution of paging retransmission);

    p Tracking Area list management (for UE in idle and active mode);

    p PDN GW and Serving GW selection;

    p MME selection for handovers with MME change;

    p SGSN selection for handovers to 2G or 3G 3GPP access networks;

    p Roaming;

    p Authentication;

    p Bearer management functions including dedicated bearer establishment;

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    l The Serving Gateway (S-GW) hosts the following functions:

    p The local Mobility Anchor point for inter-eNB handover;

    p Mobility anchoring for inter-3GPP mobility;

    p E-UTRAN idle mode downlink packet buffering and initiation of network triggered service request procedure;

    p Lawful Interception;

    p Packet routeing and forwarding;

    p Transport level packet marking in the uplink and the downlink;

    p Accounting on user and QCI granularity for inter-operator charging;

    p UL and DL charging per UE, PDN, and QCI.

    l The PDN Gateway (P-GW) hosts the following functions:

    p Per-user based packet filtering (by e.g. deep packet inspection);

    p Lawful Interception;

    p UE IP address allocation;

    p Transport level packet marking in the downlink;

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  • LTE System Overview

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    l The radio interface described in this specification covers the interface between the User Equipment (UE) and the network. The radio interface is composed of the Layer 1, 2 and 3.

    l The physical layer offers a transport channel to MAC. The transport channel is characterized by how the information is transferred over the radio interface. MAC offers different logical channels to the Radio Link Control (RLC) sub-layer of Layer 2. A logical channel is characterized by the type of information transferred.

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  • LTE System Overview

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    l In the user-plane, where PDCP, RLC and MAC sublayers (terminated in eNB on the network side) perform the functions for the user plane such as header compression, ciphering, scheduling, ARQ and HARQ;

    l In the control plane, where:p PDCP sublayer (terminated in eNB on the network side) performs the functions

    listed for the control plane in subclause 6, e.g. ciphering and integrity protection;

    p RLC and MAC sublayers (terminated in eNB on the network side) perform the same functions as for the user plane;

    p RRC (terminated in eNB on the network side) performs the functions:p Broadcast;

    n Paging;

    n RRC connection management;

    n RB control;

    n Mobility functions;p UE measurement reporting and control.p NAS control protocol (terminated in MME on the network side) performs

    among other things:

    n EPS bearer management;

    n Authentication;

    n ECM-IDLE mobility handling;

    n Paging origination in ECM-IDLE;

    n Security control.

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    l UL physical signals

    p Reference signal (RS)

    l Available modulation for data channel

    p QPSK, 16-QAM, and 64-QAM

    l Single user MIMO not supported in current release

    p But it will be addressed in the future release

    l Multi-user collaborative MIMO supported

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  • LTE System Overview

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    l A transport channel is defined by how and with what characteristics the information is transmitted over the radio interface.

    l Following the notation from HSPA, which has been inherited for LTE, data on a transport channel is organized into transport blocks. In each Transmission Time Interval (TTI), at most one transport block of a certain size is transmitted over the radio interface in absence of spatial multiplexing. In case of spatial multiplexing (MIMO), there can be up to two transport blocks per TTI.

    l Associated with each transport block is a Transport Format (TF), specifying how the transport block is to be transmitted over the radio interface. The transport format includes information about the transport-block size, the modulation scheme, and the antenna mapping. Together with the resource assignment, the resulting code rate can be derived from the transport format. By varying the transport format, the MAC layer can thus realize different data rates. Rate control is therefore also known as transport-format selection.

    l Broadcast Channel (BCH) has a fixed transport format, provided by the specifications. It is used for transmission of the information on the BCCH logical channel.

    l Paging Channel (PCH) is used for transmission of paging information on the PCCH logical channel. The PCH supports discontinuous reception (DRX) to allow the mobile terminal to save battery power by sleeping and waking up to receive the PCH only at predefined time instants..

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  • LTE System Overview

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    l Downlink Shared Channel (DL-SCH) is the transport channel used for transmission of downlink data in LTE. It supports LTE features such as dynamic rate adaptation and channel-dependent scheduling in the time and frequency domains, hybrid ARQ, and spatial multiplexing. It also supports DRX to reduce mobile-terminal power consumption while still providing an always on experience, similar to the CPC mechanism in HSPA. The DL-SCH TTI is 1 ms.

    l Multicast Channel (MCH) is used to support MBMS. It is characterized by a semi-static transport format and semi-static scheduling. In case of multi-cell transmission using MBSFN, the scheduling and transport format configuration is coordinated among the cells involved in the MBSFN transmission.

    l Uplink Shared Channel (UL-SCH) is the uplink counterpart to the DL-SCH.

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  • LTE System Overview

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    l A logical channel is defined by the type of information it carries and are generally classified into control channels, used for transmission of control and configuration information necessary for operating an LTE system, and traffic channels, used for the user data. The set of logical-channel types specified for LTE includes:

    l Broadcast Control Channel (BCCH), used for transmission of system control information from the network to all mobile terminals in a cell. Prior to accessing the system, a mobile terminal needs to read the information transmitted on the BCCH to find out how the system is configured, for example the bandwidth of the system.

    l Paging Control Channel (PCCH), used for paging of mobile terminals whose location on cell level is not known to the network and the paging message therefore needs to be transmitted in multiple cells.

    l Dedicated Control Channel (DCCH), used for transmission of control information to/from a mobile terminal. This channel is used for individual configuration of mobile terminals such as different handover messages.

    l Multicast Control Channel (MCCH), used for transmission of control information required for reception of the MTCH, see below.

    l Dedicated Traffic Channel (DTCH), used for transmission of user data to/from a mobile terminal. This is the logical channel type used for transmission of all uplink and non-MBMS downlink user data.

    l Multicast Traffic Channel (MTCH), used for downlink transmission of MBMS services.

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  • LTE(TDD) System Overview

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    l In LTE, devices are allocated blocks of subcarriers for a period of time. These are referred to as a PRB (Physical Resource Block). The resource blocks are contained within the LTE generic frame structure of which two types are defined; Type 1 and Type 2 radio frames.

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    l FDD

    p 10 subframes are available for DL in each 10 ms interval

    p 10 subframes are available for UL in each 10 ms interval

    l TDD

    p a subframe is either allocated to DL or UL transmission

    p Subframe 0 and subframe 5 are always allocated for DL transmission.

    l In case of FDD, that is operation in paired spectrum, all subframes of a carrier are either used for downlink transmission (a downlink carrier) or uplink transmission (an uplink carrier).

    l On the other hand, in case of operation with TDD in unpaired spectrum the first and sixth subframe of each frame (subframe 0 and 5) are always assigned for downlink transmission while the remaining subframes can be flexibly assigned to be used for either downlink or uplink transmission. The reason for the predefined assignment of the first and sixth subframe for downlink transmission is that these subframes include the LTE synchronization signals. The synchronization signals are transmitted on the downlink of each cell and are intended to be used for initial cell search as well as for neighbor-cell search.

    l As the subframe assignment needs to be the same for neighbor cells in order to avoid severe interference between downlink and uplink transmissions between the cells, the downlink/uplink asymmetry cannot vary dynamically.

    l To provide consistent and exact timing definitions, different time intervals within the LTE radio access specification can be expressed as multiples of a basic time unit Ts =1/30720000.1 The time intervals outlined in Figure 16.1 can thus also be expressed as Tframe =307200 Ts and Tsubframe =30720 Ts.

    l Each 1 ms subframe consists of two equally sized slots of length Tslot =0.5 ms (15360 Ts).

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    l A separate uplink and downlink channel are utilized enabling a device to transmit and receive data at the same time (assuming the device incorporates a duplexer). The spacing between the uplink and downlink channel is referred to as the duplex spacing.

    l Normally the uplink channel (mobile transmit) operates on the lower frequency. This is done because higher frequencies suffer greater attenuation than lower frequencies and therefore it enables the mobile to utilize lower transmit power levels.

    l Some systems also offer half-duplex FDD mode, where two frequencies are utilized, however the mobile can only transmit or receive, i.e. not transmit and receive at the same time. This allows for reduced mobile complexity since no duplex filter is required.

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    l TDD mode enables full duplex operation using a single frequency band with time division multiplexing for the uplink and downlink signals. One advantage of TDD is its ability to provide asymmetric uplink and downlink allocations. Depending on the system, other advantages include dynamic allocation, increased spectral efficiency, and improved use of beamforming techniques. The later being due to the carrier having the same uplink and downlink frequency characteristics.

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    l The LTE-FDD frame format defined in 3GPP is Type 1. The Type 1 radio frame structure is used for FDD and is 10ms in duration. It consists of 20 slots, each lasting 0.5ms. Two adjacent slots form one subframe. For FDD operation, 10 subframes are available for downlink transmission and 10 subframes are available for uplink transmission, with each transmission separated in the frequency domain.

    l CP (Cyclic Prefix) within OFDM systems has already been discussed. In LTE, two different cyclic prefix sizes, namely Normal and Extended were selected. In order to facilitate these, two different slot formats are required. Figure 2-28 illustrates the seven and six ODFM symbol options. Obviously, to facilitate a larger cyclic prefix, one of the symbols is sacrificed, thus the symbol rate is reduced.

    l The use of the extended cyclic prefix is intended for scenarios where the range of the cell needs to be extended, e.g. for coverage planning purposes or when multicast services are being employed in the cell.

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    l The LTE-TDD frame format defined in 3GPP is Type 2. The Type 2 radio frame structure is used for TDD. One key addition to the TDD frame structure is the concept of special subframes. This includes a DwPTS (Downlink Pilot Time Slot), GP (Guard Period) and UpPTS (Uplink Pilot Time Slot). These have configurable individual lengths and a combined total length of 1ms.

    l For TDD operation, the ten subframes are shared between the uplink and the downlink.

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    l The DwPTS can be considered as a special downlink subframe, which contains 12 symbols at most and 3 symbols at least. The DwPTS is used to transmit downlink data and signaling messages.

    l No control signaling message or data is transmitted in the UpPTS.

    l The length of the UpPTS is two symbols or one symbol.

    p When the UpPTS contains two symbols, it is used for short RACH or sounding RS.

    p When the UpPTS contains one symbol, it is used only for sounding.

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    l There are various frame configuration options available for TDD operation. Table illustrates these different options. Configuration options 0, 1, 2 and 6 have a 5ms switching point and therefore require two special subframes. The remainder are based on a 10ms switching point. In the table, the letter D is reserved for downlink transmissions, U uplink transmissions and S denotes a special subframe with the three fields DwPTS, GP and UpPTS.

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    l Note instruction

    p Font: FrutigerNext LT Regular

    p Font Size: 11

    p Item symbol Size: 70%

    p Row Space: 1.25

    p Segment: 3 pound

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    l Operating bands defined for FDD in 3GPP

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    l Operating bands defined for TDD in 3GPP

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    l ICIC:Inter-cell Interference Coordination

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  • LTE System Overview

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    l In the frequency domain the downlink subcarriers are grouped into resource blocks, where each resource block consists of 12 consecutive subcarriers3 corresponding to a nominal resource-block bandwidth of 180 kHz. In addition, there is an unused DC-subcarrier in the center of the downlink spectrum. The reason why the DC-subcarrier is not used for any transmission is that it may coincide with the local-oscillator frequency at the base-station transmitter and/or mobile-terminal receiver. As a consequence, it may be subject to un-proportionally high interference, for example, due to local-oscillator leakage.

    l Downlink scheduling is carried out on a subframe (1 ms) basis. Thus, as a downlink resource block is defined as a number of subcarriers during one 0.5 ms slot, the downlink resource-block assignment is always carried out in terms of pairs of resource blocks, where each pair consists of two, in the time domain, consecutive resource blocks within a subframe.

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  • LTE System Overview

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    l For 10 MHz system, the data will be doubled.

    l For the LTE downlink, the OFDM subcarrier spacing has been chosen to f =15 kHz. Assuming an FFT-based transmitter/receiver implementation, this corresponds to a sampling rate fs =15000 NFFT, where NFFT is the FFT size. The time unit Ts defined in the previous section can thus be seen as the sampling time of an FFT-based transmitter/receiver implementation with NFFT =2048.

    l In practice, an FFT-based transmitter/receiver implementation with NFFT =2048 and a corresponding sampling rate fs =30.72MHz is suitable for the wider LTE transmission bandwidths, such as bandwidths in the order of 15MHz and above. However, for smaller transmission bandwidths, a smaller FFT size and a correspondingly lower sampling rate can very well be used. As an example, for transmission bandwidths in the order of 5 MHz, an FFT size NFFT =512 and a corresponding sampling rate fs =7.68MHz may be sufficient.

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  • LTE System Overview

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  • LTE System Overview

    Confidential Information of Huawei. No Spreading Without Permission

    l The E2E LTE solution, showing the eUTRAN position within network/architecture.

    p Terminal

    p E-UTRAN

    p EPC

    p IP Transmission

    p Application Server

    l LTE is end to end native IP.

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  • LTE System Overview

    Confidential Information of Huawei. No Spreading Without Permission

    l IPv4 IPv6 HW ready

    l Synchronization: GPS, 2MHz,1588v2, SyncE, etc

    l Ethernet performance monitoring for FE/GE

    l QoS: DSCP marking, Traffic shaping, congestion control, flow control, etc

    l Transport security: IPSec

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  • LTE System Overview

    Confidential Information of Huawei. No Spreading Without Permission

    l uniNodeB series.

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  • LTE System Overview

    Confidential Information of Huawei. No Spreading Without Permission

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  • LTE System Overview

    Confidential Information of Huawei. No Spreading Without Permission

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  • LTE System Overview

    Confidential Information of Huawei. No Spreading Without Permission

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  • LTE System Overview

    Confidential Information of Huawei. No Spreading Without Permission

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  • LTE System Overview

    Confidential Information of Huawei. No Spreading Without Permission

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