LTE Dimensioning


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LTE

In September 2007, 3GPP started the Evolved UTRAN (E-UTRAN) work Group. The work with creating the concept was officially started in the summer of 2006 when the study phase was successfully completed and the 3GPP work item “3G Long Term Evolution – Evolved Packet System RAN” (LTE) commenced. More than 50 companies and research institutes are participating in the largest joint standardization effort ever to specify the new world wide radio access and the evolved core network technology.

The standard development in 3GPP is grouped into two work items, where Long Term Evolution (LTE) targets the radio network evolution and System Architecture Evolution (SAE) targets the evolution of the packet core network. Common to both LTE and SAE is that only a Packet Switched (PS) domain will be specified. The result of these work items are the Evolved UTRAN (EUTRAN) and the Evolved Packet Core (EPC). These together (E-UTRAN+EPC) builds the Evolved Packet System (EPS).

LTE/SAE is specified from 3GPP Release 8 onwards. L12A supports the mandatory features and functionalities specified in 3GPP release 9 such as Dual Band operation and IRAT to WCDMA, coverage based. LTE and SAE refer to the work items in 3GPP. The name of the actual Radio Access Network (RAN) is E-UTRAN and the name of the Core Network (CN) is EPC. A parallel Partnership Project was also established – “3GPP2,” which, quite similar to its sister project 3GPP, also standardizes International Telecommunication Union’s (ITU) International Mobile Telecommunications “IMT-2000” based networks. 3GPP2 focuses on the evolution of cdmaOne with cdma2000 and EV-DO (HRPD) while 3GPP focuses on the evolution of GSM,WCDMA, HSPA and LTE. 3GPP2 is divided into four Technical specification groups comprised of representatives from the Project’s Individual Member companies.

The TSGs are:

  • TSG-A for Access Network Interfaces
  • TSG-C for cdma2000
  • TSG-X Core Networks

The E-UTRAN standard is based on Orthogonal Frequency Division Multiplexing (OFDM) and OFDMA (Orthogonal Frequency Division Multiple Access) downlink operation and Single Carrier Frequency Domain Multiple Access (SC-FDMA) uplink operation. These choices support great spectrum flexibility with a number of possible deployments from 1.4 MHz up to 20 MHz spectrum allocations. It will support both FDD and TDD mode of operation and targets both a paired spectrum allocation with uplink and downlink separated in frequency, and unpaired spectrum with uplink and downlink operating on the same frequency.

E-UTRAN supports use of different MIMO (Multiple Input Multiple Output) multiple antenna configurations. This increases the data rates and spectrum efficiency.

LTE is sometimes referred to as 3.9G. Why not 4G? Well, ITU has defined IMT Advanced, which is the follower to IMT2000. IMT Advanced is regarded as 4G and is meant to support theoretical bitrates up to approximately 1Gbit/s and may be deployed with LTE Release 10 (also referred to as LTE Advanced).

LTE Release 10 will probably fulfill the IMT Advanced requirements. LTE Release 10 will simply be called LTE in Release 10, since it is built on the same solutions as LTE in Release 8, but with some extra features like simultaneous communication with different base stations (COMP) and spectrum aggregation. The first LTE networks based on 3GPP Release 8 was implemented in 2009. EPS in 3GPP Release 8 is based on a simplified network architecture compared to Release 6. The number of user-plane nodes is reduced from four in Release 6 (NodeB, RNC, SGSN and GGSN) to only two (e-NodeB and S-GW) in EPS. Only a Packet Switched (PS) domain is defined in LTE. This means that the traditionally Circuit Switched (CS) services will be carried by PS bearers. 

The E-UTRAN architecture consists of eNBs that provide the air interface user plane and control plane protocol terminations towards the UE. On one side, the user plane protocols consist of Packet Data Control Plane (PDCP), Radio Link Control (RLC), Medium Access Control (MAC) and Physical Layer (PHY) protocols. On the other side, the control plane protocol refers to the Radio Resource Control (RRC) protocol.

Each of the eNBs are logical network components that serve one or several E-UTRAN cells and are interconnected by the X2 interface. Additionally, Home eNBs (also called femtocells), which are eNBs of lower cost, can be connected to the EPC directly or via a gateway that provides additional support for a large number of HeNBs.

The main functionalities for E-UTRAN is the following:

  • Inter-cell Radio Resource Management (RRM)
  • Resource Block control
  • Connection mobility control
  • Radio admission control
  • eNB measurement configuration and provisioning
  • Dynamic resource allocation (scheduling)

Background

eNB functionality

eNB is the RAN node in the EPS architecture that is responsible for radio transmission to and reception from UEs in one or more cells. The eNB is connected to EPC nodes by means of an S1 interface. The eNB is also connected to its neighbor eNBs by means of the X2 interface. Some significant changes have been made to the eNB functional allocation compared to UTRAN. Most Rel-6 RNC functionality has been moved to the E-UTRAN eNB. A description of the functionality provided by eNB.

Cell control and MME pool support: eNB owns and controls the radio resources of its own cells. Cell resources are requested by and granted to MMEs in an ordered fashion. This arrangement supports the MME pooling concept. S-GW pooling is managed by the MMEs and is not really seen in the eNB.

Mobility control: The eNB is responsible for controlling the mobility for terminals in active state. This is done by ordering the UE to perform measurement and then performing handover when necessary.

Control and User Plane security: The ciphering of user plane data over the radio interface is terminated in the eNB. Also the ciphering and integrity protection of RRC signaling is terminated in the eNB.

Shared Channel handling: Since the eNB owns the cell resources, the eNB also handles the shared and random access channels used for signaling and initial access.

Segmentation/Concatenation: Radio Link Control (RLC) Service Data Units (SDUs) received from the Packet Data Convergence Protocol (PDCP) layer consist of whole IP packets and may be larger than the transport block size provided by the physical layer. Thus, the RLC layer must support segmentation and concatenation to adapt the payload to the transport block size.

HARQ: A Medium Access Control (MAC) Hybrid Automatic Repeat reQuest (HARQ) layer with fast feedback provides a means for quickly correcting most errors from the radio channel. To achieve low delay and efficient use of radio resources the HARQ operates with a native error rate which is sufficient only LTE Radio Interface General Principles for services with moderate error rate requirements, such as, for instance, VoIP. Lower error rates are achieved by letting an outer Automatic Repeat reQuest (ARQ) layer in the eNB handle the HARQ errors.

Scheduling: A scheduling with support for QoS provides for efficient scheduling of UP and CP data.

Multiplexing and Mapping: The eNB performs mapping of logical channels on to transport channels.

Physical layer functionality: The eNB handles the physical layer such as scrambling, Tx diversity, beamforming processing and OFDM modulation. The eNB also handles layer one functions like link adaptation and power control.

Measurements and reporting: eNB provides functions for configuring and making measurements on the radio environment and eNB-internal variables and conditions. The collected data is used internally for Radio Resource Management (RRM) but can be reported for the purpose of multi-cell RRM.

Automated operation and maintenance: eNB provides functions for Automated Neighbor Relations (ANR) and Automatic Integration of RBS.

Air Interface

The LTE E-UTRA work item is essential so that an optimized packet-based access system can achieve the expected system performance in terms of high data rates and low latency. EUTRA is also expected to support mobility up to 350 km/h, conserve mobile station’s power consumption through micro-sleep, and provide seamless integration of unicast and enhanced
broadcast transmission. Key techniques for the LTE air interface are summarized as follows:

Orthogonal Frequency Division Multiplexing Access: (OFDMA) for the Downlink OFDMA allows data to be transmitted in parallel in a set of narrowband, orthogonal, and tightly close sub-carriers, providing an efficient use of the available bandwidth. The use of cyclic prefix in OFDMA makes it robust to time-dispersion (multipath) without the need of complex equalizers in the receiver end, which reduces complexity, cost and power consumption.

Single-Carrier Frequency Division Multiple Access: for Uplink One of the disadvantages of OFDMA is that it produces large output variations, which require highly linear power amplifiers that are inherently low power efficient. Since power consumption is extremely important for the UE, plain OFDMA is not used for the uplink but a DFT-precoded OFDM, also known as Single-Carrier OFDMA (SCFDMA). SC-FDMA comes as a power efficient alternative of OFDMA that keeps most of the advantages of OFDMA.

Multiple-Input Multiple-Output (MIMO) transmission: MIMO techniques enhance system performance, service capabilities, or both. At its highest level, LTE multi-antenna transmission can be divided into transmit diversity and spatial multiplexing. The former can can be seen as a technique for averaging the signals received from the two antennas, thereby avoiding the deep fading dips that occur per antenna. The latter employs multiple antennas at the transmitter and receiver side to provide simultaneous transmission of multiple parallel data streams over a single radio link, therefore increasing significantly the peak data rates over the radio link. Additionally, LTE supports SDMA (Spatial Division Multiple Access) and beamforming.

Channel-dependent scheduling: A common property of the radio channel is its variation in both frequency and time. Instead of trying to fight against and overcome these channel variations, LTE advocates to utilize these channel variations as input to the scheduler. In other words, by taking into account the channel conditions at each time and frequency block (called radio resource block or RRB), the scheduler can select the users that experience the best channel condition in each RRB, achieving the maximum possible performance. In order to perform this scheduling, the scheduler requires feedback from the UEs regarding the channel state that they are experiencing.

Retransmission scheme: In order to handle transmission/reception errors, LTE uses a combination of selective-repeat ARQ and hybrid-ARQ. In this way it can rapidly recover from errors through the hybrid-ARQ maintaining a low feedback overhead, while at the same time having a robust fallback recovery method (ARQ) when hybrid-ARQ is not enough to recover from the error. In this way, a combination of low overhead/latency from hybrid-ARQ (which will manage most of the errors) and high reliability from ARQ is obtained.

Spectrum flexibility: LTE provides a single radio interface supporting both FDD and TDD. Most of the processing for TDD and FDD is the same, except for the frame structure. This allows easier and lower cost implementation of devices that support both TDD and FDD. In addition, to provide great operational flexibility, E-UTRA physical layer specifications are bandwidth agnostic and designed to accommodate up to 20 MHz system bandwidth. The following table shows the downlink parameters for the different bandwidth allocations.

Inter-cell interference coordination: Since UEs utilize OFDMA and SC-FDMA for downlink and uplink, respectively, their transmission are orthogonal and should not interfere with each other within a cell
(intra-cell interference). However, since LTE advocates for full frequency reuse, a UE could receive interference from other UEs that have been assigned the same RB in a
different cell. This problem will affect the most to the UEs that are located at the celledge since they will be farther away from the base station of the cell that they belong
and nearer to the UEs and base station in a neighbor cell. To reduce this interference, LTE allows coordination between different base stations so that they can identify which UEs are located near the cell-edge and dynamically assign preferably complementary parts of the spectrum to reduce the inter-cell interference. Inter-cell interference coordination techniques are applied both for uplink and downlink.

The following terms are used in describing dimensioning:

Average user bit rate: Bit rate achievable by a single user, as an average over the cell area.When all resources in a cell are used, the average user bit rate can be the average throughput in one cell. It is a measure of average potential in a cell while all interfering cells are loaded to the dimensioned level.

Air path loss: Part of the signal attenuation that is sustained in the air. This is the quantity that is converted to geographical distance.

Cell edge: Geographical location where the air path loss between the Radio Base Station (RBS) and the UE fulfills the quality requirement imposed on the network. Cell edge includes both the median signal attenuation at the cell border and the log-normal fading margin. For example guaranteeing a specified bit rate at a certain probability.

Cell throughput: Throughput obtained in one cell when all cells are loaded to the dimensioning level, and the resource use is equal to the traffic load, in interfering cells as well as in interfered cells. It is the average throughput per cell as calculated across the entire network.

Coverage (area): Percentage of cell area that can be served in accordance with a defined quality requirement. With a uniform subscriber density (as often assumed in a dimensioning exercise), the percentage of served area equals the percentage of served users.

Interference due to control channels: Fraction of PDSCH resources elements interfered by control channels.

Radio unit: Receiver and converter of digital data to analog signals and radio signals to digital signals.

Resource block: Two-dimensional unit in the time-frequency plane, consisting of a group of 12 carriers, each with 15 kHz bandwidth, and one slot of 0.5 ms

Signal attenuation: Attenuation of the radio signal between the TX reference point and the RX reference point.

SINR: Quotient between the average received modulated carrier power and the average received co-channel interference power including the thermal noise power

Resource Block Flexible Bandwidth

A transmitted OFDMA signal can be carried by a number of parallel subcarriers. Each LTE subcarrier is 15 kHz. Twelve subcarriers (180 kHz) are grouped into a resource block. The downlink has an unused central subcarrier. Depending on the total deployed bandwidth, LTE supports a varying number of resource blocks.

A resource block is limited in both the frequency and time domains. One resource block is 12 subcarriers during one slot (0.5 ms). In the downlink, the time-frequency plane of OFDMA structure is used to its full potential. The scheduler can allocate resource blocks anywhere, even non-contiguously. A variant of OFDMA is used in the uplink. This variant requires the scheduled bandwidth to be contiguous, forming in effect a single carrier. The method, called SC-FDMA, can be considered a separate multiple access method. A user is scheduled every Transmission Time Interval (TTI) of 1 ms, indicating a minimum of two consecutive resource blocks in time at every scheduling instance. The minimum scheduling in the frequency dimension is 12 subcarriers, that is the width of one resource block in the frequency dimension. The scheduler is free to schedule users both in the frequency and time domain. The illustration below shows an example of two users scheduled in the time and frequency domain for the downlink and the uplink:

The defined LTE bandwidths in 3GPP are the following:

User Equipment, states and Area Concepts

LTE is developed to have a simpler architecture (fewer nodes) and less signaling (fewer messages) than UTRAN. Also, the number of states which the UE can be in (corresponding to RRC states) are reduced from 5 in UTRAN (DETACHED, IDLE, URA_PCH, CELL_FACH, CELL_DCH) to only 3 in E-UTRAN (DETACHED, IDLE and CONNECTED). Furthermore, the area concept is somewhat simplified in LTE compared to UTRAN. In LTE only one area for idle mode mobility is defined: the Tracking Area (TA). In UTRAN, Routing Area (RA) and UTRAN Registration Area (URA) is defined for PS traffic and Location Area (LA) for CS traffic. In ECM-IDLE (EPS Connection Management IDLE) the UE position is only known by the network on TA level in case the UE is EMM-Registered. In ECM-CONNECTED the UE location is known on cell level by the eNB.


When a UE attaches to the network it is assigned an IP address from a P-GW. The IP-address is kept regardless of whether the UE enters idle mode or not, as long as it is attached to the network, but is released if the UE detaches from the network.

 

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