Maciej Stasiak, Mariusz GłąbowskiArkadiusz Wiśniewski, Piotr

Zwierzykowski

LTE

Modeling and Dimensioning of Mobile Networks: from GSM to

LTE

LTE – Introduction 1/1• The International Telecommunications Union has defined the

requirements for a fourth generation system• One of the systems for which the standardization process is already

much advanced—in line with the requirements set up by ITU-R for 4G systems—is the Third Generation Partnership Project Long-Term Evolution (3GPP LTE)

• The most important requirements identified for the LTE systems can be itemized in the following way:o cost reduction in network data transmission (per bit):

• improvement of spectrum efficiency• cost reduction in backhaul data transmission

o reduction in setup time and round trip delayo improvement in functioning of quality of service (QoS) mechanisms for various serviceso focus on services utilizing the IP protocolo broadening of multimedia multicasting services for selected groups of users (enhanced MBMS)o increase in the transmission rate to over 100 mbps in the downlink and 50 mbps in the uplink directiono flexibility in the use of existing and new spectral resourceso possibility of carrier allocation with different bandwidth, ranging from 1.25 to 20 MHz

System Architecture 1/1• The following elements can be distinguished in LTE architecture:

o eNB—E-UTRAN Node Bo eGW—access gatewayo MME—mobility management entityo UPE—user plane entity

• The tasks to be assigned tome include management and context storage for the control of users’ terminals, authorization procedures for terminals, and mobility management schemes for terminals

• The tasks of the UPE unit encompass management and subscriber terminal context storage. The unit also performs functions such as encoding, securing integrity of data blocks, and automatic repeat request (ARQ) to reduce data block discard

• Individual data blocks located in network nodes (eGW) communicate with one another and with external networks

• The blocks cooperate with base stations (eNB), which carry on all functions related to radio transmission

Transmission Techniques in the LTE System 1/1 • One of the key elements of LTE is the use of orthogonal

frequency division multiplexing (OFDM) as the signal bearer and the associated access schemes, orthogonal frequency division multiple access (OFDMA) and single carrier frequency division multiple access (SC-FDMA)

• The actual implementation of the technology will be different in the downlink and the uplink direction as a result of the different requirements between the two directions and the equipment at either end

• However OFDM was chosen as the signal-bearer format because it is very resilient to interference. It is also a modulation format that is very suitable for carrying high data rates—one of the key requirements for LTE

Long-term Evolution OFDMA in the Downlink Direction 1/6• The principle of the OFDMA is based on the use of narrow,

orthogonal subcarriers • In LTE the sub-carriers spacing is 15KHzregardless of the total

transmission bandwidth• The transmission is done after a fast fourier transform (FFT) block,

which is used to change between the time and frequency domain representation of the signal

• Within the OFDM signal it is possible to choose between three types of modulation:o QPSKo 16QAMo 64QAM

• The duration of the LTE system frame is 10 ms and the system consists of ten sub-frames with two slots each. Within one slot, seven OFDM symbols are transmitted, with a short cyclic prefix, and six OFDM symbols, when a longer cyclic prefix is used

Long-term Evolution OFDMA in the Downlink Direction 2/6

Long-term Evolution OFDMA in the Downlink Direction 3/6• The OFDM signal includes NBW subcarriers. The signal on a single

subcarrier in one OFDM symbol is thus of fundamental importance. • There are therefore 6NBW or 7NBW such elements depending on the length of

the cyclic prefix• The total resources of a single slot are divided into the so-called physical

resource blocks, each being composed of 12 subsequent subcarriers allocated in a given time slot

• The physical resource block is the basic unit for radio resource allocation • Figure on the next slide shows a block diagram of the transmitter and the

receiver of the signal in the downlink direction in the case where single antennas are used

• A data block from the n-th interval of the modulation can result from multiplexing several streams generated by users

• Then it is arranged in the radio resource allocation block and the data is mapped in symbols from the constellation of elementary symbols (QPSK, 16-QAM, 64-QAM), which are then attributed to appropriate subcarriers

• Signal samples in the time domain are performed with the help of M-point IFFT transformation

Long-term Evolution OFDMA in the Downlink Direction 4/6• A cyclic prefix is added at the head of block of samples thus generated.

This prefix is a sub-block of the sequence of samples copied from the rear of the block (trailer)

• The signal constructed in this way, with the inphase and quadrature components (the real and the imaginary part of the complex block of IFFT samples respectively), is then converted into analog form, converted to radio band and enhanced to be emitted through the antenna

• On the receiver side the dual processes take place. So, after signal conversion to base band and conversion to a block of digital samples, the cyclic prefix is removed

• Then, the subcarrier signals are correlated with reference signals using the FFT method

• The sample block from the FFT output, corresponding to the frequency domain, is correlated on the basis of the estimated characteristics of the transmission channel

• The samples obtained in this way from individual subcarriers are then mapped in the constellation points that indicate the positions of the symbols mapped in the constellation

• Eventually, the binary block is determined on the basis of the symbols. The block includes the final decision on the properly received signal block

Long-term Evolution OFDMA in the Downlink Direction 5/6

Long-term Evolution OFDMA in the Downlink Direction 6/6

Long-term Evolution SC-FDMA in the Uplink 1/2• For the LTE uplink, a new concept is used for the access technique• Although still using a form of OFDMA technology, the implementation is called single

carrier frequency division multiple access• A fundamental problem for the OFDM transmission, from the mobile station

perspective, is limited power resources derived from battery energy. Hence, efficiency of radio blocks is of fundamental significance, in particular that of the power enhancer

• The OFDM signal is characterized by a high value of peak power ratio to average power, which implies the need for a high degree of linearity in the power enhancer and a decrease in the average power of emitted signal as compared to signals for which the peak power ratio to the average ratio is insignificant. In the uplink SC-FDMA transmission is therefore used

• The binary data are first mapped into constellation points. The constellation is chosen according to the quality of the modulation channel (QPSK, 16-lub 64-QAM)

• The symbols are then arranged in blocks with the length N. Such a block is treated as a sequence of samples in the time domain and undergoes a frequency transformation according to the DFT algorithm (FFT)

• The obtained frequency samples are then mapped in selected subcarriers of SC-FDMA modulator

• The block of samples thus obtained within the frequency domain is then transformed into the time form by the block that performs the IFFT algorithm. The block of samples in time is preceded with a cyclic prefix and the whole of the block is filtered so that spectral properties of the signal are shaped. Dual operations are performed in the receiver

Long-term Evolution SC-FDMA in the Uplink 2/2

Long-term Evolution MIMO 1/2• Multiple input multiple output (MIMO) is another major LTE technology innovations used to

improve the performance of the system• It uses multiple antennas at both the transmitter and receiver to improve communication

performance• It offers significant increases in data throughput and link range without using additional

bandwidth or transmission power. This is achieved by higher spectral efficiency (more bits per second per hertz of bandwidth) and link reliability or diversity (reduced fading)

• Although MIMO adds complexity to the system in terms of processing and the number of antennas required, it enables high data rates to be achieved along with much improved spectral efficiency

• As a result, MIMO has been included as an integral part of LTE. The basic concept of MIMO uses the multipath signal propagation that is present in all terrestrial communications

• For the downlink direction, a configuration of two transmit antennas at the base station and two receive antennas on the mobile terminal is used as baseline, although configurations with four antennas are also being considered

• Figure on the next slide presents a configuration of reference symbols within two subsequent physical resource blocks included in a subframe for a single antenna transmission (SISO) and for two antenna transmission

• A similar configuration is also expected with the case of the application of four transmitting antennas

Long-term Evolution MIMO 2/2

Channels in the Radio Interface of the LTE System 1/1• Like UMTS, channels in the radio interface of the LTE

system can be divided into three types: o logical channelso transport channelso physical channels

Radio Resource Management in LTE 1/4• Radio resource management (RRM) aims at efficient usage of all available

algorithms and techniques offered by the system and supports appropriate level of quality of service (QoS). Some examples of mechanisms for radio resource management in LTE are presented below

• Admission Control - AC algorithm in eNB decides whether or not a new call in the cell can be serviced. Admission control takes into account available resources in a cell, call priorities, quality requirements for a new call as well as current quality parameters in the cell. A new call can only be serviced when the expected quality of currently serviced calls with the same, or higher, priorities will be retained and the assumed QoS parameters for the new call will be satisfied . Admission control algorithms have not yet been specified by 3GPP. Possible solutions with regard to the issue are to be decided by hardware suppliers

• Frequency Domain Packet Scheduling - FDPS uses frequency selective power variations on either the desired signal (frequency selective fading) or interference (fading or due to fractional other cell load) by only scheduling users on the physical resource blocks with high channel quality. Thanks to this solution, transmission in the radio channel is executed only when it is most efficient. The working principle for the frequency domain packet scheduling is presented on the next slide

Radio Resource Management in LTE 2/4

Radio Resource Management in LTE 3/4• Interference Management and Power Settings - the LTE system

offers a number of mechanisms that underlie control of interference between neighboring cells. Version 8 of the system (Release 8) uses the distribution of frequencies between neighboring cells and the possibility of power control in the radio channel. The X2 interface supports information transmission about inter cell interference between eNB base stations. In LTE two methods of interference level management in the radio interface can be distinguished:o The reactive method, based on monitoring the network quality. If interference is

detected that is too high the system decreases the level of transmitted power in the radio channel or modifies the packet transmission process

o The proactive method, based on an exchange of information between eNBs regarding how they plan to schedule packets to its users in the future. This allows neighboring eNBs involved in packet transmission to take into account the current load of the radio interface effected by neighboring cells. This communication is performed by means of the X2 interface

• Interference management in 3GPP Release 8 aims primarily at a constant improvement of the quality of shared data channels in the uplink and in the downlink direction (PDSCH and PUSCH)

Radio Resource Management in LTE 4/4• Discontinuous Transmission and Reception (DTX/DRX) -radio

resources management mechanism in the LTE system offers discontinuous signal transmission and reception (DTX/DRX). Discontinuous transmission/reception in LTE allows a mobile station (MS) to enter the power-saving mode and to cease monitoring the PDCCH channel in a given sub frame. This operation can be effected based on activity requirements for a given mobile station in the uplink and in the downlink direction. For traffic in which a cycle pattern can be predicted with high probability, discontinuous reception can last 20 or 40 ms. His functionality lowers energy consumption in the mobile station and the level of interference in the radio channel. However, it should not be forgotten that too aggressive parametrization of the function, such as maximization of the periods of discontinuous transmission and reception, can eventually reduce the quality of service offered