4G LTE Networks Modulation Technique,Cell Planning,physical layer & Propagation Modeling
RF Planning and Optimizationfor LTE Networks
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4g Network Architecture |
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Detailed 4G Architecture
Introduction 4G Network.
Long-term evolution (LTE) is the next generation in cellular technology to follow
the current universal mobile telecommunication system/high-speed packet access
(UMTS/HSPA)∗. The LTE standard targets higher peak data rates, higher spectral
efficiency, lower latency, flexible channel bandwidths, and system cost compared
to its predecessor. LTE is considered to be the fourth generation (4G) in mobile
communications [1, 2]. It is referred to as mobile multimedia, anywhere anytime,
with global mobility support, integrated wireless solution, and customized personal
service (MAGIC) [1]. LTE will be internet protocol (IP) based, providing higher
throughput, broader bandwidth, and better handoff while ensuring seamless services
across covered areas with multimedia support.
Enabling technologies for LTE are adaptive modulation and coding (AMC),
multiple-input multiple-output systems (MIMO), and adaptive antenna arrays. LTE
spectral efficiency will have a theoretical peak of 300 Mbps/20 MHz = 15 bits/Hz
(with the use of MIMO capability), which is six times higher than 3G-based networks
- MIMO BLOG
that have 3.1 Mbps/1.25 MHz = 2.5 bits/Hz [i.e., evolution data only, (EV-DO)].
LTE will have a new air interface for its radio access network (RAN), which is based
on orthogonal frequency division multiple access (OFDMA) [3].
This post will focuses on the radio frequency (RF) planning and optimization of
4G LTE cellular networks, or the so-called evolved universal terrestrial radio access
networks (E-UTRAN) and discusses the physical layer modes of operation for the
user equipment (UE) as well as base stations (BS) or the so called evolved node B
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Comparison 3G and 4G networks. |
(TDD) modes of operation and their frequency bands are also discussed and
illustrated according to the 3GPP specification
RF aspects of cell planning such as cell types, diversity, antenna arrays and
MIMOsystem operation to be used within this architecture will be discussed. Various
wireless propagation models used to predict the signal propagation, strength, coverage
and link budget are to be explained. The main performance and post deployment
parameters are then discussed to assess the RF network performance and coverage.
Model tuning according to field measurements is discussed to optimize the network
performance. These will follow the standard recommendation for mobile and
stationary users. All these aspects are essential for the RF planning process.
LINK:- Complete 3G RF-Optimization
LINK:- 3G WCDMA CHANNELS DETAILED TUTORIAL
LINK:- 3G Network Architecture and Techniques TUTORIAL
LTE 4G Architecture and the Physical Layer
LTE Network Architecture
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General comparison between 3G and 4G Network |
The LTE network architecture is illustrated in Figure. The data are exchanged
between the UE and the base station (eNB) through the air interface. The eNB is
part of the E-UTRAN where all the functions and network services are conducted.
Whether it is voice packets or data packets, the eNB will process the data and route
it accordingly. The main components of such a network are [4]:
Evolved 4G Cellular Network Planning and Optimization
User Equipment (UE):
This is the user device that is connected to the,LTE network via the RF channel through the BS that is part of the eNB subsystem.
Evolved NodeB (eNB):
The eNB functionalities include radio resource management (RRM) for both uplink (UL) and downlink (DL), IP header compression and encryption of user data, routing of user data, selection of MME, paging, measurements, scheduling, and broadcasting.
Mobility Management Entity (MME):
This portion of the network is responsible for nonaccess stratum (NAS) signaling and security, tracking UE, handover selection with other MMEs, authentication, bearer management, core network (CN) node signaling, and packet data network (PDN) service and selection. The MME is connected to the S-GW via an S11 interface [5].
Serving Gateway (S-GW):
This gateway handles eNB handovers, packet data routing, quality of service (QoS), user UL/DL billing, lawful interception, and transport level packet marking. The S-GW is connected to the PDN gateway via an S5 interface.
PDN Gateway (P-GW):
This gateway is connected to the outside global network (Internet). This stage is responsible for IP address allocation, per-user packet filtering, and service level charging, gating, and rate enforcement.
Evolved Packet Core (EPC):
It includes the MME, the S-GW as well as the P-GW.
Evolved Cellular Network Planning and Optimization
Logical, functional, and radio protocol layers are graphically illustrated in
Figure. The logical nodes encompass the functional capabilities as well as radio
protocols and interfaces. Interfaces S1–S11 as well as X2 are used to interconnect
the various parts of the LTE network and are responsible for reliable packet routing
and seamless integration. Details of such interfaces are discussed in the 3GPP
specification and is discussed in this chapter. Radio protocol layers are the shaded
ones in Figure. After a specific eNB is selected, a handover can take place based
on measurements conducted at the UE and the eNB. The handover can take place
between eNBs without changing the MME/SGW connection. After the handover
is complete, the MME is notified about the new eNB connection. This is called
an intra-MME/SGW handover. The exact procedures for this operation as well as
inter-MME/SGW handover are discussed in detail in [4]. Handovers are conducted
within layer-2 functionality (i.e., radio resource control (RRC)).
When comparing the new LTE standard release 8 to the currently deployed cellular
systems in terms of maximum data rates, modulation schemes, multiplexing,
among other system specific performance parameters, several improvements can be
easily observed. Table above lists the major technologies and system performance for
different networks evolved from 2.5G up to 4G. The North American system (based
on CDMA) is shown in the shaded columns. The RF channel that connects the UE
to the eNB is the focus of RF planning for LTE network design. The duplexing,
multiplexing, modulation, and diversity are among the major aspects of the system
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Evolved 4G Cellular Network Planning and Optimization |
architecture that affect the planning process. Also, the wireless propagation model,
antenna types and number (LTE supports multiple antennas in theUEand eNB), and
semiconductor technology used are key components in RF planning and design. The
UE as well as the eNB (UL and DL) have to be designed, analyzed, deployed, and optimized
in order achieve the system performance metrics defined within the standard.
Duplexing, Coding, and Modulation in 4G LTE
In LTE, time division duplexing (TDD) and frequency division duplexing (FDD)
are supported. If the cellular system is using two different carrier frequencies for the
UL and DL, then the duplexing is called FDD. In this case, both the UE and the
eNB can transmit at the same time. For FDD, a channel separation is needed to
reduce the interference between the UL and DL traffic. Another precaution should
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4G Modulation Technique |
Evolved Cellular Network Planing & Optimization
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4G Network Modulation Technique
SC-FDMA/OFDMA tranceiver block diagram (DL/UL).
In LTE, adaptivemodulation and coding (AMC) is implemented on the UL/DL
streams according to channel conditions. Thus, the modulation scheme as well as the
coding scheme are changed automatically for best transmission performance for the
given channel conditions. Multicarrier multiplexing is attractive because a frequencyselective
channel would appear as a flat-fading one for individual orthogonal carriers
(subcarriers). Thus compensation for channel impairments would become easier to
realize in hardware [1,9].Twomultiplexing schemes are used in theLTEarchitecture.
shows a block diagram for a SC-OFDMA transceiver and the modification required
to obtain that of an OFDMA .
CP is needed to overcome inter-symbol interference (ISI) that is introduced on
the data by the wireless channel. The cyclic bit extension (adding a copy of the last
data portion of the OFDMA symbol to the beginning of it instead of a guard band)
to the FFT output adds a guard interval to the data to be transmitted.
In a typical OFDMA system block diagram in FIG, the serial input binary
data is converted into a parallel (S/P) stream that is mapped into a complex constellation
(modulation and coding) before being formatted for subcarrier mapping
through an IFFT operation. This process is followed by the addition of a CP before
being passed to a digital-to-analog converter (DAC). The data are then passed to
the RF part and the antenna elements (in case if more than 1 is used, i.e., MIMO).
In the receiver, the opposite sequence of operations is followed with the use of an
FFT processor. A SC-FDMA system includes an extra FFT/IFFT operation in the
transmitter/receiver, respectively. The size of the former FFT/IFFT processor is less
than the latter one (M > N). This change in the signal chain of the system diagram
gives several advantages of SC-FDMA over OFDMA. The major advantage of using
SC-FDMA over OFDMA is the lower peak-to-average power ratio (PAPR) that
minimizes problems to power amplifiers within the terminals. Thus, although the
SC-FDMA will entail more signal processing complexity (which can be handled with
today’s DSP), it will allow the creation of cheaper UE (RF portion is still relatively
expensive) and a better link budget because a lower PARP is achieved.
4G Cell Planning
The aim of the cell planning engineer is to establish the proper radio network in
terms of service coverage, QoS, capacity, cost, frequency use, equipment deployment,
and performance. In order to plan a cellular radio network, the designer has to
identify specifications, study the area under consideration and create a database with
geographic information (GIS), analyze the population in the service area, create
models (i.e., cell types, IDs, locations, etc.), and perform simulations and analysis
using proper propagation scenarios and tools. Afterward, simulation and coverage
results are analyzed, followed by cell deployment and drive testing. The results of
field measurements are compared against the simulation model results, and the model
is tuned for performance optimization. Each of the aforementioned stages in turn
consist of a number of steps that need to be performed.
4G Coverage
Coverage planning is an important step in deploying a cellular network. This process
includes the selection of the proper propagation model based on the area’s terrain,
clutter, and population. Propagation models (empirical models) are too simplistic
to predict the signal propagation behavior in an accurate fashion; they provide us
with some relatively good accuracy of how things would behave. Field measurements
are the most accurate in predicting radio coverage in a certain area. For example, in
buildings coverage will add about 16 to 20 dB of extra signal loss, and inside vehicle
ones can increase the loss by an extra 3 to 6 dB.
Engineers rely on prediction tools to study and analyze the performance of the
network for a geographic area via its coverage. In LTE, the air interface and radio
signal electronics are going to be different than those already deployed (in terms of
multiplexing,AMC,andMIMOcapability for both theUEand eNB). Modeling and
simulation using some current RF planning tools (i.e., Atoll [12]) for LTE cells will
give a good idea about the coverage performance of a certain grid within a specific area.
Based on the simulations made, the planning engineer would change eNB locations,
add more towers, replace antenna types, add more sectors to some towers, and so on.
Most cells are designed to be hexagonal in theory; in reality, this is not the case, as
several factors affect the location selection decision (political, humanitarian, economical).
Figure shows simulated signal power levels (color coded) of four LTE RF
cells in downtown Brussels, Belgium. Note the description of one of the sites where
the frequency band and bandwidth are shown along with the RF equipment characteristics:
antenna parameters, tower-mountedamplifier (TMA) characteristics, and
feeder loss.
Cell IDs
For LTE cells, the eNB antenna is 45mtall in rural areas and 30mtall in urban areas.
Typically eNBs (or sites) in a macrocellular deployment are placed on a hexagonal grid with an intersite distance of 3 × R, where R =500 m is the cell radius. Each eNB has three sectors with an antenna placed at each sector. In a multioperator cellular layout, identical cell layouts for each network shall be applied, with second network sites located at first network cell edges [13]. In an LTE system, the same carrier frequency is used, and thus the system relies on scrambling and pseudo-noise (PN) codes to distinguish between users and sites as well as to establish synchronization between the UE and eNB. A cell ID and scrambling code is to be given to each site. There are 504 unique cell IDs that can be used within the LTE physical layer. These IDs are grouped into three 168 groups, each group contains three identities. The cell ID is found from:
[ Ncell = 3NG + NID (11.5) ]
where NG ∈ [0, 167] is the physical layer cell group ID, and NI D ∈ [0, 2] is the
identification number within the group. NI D is also used to pick one of the 64
Zandoff-Chu scrambling codes used for the primary and secondary synchronization
channels (reference channels). A Zandoff-Chu sequence is a complex-orthogonal
sequence that is used to give unique signatures to radio signals. Orthogonal codes
are used to distinguish between radio transmissions and thus distinguish between
surrounding eNBs. In UMTS, Walsh codes were used for this purpose. In LTE,
Zandoff-Chu sequences are used. These give rise to constant amplitude radio signal
after the scrambling process. A root Zandoff-Chu sequence can be found using:
du (n) = e−j _un(n + 1)Nzc (11.6)
where, Nzc = 63 in LTE, and 0 ≤ n ≤ Nzc ; the root index u is related to NI D. In
the UE, GC sequences with different shifts are used based on the subscriber identity
and the physical channel type [8].
Atoll coverage EXAMPLE
For cell ID and scrambling code planning, several strategies exist based on minimum
reuse distance, domain constraints, minimum Ec /I0 levels, number of codes
per cluster, etc. Several automatic scrambling code planning algorithms exist within
RF planning packages that can be used as well. The fact that there are plenty of
cell IDs that can be used allows for a large pool of sequences and thus a larger area
between similar reused sequences. Some of these strategies are [12, 14]:
4G Cluster Reuse-Based Method:
This method assigns code sets according to a code set reuse pattern that is predefined (i.e, 13 cell clusters). Then, based on the propagation loss exponent and the processing gain of the radio scheme, the minimum reuse distance is found.
4G Graph Optimization Technique:
In this method, heuristic algorithms are used to assign cell IDs and scrambling codes by minimizing the number of sets to be used based on an optimization criteria. The algorithm first finds the inter-cell distances, and then starts automatic code assignments based on the
optimization criteria and their priorities.
Distributed Per-cell/Per-site:
In the per-cell strategy, the pool of codesis distributed among as many cells as possible, thus increasing the minimum reuse distance. The distribution per site allocates a group of different codes to adjacent sites, and from these groups, one code per transmitter is assigned.
4G Cell Types
Third-generation cellular networks utilize three cell types: macro, micro, and pico
based on their coverage area and user capacity . In LTE as well as WiMAX, a
fourth type is introduced to serve a single household—femtocell. According to ,
these four cell types are defined as:
_ 4G Macrocells: The largest cell types that cover areas in kilometers. These eNBs
can serve thousands of users simultaneously. They are very expensive due to
their high installation costs (cabinet, feeders, large antennas, 30–50 m towers,
etc). The cells have three sectors and constitute the heart of the cellular network.
Their transmitting power levels are very high (5–40 W).
_4G Microcells: Provide a smaller coverage area than macrocells, and are added
to improve coverage in dense urban areas. They serve hundreds of users and
have lower installation costs than macrocells. You can find them on the roofs
of buildings, and they can have three sectors as well, but without the tower
structure. They transmit several watts of power.
_ 4G Picocells: Used to provide enhanced coverage in an office like environment.
They can serve tens of users and provide higher data rates for the covered area.
The 3G networks use picocells to provide the anticipated high data rates. They
have a much smaller form factor than microcells and are even cheaper. Their
power levels are in the range of 20 to 30 dBm.
_ 4G Femtocells: Introduced for use with 4G systems (LTE and WiMAX). They
are extremely cheap and serve a single house/small office. Their serving capacity
does not exceed 10 users, with power levels less than 20 dBm. A femtocell will
provide a very high DL and UL data rates, and thus provide multi-Mbps per
user, thus accomplishing MAGIC (see Section 11.1, Introduction).
Multiple-Input Multiple-Output Systems (MIMO)
MIMO systems are one of the major enabling technologies for LTE. They will
allow higher data rate transmission through the use of multiple antennas at the
receiver/transmitter. Let the number of transmitting antennas beMT and the number
of receiving antennas be NR where NR ≥ MT.
In a single-input single-output system (SISO)—used in current cellular systems,
3G and 3.5G—the maximum channel capacity is given by the Shannon-Hartley
relationship:
C ≈ B × log2(1 + SNRavg) (11.7)
A MIMO system block diagram 4G Diversity
For MIMO-based systems, different kinds of diversity techniques are used. MIMObased diversity systems can be described as follows.
_ Transmit Diversity: The signal to be transmitted is forwarded and sentover all antennas, the same signal that is sent on all transmit antennas reachesthe receiver, and the combined signal level will be higher if only one transmit antenna was used, making it more interference resistant. Transmit diversity will increase the carrier-to-interference plus noise ratio (CINR) level, and isused at cell locations with low CINR (i.e., further from eNB toward the cell edges).
_ Spatial Multiplexing: Different signals are passed to different transmit antennas in this diversity technique. If the transmit terminal has M antennas and the receive terminal had N antennas, the throughput through the transmitreceiver link can be increased by [min(M, N)]. This diversity technique will increase the channel throughput provided that good CINR levels exist.
_ Adaptive MIMO Switching: This technique allows switching between transmit diversity and spatial multiplexing based on the environment conditions. If the CINR exceeds a certain threshold, spatial multiplexing is chosen to provide the user with higher throughput. On the other hand, if CINR is below the defined threshold, transmit diversity is picked to improve user reception by choosing to operate at a lower throughput.
Antenna Arrays
Antenna arrays are used to provide directional radiation characteristics and higher gain to the transmitted/received signal. The outputs of individual antenna elements within the array are combined to provide a certain desired radiation pattern and gain. The more directional the antenna array, the narrower the half power beam width becomes. Relationships between these parameters are of importance to the antenna design engineer and is found in [17]. Another important parameter for cellular antennas is their polarity. Vertical polarization is used in cellular systems.The eNB antenna gain within a macrocell in urban and rural areas is to be between 12 and 15 dBi, including the feeder losses within the bands of oeration.These gain values are important in formulating the RF link budget of the system and indetermining the coverage power levels. With proper use of adaptive techniques, the weighting of signal levels coming out of each antenna element with optimized coefficients gives better signal-to-noise ratio (SNR), interference reduction, and source signal tracking. This is called antenna beam forming (BF). If BF is utilized, the eNB antenna array can keep the main lobe in the direction of the UE, thus providing maximum antenna gain. This means higher SNR (CINR) levels, and thus better throughput and higher data transmission rates. Five predefined angles for BF have been suggested [0◦, 30◦, 45◦, 60◦, 70◦] along withtheir image (negative) angles. The weights for the antenna array can be stored in a lookup table in the antenna electronics, or can be achieved using an RF butler matrix.
Propagation Modeling
In wireless communications, a multipath channel is the one that describes the medium between the UE and the eNB. A multipath channel is characterized by the delay profile that is characterized by the RMS delay spread and the maximum delay spanned by the tapped-delay-line taps, along with the Doppler spread. Four environments are defined for LTE: extended pedestrian (low delay profile), extended vehicular (medium delay profile), extended typical urban (high delay spread), and the high-speed train (nonfading). This section presents the four propagation scenarios supported in the LTE standard, followed by the propagation channel modelsused. Statistical and deterministic channel modeling is presented. Creation of the LTE link budget is discussed as well.
Propagation Environments
Multipath channel characteristics can be described by a combination of a delay spread profile, the Doppler spectrum, and the effect of multiple antennas in aMIMOsystem through the use of correlation matrices. The delay spread profile can be modeled as a tapped delay line with predefined delay elements and relative power contributions.There are four propagation scenarios in LTE:
1. Extended Pedestrian A Model: This model covers walking users with
speeds up to 3 km/h. The tapped delay line model consists of 7-taps with
delays ∈ [0, 30, 70, 90, 110, 190, 410] ns, and relative power ∈ [0.0, −1.0,
−2.0, −3.0, −8.0, −17.2, −20.8] dB. The maximum Doppler shift is 5 Hz.
2. Extended Vehicular A Model: This model covers moving vehicles with
speeds up to 50 km/h. The model consists of 9-taps with delays ∈ [0, 30, 150,
310, 370, 710, 1090, 1730, 2510] ns and relative power ∈ [0.0, −1.5,
−1.4, −3.6, −0.6, −9.1, −7.0, −12.0, −16.9] dB. The maximum Doppler
shift is between 5 and 70 Hz.
3. Extended Typical Urban Model: This model covers moving vehicles with
speeds up to 90 km/h. The model consists of 9-taps with delays ∈ [0, 50, 120,
200, 230, 500, 1600, 2300, 5000] ns and relative power ∈ [−1.0, −1.0,
−1.0, 0.0, 0.0, 0.0, −3.0, −5.0, −7.0] dB. The maximum Doppler shift is
between 70 and 300 Hz.
4. High-Speed Train: This model covers train users with speeds of 300 km/h.
This is considered a nonfading model with a 1-tap delay line. The maximum
Doppler shift is 750 Hz.
Empirical/Statistical Path Loss Models
Path loss models are important in the RF planning phase to be able to predict coverage and link budget among other important performance parameters. These models are based on the frequency band, type of deployment area (urban, rural, suburban, etc.), and type of application. Two path loss models for macro/microcell propagation are listed in that are accurate if used beyond 100 m distances from the site for both urban and rural areas. Table lists the most widely used propagation models in current cellular systems. Most of these models are a fusion of empirical formulas extracted from field measurements and some statistical prediction models. Three of the listed models that will be used in LTE are discussed in detail in the rest of this section.
Deterministic Path Loss Models
The previous section discussed three of the most widely used empirical/statistical path loss models used in3Gmodels that will also be used in LTE. These models are derived from extensive measurement scenarios from which the wireless channel is described by probability functions of statistical parameters. Empirical/statistical models provide general results. Another group is based on deterministic channel modeling. The channel characteristics are obtained by tracing the reflected, diffracted, and scattered rays based on a specific geometry with a database what includes the sizes of the physical objects and their material properties.
CW Testing
Continuous wave (CW) testing, also called CW drive testing, is essential to the RF planning process and deployment of cellular networks. A CW test should be conducted to examine the signal levels in the area of interest: indoor, outdoor, and in vehicle. There are two types of drive tests:
( 1.) CW Drive: A CW drive test is conducted through different routes in the area to be covered before the network is deployed. A transmit antenna is placed in the location of interest (future site), and is configured to transmit an unmodulated carrier at the frequency channel of choice. A vehicle with receiver equipment is used to collect and log the received signal levels.
( 2.) Optimization Drive: This drive test is conducted after the cellular network is in operation (different call durations, data uploads, and data downloads). Thus, the modulated data signal is transmitted and then collected by the on-vehicle receiver equipment, then the data are analyzed for different performance parameters like reference channels (similar to the pilot in 3G systems), power measurements, scrambling codes, block error rates, and error vector magnitudes.
Model Tuning
Model tuning is the step that follows CW testing. The logged CW data are used to come up with a tuning factor for the initially picked propagation model used for the area under investigation. Propagation model optimization/tuning is performed using various curve fitting and optimization algorithms that are proprietary to the planning tool, and after the process is complete, statistical performance measures are obtained to illustrate the effect of optimization on the model behavior in terms of the mean, standard deviation, and RMS error. This process will provide a mode accurate channel model.
4G Network Performance Parameters4G Performance Parameters
Several types of parameter measurements are made at the UE or the eNB. These measurements are used to quantify the network performance and thus will aid in the adaptation of the appropriate coding/modulation as well as the link/cell traffic and capacity. In idle mode, eNB broadcasts the measurements within messages in the frame protocol. To initiate a specific measurement from the UE, the eNB transmits an “RRC connection configuration message” to the UE, along with the measurement type and ID, objects, command, quantity, and reporting criteria. The UE performs the measurement and responds to the eNB request with the measurement ID and results via a “measurement report message”.
Some of the most common performance metrics in LTE are:-
Received Signal Strength Indicator (RSSI): This measures the wideband received power within the specified channel bandwidth. This measurement is performed on the broadcast control channel (BCCH) carrier. The measurement reference point is the UE antenna connector. This measurement is easy to perform, as it does not need any data decoding, rather it shows whether a strong signal is present or not. It does not give any details about the channel or signal structure.
_ Received Signal Code Power (RSCP): measures the received power on one code on the primary common-pilot channel (CPICH). If the measurement is made while the equipment is in spatial multiplexing, the measured code power from each antenna is recorded, and then all are summed together. If transmit diversity is chosen, the largest measurement from all antennas is picked. The measurement reference point is the UE antenna connector.
_ Ec/N0(Ec/I 0): This is the received energy per chip divided by the noise power density (Ec /N0) (interference power density Ec /I0) in the band. When spatial multiplexing is used, the individual received energy per chip is measured for each antenna, and then summed together. The sum is divided by the noise power density in the band of operation. If transmit diversity is used, the measuredEc /N0 for antenna i should not be lower than the corresponding RSCP level. The measurement reference point is the UE antenna connector. Usually the Ec /I0 level is indicated as the interference levels are more profound and affect signal quality than noise levels (i.e., thermal noise).
_ Block Error Rate (BLER): This is used to measure error blocks within a specific channel transmission as a measure of transmission quality. This is performed on the transport and dedicated channels (TCH, DCH). _ Carrier–Interference Plus Noise Ratio Power Level [CINR (C/(I + N ))]: The CINR is measured in both the UE and eNB to determine the radio bearer to be used based on some predefined set of thresholds. The radio bearer defines which modulation and coding scheme to use for the data to be transmitted. The higher the CINR, the higher the spectrum efficiency by using a higher constellation modulation and coding scheme. The calculation of CINR is more involved than the RSSI, and it provides a better indication on the channel and signal qualities. CINR is sometimes referred to as the G-factor.
_ Error Vector Magnitude (EVM): It measures of the difference between the measured symbol coming out of the equalizer to that of the reference. The square root ratio of the mean error vector power to the mean power of the reference symbol is defined as EVM. The required EVM percentage over all bandwidths of operation performed over all the resource blocks and subframes for LTE is based on the modulation scheme used. Thus, for QPSK, 16-QAM, and 64-QAM modulation is given by 17.5%, 12.5%, and 8%,respectively.
4G Postdeployment Optimization and Open IssuesPostdeployment Optimization
As with all currently deployed cellular networks, whether it be a 2GGSM network or even a 3G UMTS one, an LTE network will have to be optimized after deployment to provide better coverage, throughput, lower latency and seamless integration as thespecification asks for. The optimization process contains several steps. It starts with data drive testing, where all performance parameters are tested and logged in the field after the network is active. This test should also include the different coverage/propagation scenarios along with their respective models (e.g., pedestrian, vehicular, indoor). The field data will then be used to tune the models for better network performance and coverage. Based on the collected data, RF planning engineers analyze the performance and maybe decide to add more eNBs for coverage, mainly pico and femtocells, in the areas that show degraded power levels or data throughput. Femtocells will be used in LTE, as they will provide service for households and small businesses. Usually, the optimization process is an iterative one with no specific steps involved, rather than a set of consistent procedures that characterizes network performance and coverage in a certain area; actions are taken accordingly.
Open Issues
There are several open issues that original equipment manufacturers (OEMs) has to take into account when designing LTE terminals and equipment. Some of the issues are being addressed, whereas others are still under extensive investigation. Here, we identify some of these open issues in two categories: UE and eNB.
UE
There are several challenges that has to be overcome in implementing LTE UE. The
use ofMIMOtechnology dictates the use of highly reliable and complex equalization
techniques. In a worse-case scenario, and using a minimum-mean-square-error
(MMSE) technique, the equalization might consume 1500 MIPS (million instructions
per second) performed on 600 subcarriers. This poses a challenge in performing
parallel computations, minimizing power consumption and silicon area. Memory
requirements for coding and decoding is also a challenge that needs to be overcome
[27, 28].
eNB
Although designers always try to minimize power consumption and silicon area in
their designs, there are less stringent requirements at the eNB side. The challenges
with complexities of hardware also exist within the eNB equipment. However, there
are other challenging aspects that have to be solved such as using BF to improve DL
performance. BF needs the use of antenna arrays, which require the use of adaptive
algorithms and electronics to be able to operate in real time and automatically. The
fact that BF will coexist within MIMO system is also a challenge. The coexistence
with legacy systems like 2G and 3G networks in the vicinity is another obstacle
to be overcome (4G-3G, 4G-2G). This coexistence will increase interference levels
and raises the thesholds of noise and interference. The LTE specification specifies
strict intermodulation levels due to this network coexistence. There are stringent
requirements within it that details the compliance levels within legacy systems bands
that OEMs should pay attention to.
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