2.4 System Capabilities:
Improved system
performance compared to existing systems is one of the main requirements from
network operators, to ensure the competitiveness of LTE and hence to arouse
market interest. In this section, we highlight the main performance metrics
used in the definition of the LTE requirements and its performance assessment.
Table summarizes the main performance requirements to
which the first release of LTE was designed.
Down link
|
Absolute requirements
|
Comment
|
Peak transmission
rate
|
>100Mbps
|
20MHz BW ,
FDD ,
2x2 spatial multiplexing,
|
Peak spectral efficiency
|
>5bps/Hz
|
|
Average cell
spectral efficiency
|
1.6-2.1 bps/Hz /cell
|
2x2 spatial multiplexing,
IRC receiver,
|
Cell edge spectral efficiency
|
0.04 -0.06 bps/Hz /user
|
As above,
Assuming 10 users/cell,
|
Broad cast spectral
efficiency
|
>1bps/ Hz
|
Dedicated carrier for
broad cast mode,
|
up link
|
Absolute requirements
|
comment
|
Peak transmission
rate
|
>50Mbps
|
20MHz BW ,
FDD ,
2x2 spatial multiplexing,
|
Peak spectral efficiency
|
>2.5bps/Hz
|
|
Average cell
spectral efficiency
|
0.66-1 bps/Hz /cell
|
2x2 spatial multiplexing,
IRC receiver,
|
Cell edge spectral efficiency
|
0.02 -0.03 bps/Hz /user
|
As above,
Assuming 10 users/cell,
|
System
|
Absolute requirements
|
comment
|
User plane latency
|
<10ms
|
|
Connection setup latency
|
<100ms
|
Idle state is the active state
|
Operating band width
|
1.4-20 MHz
|
Initial requirement
started at 1.25MHz with scalable band width
|
Cell coverage
|
Up to 100 km
|
It is found
in the standard but not achieved in
release 8
|
VOIP capacity
|
NMGN preferred target
>session /MHz /cell
|
|
The requirements shown in Table
are discussed and explained in more detail below.
2.4.1 peak rate and peak spectral efficiency:
For marketing
purposes, the first parameter by which different radio access technologies are
usually compared is the peak per-user data rate which can be achieved. This
peak data rate generally scales according to amount of spectrum used. The peak
rate can be defined as the maximum throughput per user assuming the whole
bandwidth being allocated to a single user with the highest modulation and
coding scheme and the maximum number of antennas supported .The target peak
data rates for downlink and uplink in the LTE system were set at 100 Mbps .and
50 Mbps respectively within a 20 MHz bandwidth, corresponding to respective
peak spectral efficiencies of 5 and 2.5 bps/Hz.
2.4.2 Cell throughput and spectral efficiency:
Performance at the
cell level is an important criterion, as it relates directly to the number of
cell sites that a network operator requires, and hence to the capital cost of
deploying the system. For LTE, it was chosen to assess the cell level performance
with full-queue traffic models (i.e. assuming that there is never a shortage of
data to transmit if a user is given the opportunity) and a relatively high
system load, typically 10 users per cell.
The requirements at
the cell level were defined in terms of the following metrics:
• Average cell throughput
[bps/cell] and spectral efficiency [bps/Hz/cell].
• Average user throughput
[bps/user] and spectral efficiency [bps/Hz/user].
• Cell-edge user throughput
[bps/user] and spectral efficiency [bps/Hz/user]. The metric used for this
assessment is the 5-percentile user throughput, obtained from the cumulative
distribution function of the user throughput.
The original requirements for the cell level metrics were
only expressed as relative gains compared to the Release 6 reference baseline.
The absolute values provided in Table are based on evaluations of the reference
system performance that can be found in downlink and uplink respectively.
2.4.3 Voice capacity:
It is important to
set system capacity requirements for such services – a particular challenge in
fully packet-based systems like LTE which rely on adaptive scheduling.
The system capacity requirement is defined as the number
of satisfied VoIP users, given a particular traffic model and delay
constraints. The details of the traffic model used for evaluating LTE can be
found in Here, a VoIP user is considered to be in outage (i.e. not satisfied)
if more than 2% of the VoIP packets do not arrive successfully at the radio
receiver
within 50 ms and are therefore
discarded.
This assumes an overall end-to-end delay (from mobile
terminal to mobile terminal) below 200 ms. The system capacity for VoIP can
then be defined as the number of users present per cell when more than 95% of
the users are satisfied.
2.4.4 Mobility and cell range:
In terms of
mobility, the LTE system is required to support communication with terminals
moving at speeds of up to 350 km/h, or even up to 500 km/h depending on the
frequency band.
These targets are
to be achieved by the LTE system in typical cells of radius up to 5 km, while
operation should continue to be possible for cell ranges of up to 100 km to
enable wide-area deployments.
2.4.5 Broadcast mode performance:
Although not
available in the first release due to higher prioritization of other service
modes, LTE is required to integrate an efficient broadcast mode for high rate
Multimedia Broadcast/Multicast Services (MBMS) such as Mobile TV, based on a
Single Frequency Network mode of operation. This mode is able to operate either
on a shared carrier frequency together with unicast transmissions, or on a
dedicated broadcast carrier.
To ensure efficient
broadcast performance a requirement was defined for the dedicated carrier case.
In broadcast systems, the system throughput is limited to what is achievable
for the users in the worst conditions.
Consequently, the
broadcast performance requirement was defined in terms of an achievable system
throughput (bps) and spectral efficiency (bps/Hz) assuming a coverage of 98% of
the nominal coverage area of the system.
This means that
only 2% of the locations in the nominal coverage area are in outage – where
outage for broadcast services is defined as experiencing a packet error rate
higher than 1%.This broadcast spectral efficiency requirement was set to 1
bps/Hz .
2.4.6 User plane latency:
User plane latency
is an important performance metric for real-time and interactive services.
It is defined as
the average time between the first transmission of a data packet and the
reception of a physical layer Acknowledgement (ACK).
The LTE system is
also required to be able to operate with an
IP-layer one-way data-packet latency across the radio access network as
low as 5 ms in optimal conditions.
However,
it is recognized that the actual delay experienced in a practical system will
be dependent on system
loading and radio propagation conditions.
2.4.7 Control plane latency and capacity:
In addition to the
user plane latency requirement, call setup delay is required to be
significantly reduced compared to existing cellular systems. This not only
enables a good user experience but also affects the battery life of terminals,
since a system design which allows a fast transition from an idle state to an
active state enables terminals tospend more time in the low-power idle state.
Control plane
latency is measured as the time required for performing the transitions between
different LTE states .The LTE system is required to support transision from
idle to active in less than 100 ms.
The LTE system
capacity is dependent not only on the supportable throughput but also on the
number of users simultaneously located within a cell which can be supported by
the control signalling.
For the latter
aspect, the LTE system is required to support at least 200 active-state users
per cell for spectrum allocations up to 5MHz, and at least 400 users per cell
for wider spectrum allocations;
Only a small subset
of these users would be actively receiving or transmitting data at any given
time instant, depending, for example, on the availability of data to transmit
and the prevailing radio channel conditions. An even larger number of
non-active users may also be present in each cell, and therefore able to be
paged or to start transmitting data with low latency.
2.4.8 Deployment cost and interoperability:
Besides the system
performance aspects, a number of other considerations are important for network
operators. These include reduced deployment cost, spectrum flexibility and
enhanced interoperability with legacy systems – essential requirements to
enable deployment of LTE networks in a variety of scenarios and to facilitate
migration to LTE.
2.4.9 Terminal complexity and cost:
A key consideration
for competitive deployment of LTE is the availability of low-cost terminals
with long battery life, both in stand-by and during activity. Therefore, low
terminal complexity has been taken into account where relevant throughout the
LTE system, as well as designing the system wherever possible to support low
terminal power consumption.
2.4.10 Network architecture requirement:
LTE is required to
allow a cost-effective deployment by an improved radio access network
architecture design including:
• flat architecture consisting of just one type of
node, the base station, known in LTE as the
eNodeB.
• effective protocols for the
support of packet-switched services;
• open interfaces and support
of multivendor equipment interoperability;
• efficient mechanisms for
operation and maintenance, including self-optimization functionalities.
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