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Nokia Networks
LTE networks for
public safety services
Public safety agencies and organizations have started
planning to evolve their networks to LTE-based public
safety solutions.
LTE supports a wide variety of services, from high
bandwidth data services to real-time communication
services – all in a common IP based network.
Mission critical communication in demanding conditions, for
example after a natural disaster, sets strict requirements,
which are not necessarily supported by regular commercial
mobile networks.
In this paper we present the technology evolution for LTE
public safety services, including standardization activities
in 3GPP and highlight selected public safety requirements
that affect LTE networks.
Nokia Networks white paper
LTE networks for public safety services
Contents
Introduction - LTE public safety momentum
3
Spectrum and network deployment options
5
Group Communication
8
Proximity Services
10
Mission Critical Push to Talk
12
Prioritization of emergency responders
13
Security
16
Network resilience
18
Network coverage and capacity
20
Conclusions
22
References
23
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Introduction - LTE public safety
momentum
This paper provides some background and looks at issues of interest
to parties considering implementing an LTE network for public safety
services.
LTE is the most quickly adopted mobile technology so far, with over 300
commercially launched networks globally. Current public safety networks
such as TETRA or Project 25 (P25) support mission critical voice
communication, but are limited to narrowband data. Mobile broadband
can help emergency services significantly, for example with live mobile
video, situation aware dispatching and remote diagnostics. In 2012 the
US government formed FirstNet and committed USD 7 billion to the
venture. FirstNet was given responsibility for coordinating the use of
Band 14 (at 700 MHz), which was reserved for public safety in 2007 by
the local regulator. The TETRA and Critical Communications Association
(TCCA) announced that LTE was the selected technology for mission
critical mobile broadband communications and public safety became the
key theme of 3GPP Release 12. Nokia took key rapporteurships in 3GPP
to make the vision part of the standards.
Public safety use cases rely heavily on the existing E-UTRAN and EPS
capabilities from 3GPP Release 8 onwards. There are even public
safety specific requirements covered in the completed 3GPP releases.
Highlights of public safety evolution in 3GPP are shown in Figure 1.
3GPP Rel-8
3GPP Rel-9
3GPP Rel-10
3GPP Rel-11
3GPP Rel-12
3GPP Rel-13
• Mobile data
connections
• Location services
and positioning
support for LTE
• Physical layer
enhancements to
increase data
throughput
(including LTEAdvanced features)
• High power devices
for Band 14 - 1.25
Watts for public
safety devices
significantly
improving the
coverage of an LTE
network, benefiting
public safety users
and reducing
network
deployment costs.
• Group
Communication
System Enablers
for LTE
• Mission Critical
Push-to-Talk
• Basic support for
Voice over LTE
(telephony)
• Support for LTE
Band 14
• a rich set of QoS
priority and preemption features
• Highly secure
authentication
and ciphering
• Multimedia
Broadcast /
Multicast Service
• E911 or
emergency calling
support
• Enhanced Home
LTE base station:
“Cell On Wheels”
• Self-Organizing
Networks (SONs)
• Relays for LTE, e.g.
to allow a base
station mounted
on a fire vehicle to
relay
communications
from firefighters in
a basement back
to the network.
• Proximity-based
Services
• Enhancements to
Proximity-based
Services
• Isolated E-UTRAN
Operation for
Public Safety
• MBMS
Enhancements
3GPP work ongoing completion expected
1Q2015
3GPP work started completion expected
2016
Figure 1. 3GPP Road to Public Safety
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Nokia Networks estimates that the global market for LTE public safety
networks will exceed 2 billion Euros in 2019. This estimate includes
network infrastructure and related services such as network planning,
implementation and optimization.
The main markets driving this growth are the US and the UK. FirstNet
is expected to start main deployment in late 2016 in the US. In UK, the
UK Home Office has established a program with a target to build a new
Emergency Service Network (ESN) that will provide mobile services for
the three emergency services (police, fire and ambulance). Currently, it is
estimated that the ESN will go live during 2016.
This provides significant momentum for the LTE industry as
governments start budgeting for next generation public safety
networks.
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Spectrum and network deployment
options
A network dedicated to public safety can offer guaranteed spectrum
and optimized security and resilience. This type of deployment model
exists in current narrowband public safety networks such as TETRA and
Project 25. The same deployment model is a viable option for LTE public
safety but is also the most expensive and reserves its own spectrum
allocation, which cannot then be used for other purposes. Therefore
public safety stakeholders are considering other models. Two key topics
to consider from the cost-efficiency perspective are spectrum allocation
and network infrastructure ownership.
A spectrum allocation strategy should consider the device ecosystem,
which has potentially high costs. Initially, the global LTE market was
very fragmented due to different frequency bands in different markets.
However, this is no longer such an issue, as mobile devices increasingly
have multi-band support for all major frequency bands used globally.
LTE public safety networks can benefit significantly from the commercial
LTE ecosystem if the same frequency bands are selected for public
safety use.
Currently only regulators in the US and Canada have allocated band 14
for public safety and while this band is not currently allocated anywhere
else, the result is a separate device ecosystem in North America. Other
regulators tend to prefer bands that are already selected for commercial
LTE networks such as band 20 (EU 800 MHz) and band 28 (APT 700
MHz). Even lower bands like band 31 (450 MHz) are considered in some
countries, but then the problem arises of a compromised broadband
performance due to a narrower available bandwidth, although it
could present good opportunities for “voice only” services. The World
Radiocommunication Conference 2015 has an agenda item on the
harmonization of Public Safety spectrum but the outcome is likely to be
a list of possible frequency bands (in ITU-R Res 646), which individual
regulators could consider when deciding on the spectrum for Public
Safety in their countries, as the topic is considered a national issue.
Network infrastructure deployment depends on spectrum allocation. If
dedicated spectrum is allocated, then a dedicated public safety network
can be implemented. This is possible for example in the US, where
FirstNet has a license for a nationwide public safety network. Economies
of scale improve as more users are served by the network infrastructure
and the spectrum and therefore network sharing can be considered to
optimize the cost per user – also in the FirstNet case.
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Public safety
services
Public safety
services
Public safety
services
Public safety
services
AS
AS,
IMS,
PCRF
VPN or
Internet
AS,
IMS,
PCRF
HSS
HSS
S/PGW
MME
S/PGW
MME
HSS
P-GW
SGW
MME
HSS
MME
Shared
spectrum
Mobile operator
Public safety
over MBB
eNB
Shared
spectrum
eNB
Shared
spectrum
Mobile operator
Mobile operator
Hosted
public safety
MVNO
public safety
S/PGW
HSS
MME
AS,
IMS,
PCRF
S/PGW
S/PGW
MME
eNB
AS,
IMS,
PCRF
eNB
Shared or
dedicated
spectrum
eNB
dedicated
spectrum
Mobile operator
RAN sharing for
public safety
Private LTE for
public safety
Figure 2. Examples of deployment options
In the UK, the intention is to reduce costs by selecting existing mobile
operators to offer LTE network services for LTE public safety users. This
approach enables sharing a common LTE infrastructure for consumer,
enterprise and public safety customers. The main cost in any mobile
network is the radio access network and therefore the major saving
is derived from the use of a common radio access network for both
commercial and public safety services. This can be achieved using
traditional and standardized sharing techniques, for example with the
RAN sharing model or with an MVNO model (Mobile Virtual Network
Operator). Network infrastructure and spectrum sharing can also be
implemented so that the Mobile Network Operator (MNO) hosts public
safety services in addition to the regular mobile services.
When network sharing is used, LTE network features, planning and
configuration must all take into account the requirements of public
safety agencies. Public safety services can set tighter coverage, security
and resilience requirements than is commonly planned in commercial
networks. Furthermore, prioritization of public safety subscribers and
services is critical in emergency situations.
Public safety agencies have already noticed that existing commercial
mobile broadband networks can be used to enhance emergency
communication services. For example, mobile broadband services
are available in areas without TETRA or P25 coverage. It is possible
to find public safety applications that currently exist for commercial
smartphones which allow public safety officers to communicate over
existing mobile broadband networks. Thus, public safety services are
implemented just like the Internet or so called over-the-top (OTT)
services.
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Roaming is one more dimension in LTE public safety deployment that
can be combined with the models depicted in Figure 2. Most especially,
service resilience can be extended by using national roaming i.e. allowing
public safety users to access services from any and all national LTE
networks. This model can be further developed by allowing WiFi access
to services, if no other terrestrial option is available. Satellite service is
another alternative, for example in rural areas.
The high level network architecture is similar in all deployment options.
The network architecture depicted in Figure 3 includes key components
of the LTE network and illustrates that different network elements and
functions are located at different sites. The public safety application
servers are highlighted and can be located in separate sites dedicated
for public safety services and related interworking functions. Note that
there are several options available and the decision on where different
network functions are located and distributed in different sites will
ultimately be for the authorized bodies and operators to agree. As an
example, the public safety service provider (MVNO role) can be separated
from the network provider (MNO role).
OSS, BSS
Management
Core site
Core site
HSS, SPR
PTT ,
Group Comm
Live
video sharing
Interworking
Tetra/P25
core
Charging
PCRF
MME
Cell site
BM-SC
eNodeB
IMS
S/P-GW
MBMS-GW
DNS
FW
AS
(e.g. VoLTE)
Load Bal.
Cell site
Agency 1
Control room / Dispatcher
IP backhaul
IP backbone
Agency 2
eNodeB
Tetra/P25 BTS
Internet
Control room / Dispatcher
Figure 3. Network architecture overview.
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Group Communication
To enable group communication services, 3GPP has introduced the
concept of the Group Communication Service application server, also
known as GCS AS [Reference TS 23.468] in Release 12. It provides a
means for both one-to-one and one-to-many communication services.
Figure 4 shows how the GCS AS is connected to the rest of the system,
according to the 3GPP Rel-12 GCS architecture. Although not explicitly
shown, the architecture allows the device to connect to GCS AS via IMS.
GC1
 Application control information
Rx
 Priority level, session information
PCRF
 Bearer status info
SGi
 Downlink traffic (unicast)
 Uplink traffic
S/P-GW
MB2
BM-SC
 Data related to GC1
GCS AS
MBMS bearer mgmt
Group identity mgmt
 Downlink traffic (broadcast)
Figure 4. Group Communication architecture.
Public safety devices use the GC1 reference point to initiate, modify or
terminate group communication sessions. The GC1 reference point will
be standardized as part of 3GPP Release 13. The GCS AS is the entity
which makes the decision to use either unicast or broadcast mode for
sending traffic (voice, video or data) to the public safety devices.
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In unicast mode, the GCS AS uses the information from application
control signaling (GC1 reference point) to derive an appropriate
priority level, which it further communicates to Policy and Charging
Rule Function (PCRF) over the Rx reference point, together with other
relevant data (e.g. IP addresses, port numbers, codec). The PCRF uses
this information to create an EPS bearer with desired prioritization
values (such as public safety specific QCI value, ARP, pre-emption
capability and pre-emption vulnerability).
In broadcast mode, the GCS AS uses eMBMS to deliver traffic to the
public safety devices. To establish an eMBMS bearer in a specific
geographical area, the GCS AS uses the MB2 reference point. The
eMBMS bearer can be pre-established, for example for mass events or
festivals, or it can be established in dynamically, for example when the
number of users within an area has exceeded a certain threshold.
The Public Safety device is responsible for service continuity between
unicast and broadcast modes. In other words, when the device and GCS
AS detects that downlink media can also be delivered via MBMS, it can
ask the GCS AS to stop sending traffic using the unicast bearer. When
the device detects that eMBMS coverage is becoming too weak, it asks
GCS AS for unicast delivery in the downlink instead of eMBMS delivery.
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Proximity Services
The Public Safety solution needs to support communication between
public safety users when the devices are in proximity and even if the
network is down or when the device is out of coverage. To enable
this, 3GPP is standardizing a feature called Proximity Services (ProSe)
[Reference TS 23.303]. Proximity Services allows two devices to
communicate directly, i.e. without the data path being routed via the
network infrastructure. The proximity range depends on the strength
of the radio signal and other radio conditions such as interference. The
actual range varies depending on the power level used for transmitting
the radio signal.
Public Safety is one of the ProSe use cases, while others include
commercial services such as friend finder. This ability to support direct
communication is a core requirement for public safety use cases. In
addition, public safety devices should be able to communicate directly
with other devices, whether the device is served by E-UTRAN or not.
These functionalities are enabled by ProSe in 3GPP Release 12.
Public safety devices can initiate direct communication without
performing a discovery procedure, as it is assumed that public safety
personnel know each other’s whereabouts and can thus determine
whether the other person is reachable for direct communication or not.
How do I find other
ProSe-enabled UEs in its
vicinity by using only the
capabilities of the two UEs with
Rel-12 E-UTRA technology
Direct
Connectivity
Prose Discovery
Prose Communication
Connectivity via
other UE
User Equipment to
Network Relay
Figure 5. High level Proximity functionality.
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The high-level ProSe feature set consists of:
• ProSe discovery: allows a device to find other devices in its vicinity by
using direct radio links or via the operator network. 3GPP Release 12
supports discovery only when the device has network coverage.
• ProSe Communication: allows a device to establish communication
between one or more ProSe enabled devices that are in direct
communication range. Communication is provided in a connectionless
manner (no control plane involved).
• Device to network relay: allows a device to act as a relay between
E-UTRAN and devices not served by E-UTRAN (out of coverage
devices e.g. inside the building). This functionality is expected in 3GPP
Release 13.
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Mission Critical Push to Talk
Mission Critical Push to Talk (MCPTT) provides one-to-one and one-tomany voice communication services. The idea is simple. Users select the
individuals or groups they wish to talk to and then press the “talk key” to
start talking. The session is connected in real time. Push to talk sessions
are one-way communication (also known as ‘half-duplex’): while one
person speaks, the others only listen. Turns to speak are requested by
pressing the “talk key” and are granted on a call prioritization basis, for
example a dispatcher has a higher priority than other users.
The push to talk service for group communication is based on multiunicasting and broadcasting. Each sending device sends packet data
traffic to a dedicated mission critical push to talk application server and
the server then copies the traffic to all the recipients (see Figure 6).
3GPP is in the process of standardizing MCPTT in Release 13 [Reference
TS 22.179] - here the MCPTT application server is assumed to be
part of the GCS application server. Note that GCS is a generic function
for voice, video and data, but as the name implies, MCPTT is a voice
communication service.
IMS
MCPTT AS
Control room / Dispatcher
• Group management
• Group member
GCS AS
SIP
RTP packets
UL/DL unicast
RTP packets
DL broadcast
eNodeB
BM-SC
eMBMS
eNodeB
Figure 6. Mission Critical Push to Talk.
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Prioritization of emergency responders
Regardless of the actual public safety network deployment model, public
safety subscribers must have priority access to the network. Commercial
mobile networks are dimensioned to serve typical busy hour traffic,
but the networks do not necessarily have capacity for extreme cases.
Therefore, subscribers may experience problems accessing mobile
services during mass events, for example in sports stadiums. In overload
conditions, the network signaling plane gets overloaded on the radio
interface due to frequently repeated connection attempts by potentially
thousands of smartphones in a single cell. Similar problems could also
occur in a large scale accident or disaster, as hundreds or thousands of
people attempt to make emergency calls and use mobile services at the
same time. When moving emergency services into commercial mobile
networks there must be a solution to limit the amount of connection
attempts as well as allow priority access for high priority users, including
emergency responders. This is solved with existing access class
prioritization and the possibility to invoke access class barring. Barring
of low priority users can prevent the signaling attempts and therefore
effectively give adequate radio resources to high priority users.
1. In case of high load, access class barring
can be activated for AC 0 – 9 users.
HSS/SPR
2. Admission control and pre-emption can
be used for prioritizing EPS bearers of
emergency responders (ARP)
3. Emergency calls and multimedia priority
services (MPS) calls get end-to-end
priority treatment.
4. User plane traffic is prioritized and
scheduled according to QoS parameters
(QCI, GBR/MBR, NBR).
PCRF
AC 14
X
X
AC 0-9
MME
P/S-GW
eNB
• High priority bearer with
pre-emption capability (ARP).
• Traffic prioritization matching
service requirements (QCI)
Figure 7. User and bearer prioritization tools.
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Operators can use access class barring and extended access barring
capabilities as an overload control tool to reduce the load generated by
regular users in normal operations. This mechanism can be automated
so that access class barring activates if certain load thresholds are
exceeded. Access class barring is commonly supported in LTE radio
access and a key new requirement is that network operators open an
interface for public safety authorities to quickly trigger emergency
access class barring in selected locations. Access Class (AC) must be
managed on a subscription level so that emergency responders get a
USIM with AC 14, whilst regular users are distributed to access classes
0 – 9. Access classes 12 and 13 are also relevant in general public safety
as they are meant for security services and public utilities respectively.
It should be noted that barring regular users in LTE still leaves 2G and 3G
accesses open for them.
The next level of prioritization occurs in admission control and preemption. Public safety users and services can be prioritized on an LTE
bearer level. Allocation and retention priority (ARP) defines a priority
level (1 – 15), pre-emption capability and pre-emption vulnerability.
Emergency responders must have higher priority for LTE data bearers
than other subscribers. Furthermore, pre-emption parameters must
allow public safety users and services to pre-empt other data bearers if
network resources are limited.
Access and service prioritization for emergency calls is a normal
regulator requirement for mobile networks. Furthermore, a new
capability called multimedia priority service (MPS) has uses in an
emergency situation. MPS enables end-to-end prioritization for a call,
important if an emergency officer must reach a regular subscriber. This
means that with the MPS service, the terminating leg to the regular user
is also prioritized in admission control and pre-emption.
The last level of prioritization is managed in the user plane. Data bearers
have a different QoS class, defined by the QoS class identifier (QCI)
parameter. QCI defines delay and packet loss targets for the connection
as well as whether the bearer is “guaranteed bit rate” (GBR) or a “nonGBR” connection. GBR bearers have additional parameters for the actual
guaranteed bit rates in uplink and downlink directions. In addition to the
existing standard QCIs (from 1 to 9), 3GPP has specified special GBR and
non-GBR QCIs for public safety group communication (QCIs 65, 66, 69
and 70) [Reference TS 23.203].
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It should be noted that public safety users are not necessarily, by
default, prioritized to the highest level. The default subscription priority
for a default bearer can be higher than regular subscribers’ priority, but
additionally, public safety users handling emergency incidents should
be prioritized over other public safety users. Therefore, public safety
responders and control room officers can indicate emergency priority
for specific communication sessions. One option for higher admission
priority and scheduling priority during mission critical sessions is to
dynamically modify the QoS of the default bearer. The drawback here is
that all service flows are affected, including any lower priority activities
such as potential background data transfers. A preferred option is to
differentiate mission critical sessions with dedicated bearers as shown
in Option 2 in the figure below. Establishment and release of dedicated
bearers requires the use of PCRF with a Rx reference point to the
application control function.
Option 1 – Default bearer modification
PCRF can trigger QoS
modification of default bearer
Option 2 – Dedicated bearer for mission critical QoS
HSS/SPR
PCRF can trigger setup of
dedicated bearer
PCRF
AC 0-9
MME
AC 0-9
Mission
critical
service
MME
Mission
critical
service
P-GW
eNB
AC 14
AC 14
Public safety user
in emergency
mission.
PCRF
P-GW
eNB
HSS/SPR
Public safety
APN
Non-mission
critical
services
AC 14
Dynamic modification of
default bearer impacts
on all service flows
AC 14
Public safety user
in emergency
mission.
Public safety
APN
Non-mission
critical
services
Dedicated bearer for
mission critical service
flows
Figure 8. QoS options for mission critical service flows.
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Security
Security is already important in the commercial mobile network.
The network infrastructure and related IT infrastructure, such as the
management system, must be protected, for example against illegal
access, viruses, malware and denial-of-service attacks. Therefore, there
must be controlled access authorization to management tools. Software
updates and maintenance must also be secure. The network must
be protected with firewalls and intrusion detection systems. Security
requirements for a public safety network may be more stringent than
in a normal mobile network. This also includes physical security, not
only in the data centers and core sites, but also in all distributed sites,
especially base station sites. Physical security also requires tight control
of personnel with access rights to different sites.
Authorized access to network services and adequate confidentiality
for subscribers is well standardized by 3GPP. Authentication and
authorization are based on secure USIM based methods. Furthermore,
signaling and user plane traffic are ciphered over the air interface. The
LTE network specification does not require user plane traffic encryption
in the backhaul and transmission networks, but this is possible with
optional IP security based solutions and is highly recommended when
using third party transport providers. Most especially, the control and
management plane traffic must be protected, as a potential attacker
could in some locations have relatively easy access to the physical
connections of the transmission links at base station sites.
User identity management and related user priority level and service
access rights require special attention in public safety communities.
Typically, many of the devices used are shared. Thus, the user identity
and user profile cannot be based on a common USIM inside the device.
If the USIM is kept inside the mobile device and used for network access
authentication and authorization, another method is needed for actual
user authentication and setup of the access profile (for example QoS).
Alternatively, public safety users could have individual USIM cards, but
to make it easy to change devices a Bluetooth based remote SIM access
could be considered.
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Additional security is needed in the application layer. Public safety
services must have their own user authentication and authorization,
which is managed by the public safety service provider. Public safety
communication content is highly sensitive and therefore confidentiality
in public safety communication must be based on end-to-end security in
the application layer. This guarantees confidentiality without any specific
dependency on the security solutions implemented in the network for
the user plane traffic. The same end-to-end security approach is also
used, for example, in current TETRA networks. Due to the sensitive
communication content, the public safety application servers and
content storage devices must be located at highly secure sites.
Public safety networks must also support traffic separation using VPN
technologies. If the same network is used by a public safety user and
other regular subscribers, public safety traffic must be separated,
for example using VPN solutions commonly used for enterprises.
Furthermore, different public safety agencies should be separated from
each other with controlled interconnection interfaces between agencies.
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Network resilience
Network resilience is based on high-availability and redundancy
solutions on multiple layers. Most resilience features needed in public
safety networks are commonly available from LTE manufacturers.
However, commercial LTE networks may not implement resilience
fully to fulfill all public safety requirements. Most network elements
have high-availability designs, for example, to recover from hardware
failures. Pooling of elements and load balancing between core elements
improves system reliability and guarantees network service availability,
even if a single node fails.
Centralized functions like management systems and core network
elements are usually located in at least two geographically separated
locations, in order to survive possible complete site failure. Connectivity
between sites and network elements supports resilience against link and
node failures. Backup connections and nodes can generally be designed
into the IP transport network, and specifically for IPSec tunnels, timing
synchronization, management connections and signaling (e.g. Diameter
routing).
High available nodes
• Redundant HW units
(e.g. fans, power, blades)
• Redundancy options (N+, 2N)
• Session continuity in switchover
Connectivity & routing
• Resilient IP network design with
fast re-routing
• Interface protection
• IPsec backup & emergency bypass
• Redundant Diameter routing
Inter-node resilience & pooling
• MME pooling with S1-flex
• P/S-GW load balancing
• CSCF load balancing
• AS pooling
• Redundant timing master
Management & automation
• Outage detection
• Automatic re-configuration
• Self-healing
Geo-redundancy
• OSS, registers and core
elements in geographically
distributed sites
• 3-site database replication
Cloudification
• Elastic scalability for
virtual network functions
• Automatic load balancing and
resource allocation
Figure 9. High-availability and resilience on multiple levels.
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It is not always straightforward to detect certain failures, such as
performance degradation in cells, but advanced management and Self
Organizing Network (SON) automation tools can detect events like cell
outages and activate automatic self-healing. The network must also be
prepared for power outages and solutions such as battery backup and
generators are common in current commercial networks. There is one
more topical technology that is not driven by resilience requirements,
but can further enhance system availability. This is the transfer of
network function to a “cloud”, enabling elastic scalability and automatic
load balancing.
Resilience in LTE public safety networks can be further enhanced from
typical commercial LTE networks. Public safety networks can have
dedicated core network elements for public safety services (HSS, EPC,
IMS, ASs), which simplifies the dimensioning and enables management
of peak load at a lower level for better performance and more reliable
operation.
Natural disasters can destroy base station sites and network
connections and therefore rapidly deployable cells are important for
disaster recovery [See Network coverage and capacity].
Future features also enable local communication when network coverage
is missing or the backhaul connection is disabled. For example, as
mentioned previously, 3GPP proximity services will introduce direct
device-to-device communication. 3GPP is also expected in Release 13
to support isolated E-UTRAN operation for public safety [Reference
TS 22.346]. This means the eNodeB site can continue offering network
services locally, even if backhaul connection to core network sites is lost.
One further consideration for network connectivity resilience is based
on the Internet service model, i.e. enabling access independent public
safety services. Such a model would allow authorized access to public
safety services via any broadband capable IP access. Public safety users
could have subscriptions that allow connection to the LTE access of all
national mobile operators. As a backup option, WiFi or satellite access
could be used when other services are not available.
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Network coverage and capacity
Service coverage for public safety users is critical and previous TETRA
solutions, operating in low frequency spectrum, offered coverage almost
everywhere. It is a critical requirement when moving to commercial LTE
networks that this coverage is maintained and if possible, enhanced.
Public safety networks must provide very high national geographic
coverage - not only high population coverage. Furthermore, public safety
users may have to work in various indoor locations where commercial
mobile services are not currently available.
The most basic approach, for a simple and cost efficient coverage is to
use lower frequency bands. In the case of broadband LTE this typically
means bands in 700 MHz and 800 MHz ranges. Even lower bands like
450 MHz are considered, but this typically results in a compromised
broadband performance due to a narrower available bandwidth.
The deployment of macro network coverage uses well known optimizations
employed in commercial LTE networks. Cell range is normally uplink limited
because of the low transmission power allowed in mobile devices. Wide
area coverage of high transmission power and the receiver sensitivity of
eNodeBs can be optimized with a number of techniques, such as 4-way
receiver diversity, higher-order sectorization and TTI (transmission time
interval) bundling. Cell range can be extended in the uplink by specifying
high power mobile devices (power class 1, 31 dBm) also for other than
existing band 14 (in other bands only class 3 devices, 23 dBm, specified).
Indoor coverage can be optimized with different indoor solutions such
as distributed antenna systems (DAS) and low power indoor cells, also
known as ‘small cells’. Small cells can be used for filling indoor white
spots. However, specialized in-building solutions will not solve all indoor
coverage issues except in selected buildings.
Capacity is an additional aspect that must be taken into account in
network dimensioning. Although the number of public safety users is
significantly lower than regular subscribers in any commercial mobile
network, the number of simultaneously active public safety users can
be very high, especially in a relatively small area when a major incident
occurs. Such situations are more likely to happen in dense urban areas
and therefore the network planning in urban areas should follow the
same principles as in commercial networks, i.e. a denser site grid and
smaller cells in urban locations. Capacity requirements in public safety
networks can make high demands on the network design due to new
public safety video applications. If for example there is a need for
multiple HD quality video streams in cell edge radio conditions, then
10 + 10 MHz FDD spectrum would be far from adequate and therefore
more carrier bandwidth is needed.
Regardless of how well the network is designed and implemented, there
will always be emergency incidents in locations without existing network
coverage (outdoors or indoors). Additionally, public safety network
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cell sites may be just as vulnerable in natural disasters as commercial
network sites and may be damaged. Therefore, public safety network
operators should be prepared with rapidly deployable base station
solutions. Deployable solutions should enable fast macro coverage
to provide and recover network availability in both rural and urban
locations. Rapidly deployable small cells may be required in difficult
indoor locations such as mines and caves and can be further used for
an instant capacity boost when required. Rapidly deployable small cells
can be also pre-installed in emergency vehicles in order to automatically
provide network coverage around the vehicles.
Availability of radio communication is further guaranteed by providing
direct device-to-device communication. In LTE this is enabled by 3GPP
proximity services [See Device to Device Communication]. ProSe can
partially solve network coverage white spot problems based on the
ProSe device relay solution.
Availability and reliability of service coverage can be improved using the
resilience mechanisms mentioned in the Network resilience section,
i.e. allowing service access via any national broadband capable network
including HSPA and WiFi or via satellite access. Other techniques, such
as Assured Shared Access coupled with MOCN (Multi-Operator Core
Network) could be used to improve the economics of deep rural coverage
and improve the service proposition by allowing all operators’ low
frequency spectrum to be pooled in these locations. Typically 800 MHz is
scarce and distributed amongst the operators of the country in question,
limiting the peak rates to the selected partner operator. By pooling
the spectrum, a much enhanced service is available and use of MOCN
technology would allow all operators to provide service from this same
eNodeB in a location where it was previously uneconomical to deploy.
Using the catalyst of emergency services coverage to improve the mobile
broadband offerings in rural locations can only improve the economics
and personal well being of people living in these remote locations.
Figure 10. Rapidly deployable macro eNodeB on a trailer
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Conclusions
Public safety networks provide communications for services like police,
fire and ambulance. In this realm, the requirement is to develop systems
that are highly robust and can address the specific communication needs
of emergency services. This has fostered public safety standards – such
as TETRA and P25 – that provide a set of features not supported in
commercial cellular systems. TETRA and P25 networks are implemented
in low frequency bands for better coverage, often using the 400 MHz
band range. The main disadvantage of the current systems is very limited
data connectivity. The supported data rate can be less than 10 kbps and
even in the enhanced TETRA specification the data rate is around 150
kbps. For evolution of public safety networks over mobile broadband, LTE
has been the technology of choice.
Best effort broadband data
LTE
TETRA
or
P25
Prioritized broadband data
Pre-standard
PTT
3GPP Rel-13
MCPTT
Pre-standard
interworking
3GPP Rel-13
interworking
Mission critical communication
Figure 11. Evolution to LTE based public safety services
The evolution from current narrowband systems to LTE based public
safety will take several years and will happen gradually. During the
transition period, public safety agencies are expected to use existing
TETRA and P25 systems in parallel with LTE based systems. The first and
the simplest step is to rely on TETRA and P25 in mission critical voice
and messaging, while LTE can offer enhanced data services, potentially
with slightly lower reliability. Initially officers will use separate TETRA
or P25 and LTE smart devices, but at some stage, device vendors may
implement multimode devices supporting several technologies in the
same device. In the distant future we assume that TETRA and P25
technologies will no longer be maintained and all public safety service
requirements will be fulfilled by LTE networks. Service interworking will
be crucial in the evolution to public safety solutions based on LTE alone.
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References
TS 23.4683GPP TS 23.468 Group Communication System Enablers
for LTE (GCSE_LTE); Stage 2
TS 23.303
3GPP TS 23.303 Proximity-based services (ProSe); Stage 2
TS 23.203
3GPP TS 23.203 Policy and charging control architecture
TS 22.1793GPP TS 22.179 Mission Critical Push to Talk (MCPTT);
Stage 1
TS 22.3463GPP TS 22.179 Isolated E-UTRAN operation for public
safety; Stage 1
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