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The Promise of LTE: Real-World Performance for Public Safety LTE Networks
With broadband spectrum earmarked in the 700 MHz band for public safety use, government agencies in the United States have chosen LTE as the air interface technology of choice to bring mobile wireless devices and services into their day-to-day operations. Understanding the design considerations, limitations and realistic performance of LTE is critical for the successful deployment of next-generation networks because not all solutions are created equal—or live up to their claims.
Demand for data services continues on a very steep growth curve and today’s narrowband networks are quickly running out of capacity. New spectrum and technology are needed to deliver higher throughput, better performance, and to help public safety agencies deploy the mobile broadband services they need to enhance their operations. LTE is the hottest topic right now for mobile broadband service. But how much is hype and what can we realistically expect when it is deployed for public safety? This article will discuss some of the key factors that can affect LTE performance in a real-world public safety deployment.
LTE is an OFDM, all-IP, next generation technology that achieves fixed-line broadband performance while enabling new, rich media services and a reliable mobile experience. But how do you know LTE will deliver on its performance promises? Peak data rates are often used to create excitement about the performance capabilities of every new generation of wireless data technology. These are often theoretical measurements and not realistically achievable on a live network. Peak data rates don’t take into account factors like traffic load, fading, attenuation loss and the signal-to-noise ratio that have an impact on the end user data rate.
Because we don’t know where the next public safety incident is going to occur, public safety agencies should instead look to define the minimum acceptable performance level throughout their coverage area, then design a system to meet that minimum performance level. Typically, the geographic area near the edge of site coverage is where the network delivers the lowest data performance rates. Designing a network based on the minimum throughput at the cell edge will likely yield the most reliable performance throughout the entire coverage area.
Practical throughput in RF systems is influenced by several factors. Perhaps the most significant of which is spectrum. The amount of spectrum has a direct correlation to the overall achievable data rates. Simply put, more spectrum provides better performance. But what data rates can we expect from LTE? Why is the stated performance different from paper to paper, vendor to vendor? The answer to this is often in understanding what is being stated. LTE performance can be measured differently, which can make it hard to understand what to expect in the real world. The measurement used often is dependent on the context and the purpose of the measurement.
•Physical Layer Peak Data Rate – This is a fixed measure based on the physical layer and determines the actual capacity available per sector at the base station without any error coding, media access control (MAC), signaling, or overhead due to User Datagram Protocol (UDP) / Internet Protocol (IP) or Transmission Control Protocol (TCP) / IP that enable applications access. It does not account for any data correction techniques, signal quality, interference, scheduling, terminal performance or mobility. This is the highest theoretical capacity and the cleanest way to compare technologies side by side. It does not represent the user experience.
•Application Layer Peak Data Rate – The application layer represents the top of the Open System Interconnection (OSI) reference model layer. The application layer peak data rate achieved here assumes there is only a single user on the network with the best possible atmospheric conditions. The rate is dependent on the type of error rate coding type applied on the link.
•Average Sector Throughput – This is the aggregate of individual user data rates and is used to quantify the total capacity of a site or sector. In other words, the total number of bits capable of being delivered to users distributed through the sector coverage area. This measure represents the capacity of a sector serving all users in a real-world environment. Commercial cellular operators with a widely dispersed customer base will often design their networks for average sector throughput.
•Coverage Edge Throughput – Edge throughout is quantification of the performance for the individual user and is typically specified for a percentage at the edge of coverage, such as the outer 5 percent ring of the total coverage area. This is important as it specifies a “minimum” data rate that a user will experience throughout an entire coverage area.
Designing An LTE Network
LTE features a modulation technique that is robust in the presence of a weak signal or interference at the coverage edge, trading better signal recovery with lower throughput. Assured minimum throughput rates per user up to and including the coverage edge is variable in LTE system design. Factors such as tower distance contribute to determining throughput, but also affect deployment costs. Understanding minimum end user requirements helps better identify all associated costs required to serve a defined user base.
It is important to consider the number and types of applications needed to be served within a geographic area. If a large-scale incident occurs, how many first responders might need broadband service and what types of applications will they be using? The answers to these types of questions need to be considered to properly design a Public Safety LTE network with sufficient capacity anywhere the incident occurs.
In the public safety environment, the largest load-demanding network requirements come from multi-response incidents. Multiple users at one location, each vying for sector bandwidth can have a noticeable effect on communications and overall system performance. LTE uses advanced Quality of Service (QoS) to enforce service priorities and maintain a defined level of performance. In a public safety operation, QoS can dynamically prioritize service based on incident, jurisdiction and role. Agencies have the advantage of a higher level of control over the network resources to ensure the most critical information at the moment is delivered when it is needed most.
During the heat of an incident, first responders need instant access to the most critical information. The LTE technology provides an ideal architecture for latent sensitive applications. LTE uses flat all-IP architecture to reduce the number of components between the user and the application. The result is broadband technology capable of delivering real-time video and voice services with an exceptional user experience.
A Smart, Future-Proof Network
Investment Mobile wireless broadband is more than just peak data rates and throughput. The true performance of a wireless broadband network is whether it can deliver the applications you need when you need them. Carefully considering your need for performance and application support throughout the coverage area will result in a broadband service that better fits the needs of your organizations and the citizens it serves.
Rishi Bhaskar is a Vice President with Motorola Solutions.
Published in Public Safety IT, Nov/Dec 2011
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