Critical Testing Areas for Next Generation 4G/LTE Radio Access Networks

October 17th, 2012

Mark LaPedus and Absolute Analysis

4G networks set to take off

The next-generation, 4G wireless standard known as long term evolution (LTE) is projected to see meteoric growth, but there are several challenges to deploy the networking technology.

In just one part of the critical 4G LTE equation, for example, carriers must ensure the interoperability, performance and reliability in the radio access network (RAN). In one possible RAN topology, the base stations are situated in a remote and central location, which can be miles away from the remote radio head at the cell tower. In 2G and 3G cellular networks, however, the base stations and radio head are typically located near each other, making the network easier to test and debug.

Figure 1: Separation of the RRH and the BBU has increased the complexity of testing and debugging the RAN

“In 2G, that was really easy when the radio head was next to the base station,” said Roger Paje, director of marketing for Absolute Analysis Inc., a Newbury Park, Calif.-based supplier of protocol analyzers and other test equipment. “Now, what they are doing is taking the base stations and grouping them into one centralized location called a ‘base station hotel.’ Now, the base stations exist maybe 40 to 60 kilometers away from the tower. This has created a whole host of timing and other issues that make the network hard to debug.”

Carriers and TEMs may end up spending weeks, if not months, debugging the equipment in the RAN for 4G LTE networks. The complexity of the RAN, coupled with time-to-market pressures, are fueling the need for a new class of mobile access network test solutions.

In fact, amid the deployment of 4G LTE networks, Absolute Analysis itself is seeing a sudden and strong demand for Investigator, a new network test product geared for 4G/LTE RAN communication protocols. In mobile networks, Investigator supports the protocols as defined by the Common Public Radio Interface (CPRI) and Open Base Station Architecture Initiative (OBSAI).

“Cost and bandwidth make this tool a requirement in 4G/LTE applications on the radio access network,” said Paje, who has been involved in the high speed analysis industry for more than 20 years. His current focus is in telecom radio access network testing and validation.

4G/LTE Takes Off
Indeed, the market is taking off for equipment, chips and other products in the 4G/LTE era. The shift towards data-intensive applications like video streaming, multiplayer gaming and others are driving the need for 4G/LTE. With speeds of up to 100-megabits-per-second and latencies in the tens of milliseconds, 4G LTE is theoretically up to 10 times faster on average than 3G, according to IHS iSuppli, a market research firm.

In total, the worldwide 4G LTE subscriber base is forecast to reach 73.3 million in 2012, up 334% from 16.9 million in 2011, according to iSuppli. The 4G LTE subscriber base is expected to reach 205.7 million in 2013, up 181%, according to the firm. Spending on 4G infrastructure equipment is up 132% this year to $8.6 billion, iSuppli added.

The explosion of 4G LTE is also causing a sea of change in the network. In 2G, the antenna is situated on the cell tower. The base station and radio head are typically at the bottom of the tower.

Then, in 3G, some carriers moved the radio head on the tower. To test and debug the network, the network operator would climb up the tower to read the RF signal, an expensive proposition at best.

When this type of network topology started moving towards 4G/LTE, operators took another approach to the problem. The carrier could read the digital RF signal at the bottom of the tower using Absolute Analysis’ Investigator protocol analyzer. “There is a big cost savings,” Paje said. “We have the only box in the world that can read that CPRI data, decode the RF signal and the RRU/BBU commands, and record that trace data around error events on the network.”

The shift towards 4G/LTE adds more complexity to the network. In this topology, carriers are assembling the base stations into a centralized location called “base station hotels,” which are often 40 to 60 kilometers or more away from the actual radio head. “Some people call this a cloud RAN or a cloud radio access network (C-RAN). The baseband hotel is actually a cloud,” he said.

Figure 2: Example of a Cloud RAN or C-RAN

The RAN, or C-RAN, must also meet the bandwidth and quality-of-service (QoS) requirements. One method to achieve this is using a technique called baseband pooling. “What they want to do is if one tower gets overloaded, they want to be able to use the bandwidth of multiple base stations that are located in this central hotel. So, in effect, they can allocate bandwidth to whatever tower needs it,” he said.

The move to 4G will also involve the deployment of heterogeneous networking architectures. This involves a combination of macro and micro base stations, coupled with low-powered small cells. These devices, sometimes called metro cells, could be mounted on mall structures and subway stations to provide augmented coverage for consumers when they access their smartphones or tablets.

Five Areas of Test
To ensure the successful deployment of the RAN, Absolute Analysis said there are five key elements in the test arena: compliance, inoperability, base station to radio head communication, RF modulation, and long-haul fiber performance. “We call them the five areas of testing,” Paje said. “Having the tools to pinpoint the problems in each of these areas is the trick.”

The first step in the test and debugging process is compliance. In one scenario, for example, a carrier may have a base station and radio head from separate equipment vendors, both of which claim to be CPRI-compliant. “But what happens, probably more than 70% of the time, is that the interpretation that one vendor did on the CPRI spec is a little bit different than the interpretation done by the other vendor,” he said.

A related “area of test” is interoperability among vendor equipment. The big question is if a system sends a particular command, does the other device recognize it? And does the device send back the proper response?

Adding to the problem are possible issues in the base station to radio head communications process. In this critical area, Paje said: “In a 4G LTE network, the base station will send a command to the radio head, something like: ‘Can you adjust your modulation?’ The radio head should simply adjust its modulation to some value that the base station recommends. Sometimes, you get into a situation where the radio head will keep adjusting until it is completely uncalibrated. The network operator needs to see if the commands coming out of the base station were correct. But if commands were correct, maybe the radio head is misinterpreting them.”

Another critical “area of test” is RF modulation. And not to be outdone, performance testing must also be done on the long-haul network.

The Proper Tools are Key to a Solution
Enter Absolute Analysis. To solve the multitude of problems in 4G LTE networks, the company has developed the Investigator. The multi-speed, a multi-technology system performs protocol analysis, bit-error-rate testing, compliance testing and other functions. With a 4Gb buffer size and a maximum of 32 ports, the protocol analyzer validates digital RF links, whether they are CPRI, OBSAI, or Ethernet.

The system accelerates the ability to deploy a RAN. The portable unit monitors conversations in the network and pinpoints the failures. “The typical application for Investigator is the lab or the field. For example, a carrier has a base station and a radio head. They need to make sure the base station and radio head will operate before they go out into the field,” he said.

The Number One Problem in Testing a RAN
Needless to say, the task of developing and deploying a RAN is daunting. The number one problem is locating where the problem exists in the RAN, a frustrating process that could waste valuable time.

Not knowing where the problem exists causes two major issues in RAN deployment. When a problem arises, design teams start pointing their fingers at each other. It’s the classic hardware engineer versus software engineer scenario. Hardware integration teams need the proper evidence in order to decide which design team can fix the problem.

“The only way to do that is to monitor the serial link in the network with a tool like ours,” he said. “You plug our box into a link and see what the conversation looks like. We can monitor the conversation and then help device vendors debug the communications and make sure they can interoperate with each other. Its biggest value is to provide the information as to where the problem lies, which reduces debug time significantly.”

With Investigator, the company is at the right place at the right time. “I’d say the industry is in the middle of 4G LTE deployment,” he added. “There is simply a need for our tool in 4G/LTE RAN deployment.”

You can find more information about Absolute Analysis at www.AbsoluteAnalysis.com.