Migration to the 900 MHz "A Block"

The 900 MHz "Spectrum Swap"

The legacy ATCS network has operated on six 12.5 kHz channel pairs in the licensed 900 MHz band. Operating with FSK modulation, this radio network is constrained to relatively low data throughputs suitable for applications such as CTC (Centralized Traffic Control). As individual railroads were assigned to operate on a specific channel pair, there was no potential reuse of any other available channels even when adjacent channels are unused or inactive. This rendered the legacy ATCS system to be a low throughput, purpose-built network.

As as result of the FCC Report and Order of May 13, 2020, AAR railroads are in the process of vacating the current six ATCS channel pairs operating in 900 MHz spectrum in exchange for ten new channels pairs in what is known as the 900 MHz A Block (“A Block”). The new A Block includes 250 kHz of spectrum – approximately 70% more bandwidth than the current ATCS spectrum. With the intention to become an "interoperable 900 MHz" that could be used for a variety of advanced applications, the industry recognized that a new standard needed to be developed so that all railroads could utilize this new network. Specifically, new modulations and spectrum aggregation technologies would allow the railroads to use existing frequencies more efficiently to allow the addition of new applications.

This new standard is IEEE 802.16t.

Legacy ATCS Frequencies

"A Block" ATCS Frequencies

802.16t - From Single Purpose to General Purpose Networks

The IEEE 802.16t standard is an open, interoperable standard that facilitates the deployment of secure, high-throughput private wireless networks over the narrowband channels available to the railroad industry. Unlike purpose-built legacy wireless networks, an 802.16t network can simultaneously support multiple applications such as train control, crossing alarm monitoring, and PTC system performance reporting while maintaining priorities for each type of traffic. Coupled with the new availability of the 900 MHz A Block, railroads have the opportunity to upgrade their networks with additional features such as advanced defect detection that are not supported on their current networks.

While the legacy ATCS channel pairs typically supported a single application (radio codeline), an 802.16t network supports end-to-end IP connectivity, allowing for a variety of applications to be deployed over the A Block with support for Quality of Service and security.

Maximization of Channel Utilization

Through the use of the Base Station Controller (BSC), an 802.16t network maximizes the use of all channels in a given frequency band by dynamically allocating spectrum to remote radios whenever it is needed. Any channels which are temporarily available may be allocated to other parts of the network to support temporary surges in demand. Thus, 802.16t allows railroads to maximize the utilization of the spectrum they already own rather than having to look to procure more spectrum in order to deploy new applications.

High Throughput Capability

Deploying an 802.16t network over the A Block facilitates the deployment of high throughput communications to support a wide array of safety applications. 802.16t radio technology supports up to 64-QAM supporting raw data rate of 6 bits per second per Hz. 802.16t radio technology also allows a radio to aggregate multiple adjacent or non-adjacent channels to support applications such as large file transfers and over-the-air software updates

Non-Continuous Channel Aggregation

Elimination of Costly Channel Planning

As compared to networks where applications are assigned a fixed channel to operate, remote radios communicating through an 802.16t-compliant base station are dynamically allocated frequencies and bandwidth based on traffic needs and priorities. This enables much higher spectrum utilization as unused channels may be temporarily assigned to other radios which are currently operating under heavier traffic loads.

Seamless Migration from Legacy ATCS Bands to the A Block

The Airlink BCP (Base Communications Package) is a high power, dual mode ATCS / 802.16t base station radio which enables operators to migrate applications from their legacy ATCS network to the new 900 MHz A Block. Airlink BCP is over-the-air interoperable with the Siemens legacy ATCS base station radio and Wayside Communications Package (WCP), enabling it to provide wireless IP connectivity to any wayside location.

The Airlink WCP (Wayside Communications Package) is over-the-air interoperable with the Siemens legacy ATCS base station radios and Airlink Base Communications Packages (BCP). As Airlink BCPs are installed, the Airlink WCP can switch to operate from the legacy 900 MHz frequencies to the new 900 MHz A Block using the IEEE 802.16t standard. This enables railroads to deploy an IP network to any wayside CTC bungalow.

Learn more about our solutions for application in the A Block here.