Software-defined radio comes of age

SDR can also mitigate problems carriers face when switching to a new standard. Generally, capacity is moved to the new standard slowly, so that customers are not forced to immediately upgrade their phones. Limited spectrum availability means the carrier must decide at some point to take away capacity from the old standard in order to add capacity to the new standard. A SDR base station can run two different air standards simultaneously, operating a control channel for each standard and saving an operator from having to make this decision at each tower.

Additional capacity then can be added to each standard on an as-needed basis, changing the number of channels used by each standard dynamically, depending on the number of users requiring voice channels for each standard.

SDR and frequency allocation

Base station hotelling is a new architecture for deploying cellular systems that takes advantage of SDR's flexibility to lower capital costs and make more efficient use of the spectrum. Companies are now separating the base station from the antennas in order to improve coverage in urban areas and add coverage to tunnels, stadiums, and within buildings by putting the antennas where they are most needed. These remote antennas provide the RF spectrum over a fiber optic cable back to a central location where all of the basestation processing resides.

This method also better utilizes base station resources, as channels can be allocated to different locations to match the load as it varies over the course of a day. For example, at rush hour, more resources can be applied to towers on the highway, whereas these same processing resources could be allocated to the downtown office area at other times of the day. It is no longer necessary to outfit towers with capacity for the peak load — capacity that will sit idle during off-peak hours. Adding capacity to the entire system is now as easy as adding a server to a rack in the central location, eliminating a trip to the tower.

Additionally, the benefits of a SDR base station still apply. It is possible to run multiple standards simultaneously from a single hotel site using the same hardware, even supporting multiple wireless services providers from the same infrastructure base. This ability to share infrastructure between standards or carriers greatly reduces capital costs for the providers.

Military

The U.S. military has a significant radio interoperability problem. The story is often told of army troops calling in air support to Grenada using their personal calling cards and using Fort Bragg as an intermediary to communicate. Such problems are avoided with SDR.

Interoperability problems are also an obstacle in joint operations, where each nation typically has its own radio systems. Recently, emphasis on peacekeeping, disaster relief, homeland security and other non-combat military operations has created further problems. In these roles, military units must communicate with public safety agencies, humanitarian organizations, and the civilian population. A single SDR device with the ability to support multiple waveforms significantly reduces the number of devices needed in the field. For military users, who must maintain, transport, power, and manage each device under challenging operational conditions, the benefit of a streamlined system is substantial.

SDR also promises to reduce military radio development and acquisition costs. Without SDR, new device development requires investing anew in the implementation of each supported communication standard. With SDR, the bulk of the implementation knowledge for a communication standard is captured in portable software, which can then be reused at low cost in new or different platforms. This software reuse holds the potential to revolutionize radio procurement economics by significantly increasing competition among platform vendors, leading to reduced per-unit costs.

The United States Department of Defense (DoD) recognizes the potential cost reduction of SDR, and has established the Joint Tactical Radio System Joint Program Office (JTRS JPO) to achieve that goal. The JPO has begun to acquire software implementations of a first set of 33 communication standards. The linchpin of the JTRS effort is a software standard called the software communications architecture (SCA), intended to ensure portability of the implementations across platforms from many vendors. The SCA standardizes the software's operating environment, and the control and communication mechanisms for both the hardware and the external interfaces of the radio. Many NATO allies have signed agreements to apply the SCA in their future acquisitions, and the JPO intends the SCA to become the basis for commercial SDR software standards as well.

Public safety interoperability

Public safety agencies in the United States struggle with interoperability problems when collaborating with other public safety agencies.

In major emergency situations, from floods to plane crashes, a large number of local, state and federal agencies respond to the scene. Their incompatible radios, ranging from legacy analog FM systems, to digital trunked radios and even commercial cell phones, leave them unable to efficiently communicate with each other. Decentralized purchasing decisions lead to different agencies within different municipalities acquiring the systems that best meet their particular needs. As a result of this acquisition process, their voice and data communications systems cannot interoperate, and their databases cannot share information.

In a mutual aid scenario, multiple agencies (some which may work together infrequently or may never have worked together previously) must work together and communicate with little opportunity for prior planning, frequently outside the range of fixed communications infrastructure and in difficult terrain. It is unknown when and where the response will be necessary, and who will be involved. Ensuring interoperability in this context requires an extremely flexible and rapidly deployable solution. The Federal Emergency Management Agency has identified radio interoperability as the one item that could have made the most significant difference in the rescue and cleanup effort after the 9/11 disaster.

SDR's position

SDR is currently the best solution to solve the communications interoperability problem by providing an immediate, cost-effective solution that does not require the agencies to purchase new radios. A portable SDR device brought to an emergency scene can enable interoperability between selected members of different agencies by creating communication links between different radios and establishing infrastructure where none exists, or supplementing inadequate existing infrastructure by serving as a basestation.

The unplanned nature of an emergency requires extremely flexible radio systems that are able to adapt to the situation's communications needs, making SDR the ideal technology for public safety radio systems.

Conclusion

Software Defined Radio has been in development for many years, but is only now achieving commercial viability thanks to advances in integrated circuits. Initial SDR systems will likely appear in specialized areas such as military and public safety applications; visibility in consumer markets will follow.

Eventually, as SDR is deployed in infrastructure and consumer devices, consumers will benefit from improved wireless coverage, and new services offerings.

The extent of SDR progress will be decided largely by RF technology. Digital processing exists today with enough capacity to handle many different waveforms, but SDR is limited by specialized RF chipsets that are optimized for particular frequency bands and waveforms. Once economic RF designs are developed that are tunable over broad frequency ranges and can handle several bandwidths, SDR will be able to truly demonstrate all of its advantages.

Jeffrey Steinheider is a member of the technical staff at Vanu Inc., where he has been developing software radio systems. He received his M. Eng. and B.S. in Electrical Engineering and Computer Science from M.I.T. He can be reached at [email protected].