Software Defined Radios in the 5 GHz band

Software Defined Radios (“SDRs”) are becoming an increasingly larger share of transmitting devices. SDRs offer benefits to consumers, businesses, the SDR manufacturers, and to regulators. This article provides a brief overview about SDRs.

SDRs control radio emissions using software. As a result, for a given SDR equipped with a particular chipset, an SDR can be programmed to operate on various frequencies. For example, a wireless local area network (WLAN) 802.11 a/b/g radio operating at 5.15-5.35 GHz could be upgraded via a software change to transmit on the new 5.47 – 5.724 GHz band or the 4.9 GHz band. Where regulations permit, the end user can update the software so that the radio can use new bands without having the radio shipped back to the factory. This gives the consumer a radio with improved utility, while saving time and money, and avoiding distruptions to service.

U.S. WLAN as Test Bed for SDR

As a result of the World Radio Conference 2003, a number of countries have adopted requirements that WLANs operating from 5.25 – 5.35 and 5.470 GHz avoid radar systems (using a technology known as dynamic frequency selection which requires WLANs to change channels when they hear radar systems operating). Only in the US are WLAN radios allowed to be approved and operated as SDRs, under U.S. Federal Communication Commission regulations adopted in 2001 and updated in 2004.

From a regulator’s perspective, the main issue of SDR approval is to verify that only approved software that controls radio emissions can be uploaded and that the software does not allow unauthorized modifications that would allow the operate the radio in an unauthorized manner (e.g., wrong frequencies, wrong power levels).

Speeding innovation to the marketplace

One way in which SDRs have been beneficial is in speeding innovation to the market. For example, although the U.S. FCC had adopted rules for the new 5.47 – 5.725 GHz band and set a date after which equipment would be allowed to operate there, the FCC was forced to postpone the date by 18 months during an extensive review of equipment certification and testing rules. During this period of delay, Cisco had developed products that could operate in the new band. But the new band was not yet open and available.

Rather than delay new WLAN product, Cisco certified the radios as Software Defined Radios and allow them to be shipped: (1) using the available 5.15- 5.35 GHz frequencies, (2) but with the 5.47 – 5.725 GHz frequencies and features disabled. Once the FCC issued its final rules opening up the 5.47-5.725 GHz band, Cisco was able to apply for, and receive, an SDR upgrade, thus allowing our customers in the field to upgrade their systems without major disruption to their networks. The alternative under a non-SDR approval would be for the systems to be upgraded by the factory, expensive for both the manufacturer and customer.

This not only allowed Cisco customers to upgrade the 5Ghz products in the field with little impact, but it also provided more economical incentive to upgrade the products to support DFS functionality for legacy systems operating in the 5.250-5.350 GHz band.

David A. Case NCE, NCT
Technical Leader Regulatory
Cisco Systems