Originally posted in Penton’s Wireless Systems Design magazine, June 2000
By John Blyler, Senior Technology/West Coast Editor, Wireless Systems Design
I was chatting with a few colleagues over pizza the other day when someone asked about multiple protocol designs in wireless devices. I assumed he meant multi-mode RF designs in communication devices, like cellphones, that will support both GSM and CDMA. As I started to talk about multi-mode issues, though, his raised eyebrow and slight smile told me I was off base. He continued to look at me oddly for a few seconds before explaining that his concern was about a two-protocol notebook that would run both 802.11b for wireless LAN connections, as well as Bluetooth for access to other devices on a personal area network (PAN).
It occurred to me that here was yet another challenge to be addressed before the much-touted convergence of communication and computational devices would become a reality. The former dealt with transmission protocols such as GSM and CDMA that would traditionally require two sets of RF systems to handle each of these competing protocols. Computational devices, however, needed to connect to a wired network infrastructure, using protocols like 802.11b or one of the PAN standards, like Bluetooth. This required separate RF subsystems as well, but the implementation was quite different.
The most common practice used to support different wireless access technologies in mobile computing devices, like laptops and PDAs, is to integrate 802.11 support into the device while providing Bluetooth access via an interface card. Several companies, such as Compaq and Toshiba, are beginning production on wireless-enabled notebooks with integrated 802.11b capabilities. Bluetooth would then be supported via a PC card. This approach acknowledges both the maturity of 802.11b and growing support for Bluetooth.
The operational coexistence of these two RF sources is not without its share of problems. Potential interference challenges between 802.11b and Bluetooth, when operating simultaneously on a single device, may mean that both radios cannot operate effectively together. If they do, then degradation in speed or performance may result. For example, if Bluetooth is receiving or transmitting, then 802.11b may operate at a reduced rate.
Designers of mobile computing devices are working on several approaches that will minimize the interference effects of these two transmission technologies. Since RF interference diminishes with the increase in distance between competing sources, one solution is to position each RF subsystem as far apart from one another as possible. Admittedly, the delta distance between RF sources in a laptop or PDA is small, but every little bit can help.
Another consideration is the design issues associated with the performance, type, and position of the antenna. (See the June issue of Wireless Systems Design magazine.)
Perhaps a better approach is to design the system with both RF sources in mind, i.e., a systems approach. Design options will allow switching back and forth between the two protocols in a more or less seamless fashion.
OK, so much for the competing RF problems on the mobile computing side. How about similar issues with mobile communication designs that support GSM and CDMA, for example? A traditional technique is to implement multimode RF subsystems through the use of distinct transceiver chains, i.e., a separate set of circuits for each mode. Not surprisingly, this is a costly approach that increases the chip count in the final product, thus increasing the size, weight and complexity of the device.
RF designers have long known that eliminating the Intermediate Frequency (IF) conversion stage of the receiving subsystem could decrease power consumption while reducing the form factor of the circuit. The catch has been that, in a traditional superheterodyne radio architecture, most of the IF conversion is performed with analog circuits. While effective, analog components often consume more power and are larger than their digital counterparts. Companies like Analog Devices and Texas Instruments have developed high-performance DSP chips that offer a digital solution to handling the IF conversion. Recent advances in reconfigurable hardware (see the June 12 DSP Alert) offer yet another solution to digitizing traditional analog RF functionality.
A new approach eliminates the need for IF conversion all together. Parker Vision’s RF transceiver implements a hardware-software solution, through their Direct2Data (D2D) technology, that removes the need for IF conversion in many applications.
In terms of actual ICs, several vendors are beginning to offer products that provide multi-mode RF solutions. Tropian Inc., for example, has introduced TimeStar, the first multi-mode RF transmitter in a single chipset. New products from many vendors will follow quickly in the coming months and years, as the rollout of 2.5G and 3G technologies become a reality.
But the question of interoperability of multi-mode RF communication systems and multi-protocol mobile computing devices remains largely unanswered. Nevertheless, convergence of these technologies will require a comprehensive answer in the near future. We’ll keep you posted.