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Architectures for mobile RF convergence

Posted: 01 Feb 2006 ?? ?Print Version ?Bookmark and Share

Keywords:pieter hooijmans? philips? mobile? rf? cellphone?

As mobile devices such as cellphones, pdas and laptops acquire multiband, multimode wireless connectivity, there is an ever-increasing need for greater levels of RF integration. Space, cost and power-consumption constraints will no longer make it viable to have a separate wireless transceiver for each communication pipe.

The latest GSM phones are already quad-band, giving users extended coverage on 800-, 900-, 1,800- and 1,900MHz GSM bands across five continents. Many also incorporate Bluetooth technology for headset and SIM card synchronization functions. They will soon acquire Wi-Fi connections to take advantage of VoIP connections and receive digital terrestrial television, digital audio broadcasts and GPS satellite information. After that, it will be UWB for wireless USB and WiMAX for mobile Internet access. Convergence will mean that not only mobile phones will sport these multiband, multimode wireless capabilities, but other combinations will also appear in PDAs, laptops and game consoles. Typical mobile devices may thus need to receive wireless transmissions over a total bandwidth of nearly 6GHz.

The current technique of devoting a separate transceiver with its own RF and baseband circuitry to each communication pipe works well, provided that the number of communication pipes is small. The increased integration density made possible by 90nm CMOS, passive-component integration techniques and RF SiP technology has reduced the size and power consumption of these transceivers to the point where two or three can be accommodated inside lightweight handheld portables.

However, as the number of wireless communication pipes increases, this approach will no longer be possible. Not only will the aggregate size of the required modules become difficult to accommodate, but their combined power consumption will threaten battery life and the increased silicon content will adversely impact product cost. It will also become increasingly difficult to ensure coexistence of the different radio channels due to the required number of interfering antennas.

Partitioning is key
One way of reducing the antenna problem is for communication pipes that operate in the same frequency band to share the same antenna. It would be logical, for example, to share a single antenna between Bluetooth and IEEE 802.11b/g Wi-Fi transceivers, both of which operate in the 2.4GHz band. These two interfaces go naturally together in mobile phones anyway, since both will be required to deliver VoIP servicesWi-Fi to connect into a LAN, and Bluetooth to maintain a headset connection.

However, as convergence gathers pace and many of these communication pipes become standard features, the issue of how RF and modem functionality should be partitioned becomes less clear-cut. Modern baseband modems operate in the digital domain, either using a dedicated hardware or a DSP, with the necessary ADCs and DACs positioned between the modem and the RF transceiver. Thus, it makes sense to shift their functionality either into the host's baseband chip or to at least aggregate the modems into a separate connectivity modem engine.

This reduces chip count and allows modem and baseband functionality to be quickly migrated from one CMOS process node to the next, thus reducing silicon area and cost. For 2G/2.5G/3G mobile-phone transceivers, this is still likely to be a BiCMOS process for some time to come, although the migration to RF CMOS is beginning today in the 2G segment.

The alternative is to integrate both the modems and their associated RF transceivers into one large CMOS chip. Although the advent of RF CMOS means that RF transceivers can be integrated into CMOSat least for lower-complexity wireless links such as Bluetooth and IEEE 802.11bit is a high-risk approach for multiband, multimode solutions. It has already taken the industry several years to workably integrate a single Bluetooth transceiver onto a single piece of silicon. Integrating multiple transceivers only millimeters apart on the same chip will introduce a totally new set of problems, especially when several of these transceivers will need to operate simultaneously. In addition, migrating the combined RF/digital CMOS design to next-generation CMOS processes will be harder than migrating a purely digital design. RF CMOS does not scale in the same way that CMOS logic scales and its performance after migration is less predictable. As a result, a significant amount of redesign is likely to be needed.

Thus, RF+BB SoCs will likely be a short-term solution, applicable during a phase where few additional communication pipes are connected to the cellular BB host. Over time, however, these SoCs will not support the required level of system integration.

Standard modem interface
In partitioning RF and modem/baseband functionality into separate chips, what will be needed is a cleaner and more standardized digital interface between them. Such an interface will allow a single software-programmable modem to service different RF transceivers. In the same way that antenna sharing may dictate system partitioning on the RF transceiver side, modulation schemes could dictate system partitioning on the modem side. Narrowband modems such as those required for Bluetooth and GSM/GPRS are typically hardwired. More complex modems, such as those required for OFDM/CDMA, are typically implemented in vector DSPs. It will be a standardized interface between RF transceivers and modems that will allow modem partitioning to be exploited.

Integrating RF with RF, modem with modem, and application processor with application processor makes sense rather than trying to integrate RF transceivers, modems and applications engines on the same chip and facing the challenge of keeping the RF CMOS up to speed with baseline CMOS process development. The Mobile Industry Processor Interface Alliance is already establishing specifications for standard hardware and software digital serial interfaces to mobile application processors. A set of standard RF-to-modem interface specifications similar to the DigRF and DigRF3G specifications for mobile phones is neededit should cover a wider range of wireless communication standards.

Reconfigurable radio
The flexibility and software programmability afforded by this partitioning will also provide a migration path to another goal of the future, the concept of "reconfigurable radio." Regarded as one of the most promising ways of meeting the demands of ubiquitous wireless communications, reconfigurable radios will allow the same transceiver and modem chain to be reconfigured under software control to switch between different frequency bands and modulation schemes.

One of the biggest challenges in implementing this will be the replacement of fixed frequency filters by switchable/tunable filters. To maintain the integration levels required in mobile devices, this will involve the development of new RF MEMS devices. It will also require transceiver chains to become much more highly digitized than they are today, not only to present a digital interface to the modem, but also to allow transceiver performance to be dynamically optimized to the requirements of different modulation schemes. It is in this area that RF CMOS is likely to play a critical role, together with new transceiver architectures that push the ADCs and DACs closer to the antenna. For the transmit channel, it will also mean the development of class E (switch-mode), class G (rail-switching) or class S (supply-modulated) RF power amplifiers to provide power-efficient, wide-bandwidth solutions.

The advantage of reconfigurable radio is that no dedicated solution is required for each communication standard or combination of modes, allowing designs to be easily upgraded by the addition of identical modules. In practice, such a perfect situation will unlikely exist, but to cover a very wide range of communication pipes with just a few groups of solutions is possible.

- Pieter Hooijmans
VP for RF
Philips Semiconductors

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