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China's TD-SCDMA readies for Olympic debut

Posted: 22 Jul 2008 ?? ?Print Version ?Bookmark and Share

Keywords:China 3G? wireless standard? TD-SCDMA? Beijing Olympics? teardown?

Having completed its trial evaluations in 2007, China's homegrown 3G wireless standard, TD-SCDMA, is ready to make its debut at the 2008 Beijing Summer Olympics.

TD-SCDMA represents China's efforts to reduce dependence on Western technology for next-generation cellular communications, although in fact that effort can be claimed only partially successful in the Hisense T68 cellphone. By developing TD-SCDMA, China hopes to bypass much of the royalty stream associated with wideband-CDMA, the more broadly deployed global 3G protocol. That said, the presence of the "CDMA" in the standard's protocol designation tips the use of spread-spectrum communications at some level, so perhaps the intellectual-property issues and costs will not fully be dodged.

TD-SCDMA implements a hybrid of time-division (the TD part) and frequency-division (the CDMA part) multiple access in an effort to get the benefits of dynamically allocated time slots for downlink and uplink, to support varying traffic asymmetry. Additionally, the downlink and uplink traffic can be multiplexed onto the same carrier frequency (vs. needing paired spectrum), resulting in more flexibility in channel allocation.

Within a time slot, communications use a synchronized CDMA technique. The scheme improves spectral efficiency, since multiple users can share the same channel as long as their chipping codes are suitably different to allow isolation of the distinct traffic within the common slice of spectrum.

The first examples of TD-SCDMA hardware also support legacy GSM communications as a link to the past, just as UMTS combines GSM and W-CDMA elsewhere to allow more-seamless use of old and new.

Hisense T68 is a GSM-TD-SCDMA bar-style handset, keeping assembly complexity fairly low. (Click to view larger image.)

Hisense T68
One such dual-mode GSM-TD-SCDMA handset is the Hisense T68. The phone is a bar-style handset, keeping assembly complexity fairly low. An internal frame supports the circuit boards, display and battery, while a single front shell and two-piece back shell supply decorative fascia. Most of the electronics reside on a single circuit board that runs the length of the phone. But a separate keypad board is used, in part because the electronics leave limited room for an in-built keyboard array once "dead space" for the display clearance is allocated. The display itself is a TFT LCD from Lead Communications measuring 2.4 inches on the diagonal, overlaid here with a resistive touchscreen.

Looking at the primary circuit board, the name Analog Devices hits like a ton of bricks. Despite the China-driven origin of TD-SCDMA, it seems ADI is the semiconductor force in rendering the standard to silicon hereor was. In late 2007, ADI sold its cellular chipset to Taiwan's Mediatek for $350 million, so these are legacy components, albeit a recent legacy. In fact, date codes suggest a late September or early October manufacture, timed about even with the Mediatek acquisition. So we'll refer to the devices as Mediatek's, despite the outward branding.

Starting at the baseband, the Mediatek AD6903 puts two slices of silicon on one package. The larger one is likely a Blackfin-based DSP supporting communications processing for both GSM and TD-SCDMA. The second, smaller chip would, by process of elimination, contain the ARM9 controller core and other digital aspects of the system, such as USB and memory interface. In that vein, the part is directly supported by a Numonyx (formerly Intel) two-chip memory stack with 64Mbyte NOR flash and 16Mbyte of pseudo-SRAM, along with a user-upgradable MicroSD card slot.

For the primary analog functions, the single-chip AD6857 takes care of both standards' analog/digital interfaces between radios and digital basebands, along with power management and much of the audio subsystem.

Despite the China-driven origin of TD-SCDMA, it seems ADI is the semiconductor force in rendering the standard to silicon. (Click to view full teardown)

Moving on to the radio, the duality of design becomes more apparent. Essentially, there is a dedicated transceiver for GSM in the AD6549, with an RF Micro Devices RF3166 RF power amp completing the GSM signal chain. TD-SCDMA requires a separate radio component set and splits the transmit and receive paths into separate devices. As with CDMA and W-CDMA devices early on, the challenges of spread-spectrum design have prevented monolithic integration of uplink and downlink radio traffic. The AD6541 handles transmit modulation, while the AD6547 tackles receive down conversion. The former part is followed by an RF isolator and Skyworks' SKY77161 TD-SCDMA RF power amp.

Both radio chips are based on a direct-conversion architecture and contain on-chip voltage-controlled oscillators, filters and localized power regulation to keep external component count low as an offset to the two-chip implementation. A Murata-manufactured RF front end is responsible for all the mode and time-division switching associated with both protocols, serving as the RF signal traffic cop between the radio subsystems and antenna.

Other devices support audio output (AKM AK4642), Bluetooth (CSR BC41B143A-BluCore4) and touchscreen control (AKM AK4183). The MV8602 media processor from MtekVision offloads imaging-related tasks stemming from the T68 phone's 1.3Mpixel camera and functions as the LCD interface.

TD-SCDMA may have Eastern roots, but the independence sought with the standard has yet to be fully realized. That may change now that China's Spreadtrum is getting into the TD-SCDMA processor game. In the spirit of that competition, let the (other) games begin!

- David Carey
President, Portelligent





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