The State of Qualcomm's Modems - WTR1605 and MDM9x25

MDM9x25

After the transceiver (and its downconversion from RF to baseband for receive and upconversion from baseband to RF for transmit) comes the digital baseband. In the case of Qualcomm’s architecture, there are two Rx output pairs for I/Q data, and one transmit port. This shouldn’t be surprising since again only two receive ports are needed at maximum to do the 2x2 MIMO modes for LTE or receive diversity. Even though Release 8 supports up to four layers to be transmitted on the downlink, virtually all handsets and data cards use 2 layers at most at present, and this will be the case with MDM9x25 as well. Going up to four layers will pose interesting challenges for handset design where getting enough sufficient inter-antenna spacing to yield good gains in throughput (and independent streams of data) is already a delicate balancing act. Of course operators will need to deploy 4 antennas per sector as well for 4x4.

MDM9x15 inside Nexus 4

We’ve seen the first and second generations of Qualcomm’s LTE basebands, first the MDM9x00 series which was 45nm, and now devices are making it onto the market with MDM9x15 (and its LTE-less sibling MDM8215) series which is 28nm and natively voice enabled. The major improvement between 9x00 and 9x15 was of course lower idle and active power consumption, and a smaller package thanks to the change in process geometry. Additionally, MDM9x15 no longer requires a Qualcomm SoC paired with it to be voice enabled, which opens it up for use in platforms like the iPhone where an OEM has a specific non-Qualcomm SoC it wants to use. This same IP block is again shared with MSM8960 and a few other SoCs. Likewise, in the future MDM9x25 will share an IP block with the MSM8960 successor I alluded to earlier.

Enabling voice and all the legacy fallback modes required for it is a huge task, and Qualcomm believes it has significant leadership by supporting all the combinations of handover and fallback modes required for support of voice services. This includes CSFB (Circuit-Switched Fallback), dual radio (1xRTT alongside LTE), and VoLTE (with and without SRVCC).

Recently Qualcomm announced that its third generation LTE baseband, MDM9x25 had begun sampling to device makers, and this is a particularly interesting part since it’s Qualcomm’s first LTE UE Category 4 baseband. Just like in the WCDMA release, the 3GPP has UE Categories for LTE which define what capabilities a given device has. Because LTE has variable channel bandwidths, this time around UE category is defined based on the number of resource blocks and spatial streams a device can support. In UE Category 3 this corresponded to 100 Mbps maximum on a 20 MHz downlink channel, in category 4 this is the full 150 Mbps - 100 resource block allocation - for 20 MHz channels.

At the same time, 20 MHz of contiguous spectrum is difficult for operators to come across in most regions, thus the 20 MHz FDD channel bandwidth that is supported in LTE isn’t widely used in some major markets where LTE is deployed at present. The mitigation is to allow for carrier aggregation in LTE similar to carrier aggregation for DC-HSPA+ (dual carrier). Currently shipping DC-HSPA+ configurations require carriers to be adjacent to each other in order to be aggregated, and this requires contiguous spectrum (in which case the operator could just run that channel bandwidth of LTE in the first place). What’s new in Release 10 and will be supported on MDM9x25 is inter-band and intra-band aggregation for both WCDMA and LTE. That is, aggregation of LTE carriers that don’t need to be next to each other and instead can be inside the same band (continuous or non continuous), or across multiple bands, for a number of configurations. This allows wireless operators to piece together enough bandwidth from their spectrum holdings across bands to get performance on par with one bigger contiguous carrier, for example 10 MHz FDD + 10 MHz FDD aggregation to emulate 20 MHz FDD performance. Just like in WCDMA, uplink remains unchanged in an FDD scenario, these carriers aren’t aggregated, but most of the time the traffic profile on cellular networks is similarly asymmetric to begin with. I’m told that MDM9x25 is capable of supporting aggregation of even a third WCDMA carrier which is another 3GPP proposed mode.

WCDMA also gets an improvement from the LTE side of things, support for 2x2 MIMO which increases the theoretical maximum bitrate on the downlink to 84 Mbps from 42 Mbps.

MDM9x25 should be an exciting part to keep an eye on. We’re still a ways off from seeing carriers light up LTE Advanced (release 10) features, and similarly still a ways off from seeing MDM9x25 in devices, though it could show up in tablets or be targeted at high end smartphones. As always, understanding some of the players roadmaps helps get a better grasp on what’s in store in the future.

Conclusions

The mobile device industry has come a considerable way in just two years. Previously, getting this kind of open disclosure about RF architecture and ports was largely unheard of. On the modem side, both operators and handset makers have considerable interest in making sure that the baseband is as close to a black box as possible for security reasons (security through obscurity at its finest). The platform architecture of a handset with clear separation between AP and modem as this detached and separate means of getting data, voice, and SMS also inherently fosters a black box approach to the whole cellular connectivity side. There are still more questions to answer and even more areas of the smartphone platform that should make it into daylight and out of from behind walls of NDAs, but this is a great step in the right direction. The ultimate goal for me is to have the equivalent of the transceiver and modem table filled out for some of the popular products from the other major vendors and gain a better understanding of the entire space.

The next topic is just what impact the introduction of WTR1605L and MDM9x25 will have on the space. WTR1605 introduces sorely needed additional ports which can be used for additional LTE bands. The reality of LTE at the moment is that the number of bands being proposed for 3GPP releases is only continuing to increase. Additional primary ports does in turn mean OEMs can choose to include maybe one or two more LTE bands, but roaming on all of the popular bands still is an unsolved problem. We’ve already seen designs including WTR1605 on the market, none of which have really gone above and beyond with more LTE bands. At the same time I expect to see devices with band 12/17 and 13 coexistence start popping up. Of course the TD-SCDMA story is perhaps the most under appreciated aspect, as China Mobile presents a market whose size is almost staggering in scale that everyone wants a piece of.

The reality is that the industry still needs more time for the LTE band landscape to settle down, refarming of existing 2G and 3G spectrum by operators, in addition to even more band support on handsets to enable one SKU solutions.