A little over a month ago, Qualcomm flew me out to San Diego to talk all about cellular modem, specifically their baseband lineup, testing, and later their RF and transceiver in what would become their largest RF disclosure ever. In the past few years, we’ve made considerable headway getting SoC vendors to disclose details on the CPU and GPU side of their products, and mobile enthusiasts now are starting to become increasingly cognizant of the SoC inside devices, and in turn the blocks inside that SoC. In a short term the industry as a whole went from smartphones largely being impenetrable black boxes to devices with understandable platforms inside. The days of an OEM not disclosing what SoC was inside a device at all are largely behind us, and for the most part vendors are open to discussing what’s really inside most of their silicon quite publicly.
The last real remaining black box from my point of view is the cellular connectivity side of things. So much of what drives smartphone design and OEM choice lately is, unsurprisingly, how the device gets connected to the cellular network, and baseband remains largely a black box by design for a number of reasons. The focus of this article is specifically about Qualcomm’s newest transceiver, WTR1605L, and some more details about MDM9x25 and MDM9x15.
Before we talk about what’s new, it bears going over cellular architecture at a high level to make things easier to understand, which is something we’ve admittedly never really done. Connecting a smartphone to a cellular network is a complicated goal, but like anything there are really only a few high level functional blocks to worry about. Understanding this architecture at a high level allows one to understand how OEMs build devices with support for multiple bands and modes, and why some designs are limited to one combination of features or another. We talk a lot about cellular baseband, but really this is just one part of the entire cellular chain on a handset. There’s of course an antenna array, and after that a switch or switches, filters for the appropriate bands, and then for receive there’s the transceiver, and finally baseband. Transmit takes much the same path, but instead of some low noise amplifiers in the transceiver, there are appropriate external power amplifiers to boost output signal to a given level for each specific transmit band.
So what role do each of those play? Antenna is pretty self explanatory — handsets have anywhere from one to four cellular antennas, each tuned for a specific band or purpose. Devices with LTE must include at least two receive chains since receive diversity is mandatory, and almost all LTE devices use a 2×1 configuration with two receive chains (in order to do 2×2 MIMO to the base station) and one transmit chain. The transmit chain usually shares the primary receive antenna, thus we see at a minimum two antennas for a normal LTE handset. Simultaneous transmission modes such as SVLTE (Simultaneous Voice and LTE) and SVDO (Simultaneous Voice and EVDO) thus far necessitate an additional dedicated transmit chain as well, bringing the total to three. Occasionally an OEM will also include an additional diversity antenna tailored for a specific band, bringing it to four. The geometry of an antenna fundamentally defines its gain and other characteristics for a given frequency band (wavelength), and is a huge part of the industrial design and form factor tradeoff for handset design. Often these are competing factors in a handset.
Switches are another fundamental part of almost every design, and are used for changing paths between the antenna and transceiver, either through the appropriate power amplifier or appropriate antenna. In TDD (Time Division Duplexing) systems, the switch is also fundamentally important since it is responsible for quickly switching between transmit and receive chains since transmit and receive are on the same band. In FDD (Frequency Division Duplexing) systems, there’s the need for a duplexer which isolates the receive and transmit bands from interfering each other.
Filters (think band pass) reject everything outside a given band and isolate a specific frequency range appropriate to whatever band class is being used. The performance of these filters is a continual subject of both scrutiny and improvement, and defines the size of guard bands in-between carriers, minimize harmonics from causing interference in other bands, and so on. In LTE for example a big concern is band 12/17 and 13 coexistence on the same band, something possible only with the latest filters.
Next in the chain is the transceiver, whose role is ultimately to downconvert the incoming signal on the receive side to I/Q data which then gets sent into the baseband (hence the name baseband, this is the complex representation of the RF signal), and on the transmit side synthesize and mix I/Q data from the baseband into RF signal for transmission. Put another way, transceiver converts from RF to baseband. There are usually a number of ports on the transceiver arranged into an arrangement of high and low bands which are tuned to work best at a given set of frequencies. If you’re not familiar with what I’m referring to when I bring up I/Q data, I encourage you to check out my Veer 4G article where I explain QAM and modulation and the I/Q plot.
Finally we have the baseband, which effectively functions as the controller for power amplifiers, switches, and transceiver and handles all the demodulation of received I/Q data and modulation for transmission. In addition the baseband worries about the layers above physical required to get the phone online, for example signaling required for the particular air interface. I’ve seen people refer to this as digital baseband, baseband processor, and modem interchangeably, it’s the same part they’re referring to. It’s this baseband which, at the end of the day, converts that RF signal into bits for the AP (Application Processor) to deal with. For cellular basebands, this is also where things like voice encoding and increasingly GNSS (Global Navigation Satellite System) reside, as these tasks just end up being yet another process running on what usually boils down to an ARM CPU and some DSP running a realtime OS of some kind. At some level the modem really just is another AP running a different workload with very specific DSP onboard.
In the case of a lot of Qualcomm SoCs (MSMs), the baseband processor sits alongside AP onboard the SoC, but the same design may also be reused and exist in a discrete part as well. For example the baseband IP block onboard MSM8960 is shared with MDM9x15, and there will be another SoC which is analogous with MDM9x25.
The transceiver and baseband combination fundamentally define the air interfaces that a device will or can support, and the number and configuration of bands as well. OEMs can always add support for more bands with a switch outside, but of course switches have their own insertion losses and thus affect link budget.
So that’s a very high level overview of cellular radios today, a subject whose complexity is further increasing with the addition of more and more radios, modes, and bands for even more connectivity on mobile devices. It’s the combination of all of these components that contribute to a given device being able to connect to, say, LTE versus just a flavor of WCDMA, or one band versus another.