CEVA and ARM Do LTE

If you have purchased a high-end cell-phone or tablet in the last couple of years it probably has LTE, although some carriers try and blur things by showing a symbol like 4G when you are in an area that has LTE despite the fact that your phone does not support it. Don’t you love cell-phone marketing? Talking of which, if a camel is a horse designed by a committee, then Long Term Evolution is an example of a powerful brand name developed by a committee. It could just as well describe an Energy Drink than a mobile standard.

Anyway, it is actually 8 different standards with data rates going from 10 megabits/s down and 5 megabits/s up all the way to 3 gigabits/s down and 1.5 gigabits/s up. At the higher data rates it can use multiple antennas. The above table gives some idea of the complexity. There is actually no separate voice channel as in previous standards, but since carriers will get most of their revenue from voice for the forseeable future, you can expect your phone to hide the fact that it is using voice-over-IP under the hood. To be fair, initially it will be using what is called CSFB (circuit switched fall-back) where the regular voice infrastructure is used for voice-calls (and text messages) and LTE is just used for non-voice data like internet access. But as capacity goes up and carriers need to transition, this will change and VoLTE (voice over LTE) will become more common.

By reputation LTE is a very difficult standard to implement because it requires a lot of processing to get those sorts of bandwidths but the power budget (battery life) has to remain basically the same as prior generation phones. Of course some of the power reduction comes and will come from moving to advanced process nodes but a lot has to come from using advanced digital signal processing. But a single generic DSP is not enough.

CEVA has a white paper, created jointly with ARM, on their LTE solution which involves an ARM Cortex-R7 to handle the higher levels of the stack (2 and 3) with Ceva DSPs to handle level 1 where all the heavy lifting is done. In fact not just one Ceva DSP but 3: an XC4110 and an XC4120 for the receive side and an XC4100 for the transmit side.

These CEVA cores are all VLIW SIMD architectures (that stands for Very Long Instruction Word, Single Instruction Multiple Data) which issue multiple instructions at a time and apply them to multiple streams of data using vector processing units. As you would expect, with an architecture involving 4 main cores and lots of other controllers, the software architecture to make everything work is also complex. In addition, VoLTE increases the complexity since the voice protocols also need to be handled.

The full block diagram gives a flavor of the complexity of an LTE implementation (and ‘full’ is a misnomer, there is a lot more detail underneath the hood). Ceva’s white paper describes the architecture of this LTE subsystem in detail allowing for a low-power rapid implementation of an LTE modem.

I first encountered what is now CEVA when I was in VLSI Technology working in the group that did M&A and licensed technology. Back then CEVA were called DSP Group (in between they were ParthusCeva) and all the cores were named after trees: Pine, Oak, Teak. Somehow XC4120 isn’t quite so catchy. The only one which seems to survive is a family of TeakLite cores. VLSI used them in VLSI’s chipsets for GSM (eventually a single chip baseband so not technically a chipset) and built up a very successful business.