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The mobile communications industry is facing an ongoing challenge to provide better bandwidth to a growing number of users. Starting as early as 1991, with the first GSM network providing a data rate of 13kbit/s (full rate speech codec), we have now moved through UMTS and HSPA to new levels of wireless communication bandwidths with the introduction of LTE. LTE – or Long Term Evolution (of GSM) – indicates promising benefits, such as ADSL-like high-speed connectivity, for both operators and consumers.
Clearly, LTE holds significant promise as one of the major mobile communication standards in the near future. However, the introduction of LTE in handsets and data cards is not without technical challenges. A demanding standard with high computational needs, LTE poses a challenge to system architects to efficiently balance three aspects in a mobile terminal: power consumption, system performance, and flexibility.
What’s more: With consumer demand for global service access, mobile terminals will have to continue to support older standards such as UMTS. Add to this, other communication protocols, like WiFi and Bluetooth, which are a given in mid- to high-end phones. As a result, a multi-RAT (Radio Access Technology) approach is required in many of the LTE solutions. In a classical solution, this is partly solved by having dedicated solutions for different modes of operation, for example, by having dedicated hard-wired chips for different standards.
However, a fully hard-wired implementation is highly unlikely as form factors continue to shrink and user applications for mobile terminals increases manifold:
- A multi-standard hardwired implementation is likely to need different blocks to implement different standards (to manage per-standard complexity); increasing time-to-market and cost (chip area)
- A hard-wired device lacks the flexibility to quickly adapt to changes in standards or algorithms
- Cross-standard optimization like hand-over, co-existence, etc. are difficult to achieve
As a result, a multi-RAT using a single-chip based on a (partly) software-based approach is attractive. It allows easy reuse of resources (which now are supplied in the form of one or more processor cores), reduces time-to-market, adds flexibility through software updates, and allows differentiation for customers like operators without the need for different silicon. For the higher protocol layers, a software approach is already common practice, implemented usually on one or more microcontroller cores.
However, programmability does have its disadvantages – power consumption being the most widely cited. The power budget of a handheld terminal is very limited (typical in the order of 1W maximum for a baseband chip and lower in typical operating mode).
The challenge is especially in the radio (PHY) part, which due to computational demands are unsuitable to be handled by a microcontroller or classical DSP. For example, just the FFT required for a single antenna 20MHz LTE channel would take a normal DSP to run in the GHz range, which is not compatible with today’s low power process capabilities. Total load for LTE is in the order of several 10’s of billion 16-bit operations per second.
We have conducted several studies in this area, to ascertain how much programmability is required in the different parts of an integrated multi-RAT solution. In our view, the demodulation part can especially add significant value to the mobile terminal, by allowing intelligence to be captured by the programmed algorithms; to supply better reception, improve on power consumption, and other similar improvements. The power savings due to this added intelligence can counter the power penalty resulting from the programmable architecture.
Kees Moerman is chief architect DSP Innovation Center at ST-NXP Wireless
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See also: Electronics Weekly's Focus on Wireless, a roundup of content related to wireless communications.