Several weeks ago I wrote a blog post about the famous Alamouti space-time block code (STBC), a complex orthogonal space-time block code (OSTBC) specialized for scenarios with two transmit antennas. The OSTBC is the only complex STBC that achieves maximum diversity while maintaining the maximum possible coding rate (i.e. R=1). This MIMO scheme is supported by the current Nutaq OFDM reference design. Nutaq’s OFDM reference design does a great job showcasing the software tools and the performance of the PicoSDR 2×2 solution while achieving a good data throughput. However, higher diversity orders are increasingly being considered in current waveform standards like LTE in order to maximize system capacity and improve the quality of service (QoS ) in multipath fading environments. To enable the development of such technology, we need the PicoSDR 2×2’s big brother: the PicoSDR 4×4.
As you can see on the PicoSDR webpage, the PicoSDR 4×4 includes two Virtex-6 FPGAs with two Radio420X FPGA mezzanine card (FMC) modules each, making the system 4×4 MIMO-capable. Basically, it’s twice the power of its little brother. To showcase the capability of the PicoSDR 4×4, the Nutaq OFDM reference design will be modified to support a higher order OSTBC. However, a compromise regarding the data rate will be needed since it has been shown that when the number of antennas is higher than three, there is no complex STBC that satisfies the two design goals of achieving the maximum diversity gain and the maximum coding rate at the same time.
As a start, the following OSTBC will be supported:
This STBC has a code rate of ½ but provides a low encoding and decoding complexity while increasing the diversity of the OFDM reference design. The following OSTBC could also be an interesting choice, if decodinghttp://nutaq.comcoding complexity is compromised:
The code rate of this OSTBC is ¾ but the encoding and the decoding process is getting more complex. However, a good thing about OSTBC is that the decoding usually ends up as simple linear processing structures, which are a good fit for FPGA processors.
Impacts on the Nutaq OFDM reference design
In practice, using these different OSTBCs will require modifications not only to the physical layer but to the data flow in the system itself, since the PicoSDR 4×4 now includes two independant FPGAs. This means that both FPGAs will need to exchange data at some point for proper MIMO transmission and reception. To achieve this goal, Nutaq implemented the Aurora protocol to exchange data between the two FPGAs (see http://nutaq.com/blog/high-speed-point-point-data-transfer-between-two-fpga-amc-chassis).
Here’s a high-level overview of the data flow on the PicoSDR 4×4:
The OFDM block in this diagram will need to be split into many blocks and shared between both FPGAs in order to evenly balance the computation load. The diagram also shows that synchronous reception/transmission between the four Radio420X FMCs will be possible using Aurora, which is required as OFDM waveforms are very sensible to synchronization (time and frequency domain).
This blog post was a simple preview of what’s coming in the next week regarding the applicative demos on the PicoSDR platform. Stay tuned for more information in the upcoming blog posts.