5G MODEM design
MODEM design and implementation is one of the core areas of OpenAirInterfaceTM development. Today we provide the most efficient software-only, open-source implementation of a LTE modem including extremely fast subroutines for Turbo decoding and FFT/IFFT computations. Moreover OpenAirInterfaceTM already supports some basic MIMO modes such as Alamouti, Beamforming and multi-user MIMO with more MIMO modes in the pipeline.
In 5G the MODEM will surely evolve – here we list a few elements that we think are important for 5G and we show how OpenAirInterfaceTM addresses them.
To support the 5G requirements such as 1000 times more capacity and less latency than 4G systems, 5G will need to provide higher spectral efficiency, the ability to support large and fragmented spectrum, dynamic spectrum access (DSA), and short packet transmissions with loose synchronization requirements. Orthogonal frequency division multiplexing (OFDM) and single-carrier frequency division multiplexing (SC-FDMA), which are the two waveforms used in current 4G systems do not fulfill all of these requirements, and therefore new waveforms have been proposed for 5G. All proposed candidate 5G waveforms are generalizations of OFDM. In the case of filter-bank multi-carrier (FBMC) additional pulse-shaping filters are applied to every subcarriers. Alternatively, Universal Filtered Multi-Carrier (UFMC) applies filtering over multiple sub-carriers, and Generalized Frequency Division Multiplexing (GFDM) uses circular convolution instead of linear convolution for the filtering of the sub-carriers. All of these waveforms have in common that they reduce the adjacent channel leakage ratio (ACLR) and the peak-to-average power ratio (PAPR) compared to an OFDM system at the expense of a more complex receiver design.
Full Duplex Radio
Currently, radio systems operate in half-duplex mode to avoid self-interference generated when high power transmission and reception coexist in time and frequency. Time Division Duplex (TDD, the same frequency band is used for transmission and for reception) and Frequency Division Duplex (FDD frequency band is used for transmission and another band for reception) are classical half-duplex schemes. Recent results have demonstrated the feasibility of wireless communication in full-duplex (or Full-duplex) at close range. The principle is to transmit and receive simultaneously in the same frequency band. The interest is obvious, since it allows in theory to double the channel capacity. The operation of this type of system is based on the mitigation of self- interference thanks to a combination of antennal techniques, and analogue and digital processing. Recent results demonstrate that the combination of these techniques makes possible the operation of wireless full-duplex nodes.
In this context, our objectives are:
- Theoretical advances on communication performance limits in Full-duplex equipment, and in particular in the case of a Multiple Inputs Multiple Outputs (MIMO) equipment
- The development of digital processing for the cancellation of the transmitted signal at the receiver
- The development of MIMO full-duplex communication equipment for the next generation of wireless communication standards
- To use Full-duplex equipment in a single wireless LTE-like link at first, and in an arbitrary network in a second step.
On a single wireless link, with traffic in both directions, full-duplex techniques have a potential to double the throughput. However, in the case of an arbitrary network, there is a need of an efficient protocol stack to exploit the benefits of full-duplex opportunities. Recent work has been done for example in the MAC layer [KIM13] [LBS15].
Massive MIMO is one of the key technologies considered for 5G and also an active area of research for OpenAirInterface. Massive MIMO takes techniques developed for multi-user MIMO to the next level by scaling up the number of antennas by one order of magnitude and thus being able to spatially multiplex to several users within the same time and frequency resources, thus increasing the spectral efficiency of a cell.