7.4 Physical Layer Impacts from Uplink MIMO
Moving from a single antenna transmission to two- or four-transmitter antennas required some partly new structures in the physical layer. It also required consideration for the resulting impact to the signal waveform properties. Also, the reference signal structure needed modification to cover the case of multiple transmit antennas per UE as well as to enable better use of the MU-MIMO functionality.
7.4.1 Uplink Reference Signal Structure
In the uplink direction there are two additional solutions introduced to the reference signal structure:
- Orthogonal Cover Codes (OCC) to complement the use of cyclic shifts of the Demodulation Reference Signal (DM RS) sequence;
- Aperiodic Sounding Reference Signal (SRS) to enhance the use of available SRS resources.
Both features are beneficial not only for SU-MIMO UEs but also for UEs with single transmit antenna.
Orthogonal cover codes are length-2 Walsh codes extended over the two DM RS in the subframe as shown in Figure 7.4. OCCs provide two benefits: they improve the reliability in separating the different RS from each other, especially when SU-MIMO or MU-MIMO transmission contains several transmission layers. Additionally, two DM RSs that overlap only partially in frequency are sufficiently separated. This is not the case when DM RS are separated by cyclic shifts. This feature is especially beneficial in the case of MU-MIMO, as now the PRB allocations of the paired UEs do not need to be fully ...