4x4 Downlink MIMO and 256QAM

Increasing bandwidth with carrier aggregation is the first and most robust technique to achieve higher 4G data rates. The next option to achieve higher 4G data rates in the DL is to upgrade to 256QAM DL Modulation. Then, increase the number of spatial streams overlaying 4X4 MIMO on one or more of the CCs that have been aggregated.

4x4 MIMO effectively reuses the same 20MHz CC four times over to transmit more data. Thus, it is a more efficient use of spectrum. Some important considerations in 4x4 MIMO deployments include the following

• ·       Only applies in the DL.

• ·       Data rates increase by a factor of two for each CC it is applied compared to 2X2.

• ·       Requires four unique data streams transmitted from a minimum of four unique antennas at the base station.

• ·       Needs four corresponding unique receiver chains in the mobile device.

·       Effectiveness is gated by the ability to de-correlate the four separate antennas in the handset — if the antennas can talkbto each other (interference), then the benefit drops.

All this means more receiver components — such as filters, switches, low noise amplifiers (LNAs), and antenna control devices — are needed in the mobile front end. Because DL is onlyvassociated to the receiver side of the RF chain, transmit power amplifiers are not affected. 256QAM DL modulation has a 1.33x multiplier on the data rate of each CC. It requires no change to the RF front end in the mobile device.

The 4G “baby” is now approaching maturity. For example, an FDD network and mobile device can achieve a theoretical peak 4G DL data rate of 1Gbps using:

• ·       3CC aggregating 3 x 20 MHz to create a 60 MHz “fat pipe”and achieve 450 Mbps

• ·       4x4 DL MIMO applied to just two out of the three CCs, to achieve 750 Mbps

• ·       256QAM DL modulation on all three CCs to achieve 1Gbps

• ·       A modem that supports ten spatial layers

The path to achieve even higher 4G DL peak data rates (in the 1.6Gbps range) is gated by the availability of transceiver/modem chipsets that support additional spatial layers (2017 chipsets can typically support up to 12 spatial layers), and the ability of hand-sets to accommodate the RF front end for more than two bands  with four receivers.