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.

MIMO (Multiple-Input Multiple-Output) Solution

MIMO Basics

MIMO systems use multiple input and multiple output antennas operating on a single channel (frequency). At the transmitter side, the signal is space–time encoded and transmitted from N_T antennas. At the receiving side, the signals are received from N_R antennas (see Figure 1). The space–time decoder combines the signal received by the N_R antennas and transmitted from the N_T antennas after having estimated the channel matrix (N_T×N_R).

Figure 1: Generic MIMO block diagram for the downlink

The objective of the MIMO solution is to exploit the space and time diversity of the channels on the different radio paths between each combination of transmit/receive antennas to improve the reception sensitivity and/or to improve the channel capacity. There are several families of MIMO solutions. The two extreme ones are the spatial diversity MIMO schemes and the spatial multiplexing MIMO schemes.

In addition, several MIMO schemes exist that are a mix between SM and spatial diversity schemes. The diversity order and capacity increase depends on the space–time code and number of antennas.

More recently, MIMO schemes using pre-coding have been defined. In these cases, the space–time code depends on a feedback from the receiver on the channel states. Indeed, this solution requires a closed-loop operation and additional signalling between the receiver and the transmitter.

Finally, MIMO can also be generated from signals transmitted from different BSs (virtual MIMO). This requires time synchronisation of the BS but also a synchronisation of the scheduler of the BS involved in the transmission. 

System Design Aspects of BS and MS

As indicated in Figure 1, MIMO operation has a significant impact on the design of the BS and the MS. Indeed, in addition to the MIMO codes, the algorithms for encoding and decoding MIMO signals, there is the requirement to implement several transmitter and several receivers both at the BS and MS sides. This may be critical for the MS side. Indeed, implementing several receiver chains implies several antennas on a small device and additional hardware and processing power. Moreover, several transmitters at the MS side also mean significant additional power consumption. Efficient implementation of MIMO at the MS side has then some technological challenges to be solved.

The preferred antenna configuration for MIMO is when the multiple antennas are perfectly uncorrelated. In that case, the performances of MIMO are optimal. Good correlation is obtained if at the BS side the antennas are separated by 10 to 20 λ and at the MS side by at least λ. The latter requirement makes solutions with more than two antennas at the MS impractical.

In some cases, the additional effort needed for the space–time decoder is significant. This also imposes some limits on the number of transmit antennas at the BS side.