In cellular network system, no matter it is 4G LTE or 5G NR, both Multiple-input multiple output (MIMO) and Carrier Aggregation (CA) are key technology, but what are the differences between them?

Carrier aggregation is a highly innovative technology introduced in LTE Advanced network, allow mobile network to combine multiple frequency carriers into one to increase overall carrier bandwidth to bring more significant data rate increasing for mobile phone users. At beginning, LTE networks based on 3GPP Release 8 didn’t support carrier aggregation, but LTE networks support flexible bandwidth configuration, which means it can use frequency carriers with different bandwidths such as 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz.

Release 10 of LTE-Advanced is the first LTE version to introduce carrier aggregation(intra-band continuous and inter-band), take 5×20 MHz as standard CA, but it is only for two operators. Release 12 of LTE-Advanced introduced in 2012, support for in-band non-contiguous carrier aggregation. Supports aggregation of up to 5 frequency carriers with flexible bandwidth, up to 100MHz bandwidth. Even 100MHz bandwidth is already impressive, but LTE-Advanced Pro network brought higher-spec carrier aggregation configuration. It aims to provide a maximum throughput of up to 3 Gbps in downlink, which requires larger carrier sizes. 3GPP Release 13 introduces LTE-Advanced Pro network that enable carrier aggregation up to 32 carriers, reach maximum bandwidth up to 640MHz.

Although carrier aggregation is a key technology already presented in LTE-Advanced networks, it enables the evolution of LTE to achieve user data rate in excess of 1Gbps, but in 5G networks, carrier aggregation will support evolution to multi-gigabit 5G, user data rate reach approximately 4 Gbps and above.

After talking about carrier aggregation, let’s take a look at what exactly MIMO is?

In wireless network, two wireless nodes communicate to send and receive data via radio channel, this channel uses a some wireless spectrum. But in many areas suitable spectrum is scarce, use more spectrum is impractical.

MIMO stands for Multiple Input Multiple Output, is an advanced antenna technology that can increase capacity but without increasing spectrum, so it is widely adopted by 4G LTE, LTE-Advanced and LTE-Advanced Pro networks. MIMO effectively improve data rate and signal quality through spatial multiplexing, antenna diversity and beamforming.

MIMO focuses on principle of spatial multiplexing in 4G LTE, means communication occurs through multiple spatially separated antennas (spatial division multiplexing). Meanwhile MIMO also improve the quality of mobile signals by minimizing the effects of multipath fading through antenna diversity.

Additionally, MIMO benefits from beamforming technology, multiple antennas transmit signals to same receiver so that receiver can receive stronger signal. LTE networks use different MIMO configurations for downlink and uplink, and both transmitters and receivers must support these configurations.

According to 3GPP Release 8, original LTE network support 4×4 MIMO configurations in downlink and 2×2 MIMO configurations in uplink. 4×4 downlink configuration means that four antenna elements in base station transmitter send signal to mobile phone, while four antenna elements in mobile phone receiver are responsible for receiving signal. Uplink transmission is from mobile phone back to base station.

LTE-Advanced and LTE-Advanced Pro network use 8×8 MIMO configuration in downlink and 4×4 MIMO configuration in uplink. To summary, MIMO and carrier aggregation play their own unique and crucial role in modern cellular networks, jointly bring users higher-speed and more stable communication experience. In 5G (NR) networks, an advanced MIMO called Massive MIMO is adopted, which uses higher configuration and multi-user MIMO. Massive MIMO has become key enabler and foundational component of ultra-fast millimeter wave 5G.

There is no fixed definition to define how many antennas must have to be considered as Massive MIMO, but systems with more than 8×8 antennas are generally considered as Massive MIMO systems, 8×8 number refers to 8 transmit antennas and 8 receive antennas. In general, MIMO act as an antenna technology, has different emphasis from carrier aggregation. 

Carrier aggregation focus on combining frequency carriers, MIMO increase data rate for mobile phone users by spatially multiplexing existing frequency carriers more efficiently. Carrier aggregation combines multiple frequency carriers together to allocate larger carrier bandwidth to mobile phones. But MIMO utilizes a large number of antenna elements at transmitter to send multiple parallel data streams to receiver. MIMO receivers also equipped with multiple antenna elements to receive these data streams, the output from user device is created by combining various data streams. In this way, each data flow acts like a virtual channel between base station and user equipment.