What is the difference between MIMO and carrier aggregation?

Modern cellular networks, including 4G LTE and 5G NR, use various technologies to improve data rates and signal quality for mobile phone users. MIMO and Carrier Aggregation are two such technologies that enhance the overall network capacity and coverage to deliver higher throughput and better radio link quality.

MIMO (Multiple Input Multiple Output) combines signals and data streams from multiple antennas to improve signal quality and data rates, whereas Carrier Aggregation (CA) combines multiple frequency carriers (channels) to enhance the bandwidth and data rates. 4G and 5G networks use both techniques.

MIMO-Multiple Input Multiple Output improves data rates through smart antenna technology, whereas carrier aggregation achieves its goals by making the best use of the available frequency carriers.

What does carrier aggregation mean?

Carrier Aggregation is a technology introduced in LTE Advanced networks to improve the achievable data rates in 4G while utilising the original LTE bandwidths. Carrier Aggregation allows 4G LTE Advanced to combine multiple carriers to increase the overall carrier bandwidth and associated data rates.

Mobile networks use frequency carriers to transmit and receive their cellular signals. Larger bandwidths allow a mobile network to offer higher data rates, as you can read in our dedicated post on bandwidth vs bit rates. Carrier Aggregation, often abbreviated as CA, allows mobile networks to combine multiple frequency carriers into one to increase the overall carrier bandwidth, resulting in higher data rates for mobile phone users. For example, suppose a mobile operator has two frequency carriers of 10 MHz each within the same base station (eNodeB – eNB). In that case, the mobile operator can increase the overall bandwidth to 20 MHz by combing the two 10 MHz channels (10 MHz + 10 MHz = 20 MHz). The combined or “aggregated” carriers can be assigned to a mobile phone user’s device to generate higher data rates.

Carrier aggregation in 4G LTE Advanced networks (4G+)

4G LTE Advanced networks (4G+), as per 3GPP release 10, can achieve carrier aggregation of up to five (5) frequency carriers to enable a maximum bandwidth of 100 MHz; LTE Advanced Pro, as per 3GPP release 13, can achieve a maximum bandwidth of 640 MHz by combining up to thirty-two (32) carriers.

When LTE networks were initially introduced according to the specifications in the 3GPP release 8, they did not have the carrier aggregation capability. However, LTE networks supported flexible bandwidths from the very beginning. Flexible bandwidth means that LTE networks can use frequency carriers of different bandwidths: 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz. The original LTE networks can support a maximum bandwidth of 20 MHz with flexible bandwidth. Carrier Aggregation was introduced in 3GPP release 10, which resulted in the LTE-Advanced enhancement. LTE-Advanced, which is shown as LTE+ or 4G+ on mobile phones, can support carrier aggregation of up to five (5) frequency carriers. It means that in LTE-Advanced, up to five frequency carriers of flexible bandwidths can be combined together to create a bigger overall bandwidth which can then be assigned to a mobile phone device. Using the 20 MHz carrier, which is the biggest one, LTE-Advanced can deliver a maximum bandwidth of 100 MHz (20 MHz x 5). While 100 MHz is a good enough bandwidth, LTE-Advanced Pro networks use an even higher carrier aggregation configuration.

LTE-Advanced Pro networks aim to deliver a maximum throughput of up to 3 Gbps in the downlink, which requires a much bigger carrier. The 3GPP Release 13 introduced LTE-Advanced Pro networks that can enable carrier aggregation of up to thirty-two (32) carriers. With a 20 MHz carrier, LTE-Advanced Pro can achieve a maximum carrier bandwidth of 640 MHz (20 MHz x 32). While the mathematics of carrier aggregation looks relatively straightforward, the allocation of the frequency bands in carrier aggregation has multiple scenarios. These scenarios depend on the frequency band used by the mobile operator for carrier aggregation. The scenarios defined by 3GPP are intra-band contiguous, intra-band non-contiguous and inter-band non-contiguous. I have written a dedicated post on carrier aggregation that goes into the details of these scenarios.

What is MIMO in 4G LTE networks?

MIMO stands for Multiple Input Multiple Output and it is an advanced antenna technology that is used by 4G LTE, LTE-Advanced and LTE-Advanced Pro networks to improve data rates and signal quality through spatial multiplexing, antenna diversity and beamforming.

MIMO is an antenna technology and is different from carrier aggregation, where the focus is on combining frequency carriers. MIMO utilises the existing frequency carriers more efficiently through spatial multiplexing to improve the data rates for mobile phone users. While carrier aggregation combines multiple frequency carriers to assign a bigger carrier bandwidth to a mobile phone, MIMO uses numerous antenna elements at the transmitter to send several parallel data streams to the receiver. MIMO receiver is also equipped with multiple antenna elements to receive the data streams. The output for a user device is created by combining the various data streams. That way, each data stream acts as a virtual channel between the base station and the user device. MIMO in 4G LTE focuses on the principle of spatial multiplexing, where communication takes place through multiple spatially separated antennas (space division multiplexing). However, MIMO also improves the mobile signal quality by minimising the impact of multipath fading through antenna diversity. Finally, MIMO also benefits from beamforming, where multiple antennas transmit the signal in the direction of the same receiver. Beamforming allows the receiver to receive a more robust signal.

LTE networks use different MIMO configurations for downlink and uplink, and they must be supported by both the transmitter and the receiver. As per 3GPP release 8, the original LTE networks support a MIMO configuration of 4 x 4 in the downlink and 2 x 2 in the uplink. To understand this concept, a downlink configuration of 4 x 4 means four antenna elements at the base station transmitter to send the signal towards a mobile phone and four antenna elements in the mobile phone receiver to pick up the signal. The uplink transmission is from the mobile phone back to the base station. LTE-Advanced and LTE-Advanced Pro networks use a MIMO configuration of 8 x 8 in the downlink and 4 x 4 in the uplink. I have written a dedicated post on MIMO in LTE networks that dives into the details of MIMO configurations and the benefits.

5G New Radio (NR) networks use an advanced type of MIMO called Massive MIMO, which uses much higher configurations and multi-user MIMO. Look at this dedicated post on Massive MIMO in 5G NR networks to find out how 5G improves capacity through MIMO. If you are wondering what multi-user MIMO is and whether it is just used in 5G or also in 4G, check out our dedicated post on multi-user MIMO technology.

Here are some helpful downloads

Thank you for reading this post. I hope it helped you in developing a better understanding of cellular networks. Sometimes, we need extra support, especially when preparing for a new job, studying a new topic, or buying a new phone. Whatever you are trying to do, here are some downloads that can help you:

Students & fresh graduates: If you are just starting, the complexity of the cellular industry can be a bit overwhelming. But don’t worry, I have created this FREE ebook so you can familiarise yourself with the basics like 3G, 4G etc. As a next step, check out the latest edition of the same ebook with more details on 4G & 5G networks with diagrams. You can then read Mobile Networks Made Easy, which explains the network nodes, e.g., BTS, MSC, GGSN etc.

Professionals: If you are an experienced professional but new to mobile communications, it may seem hard to compete with someone who has a decade of experience in the cellular industry. But not everyone who works in this industry is always up to date on the bigger picture and the challenges considering how quickly the industry evolves. The bigger picture comes from experience, which is why I’ve carefully put together a few slides to get you started in no time. So if you work in sales, marketing, product, project or any other area of business where you need a high-level view, Introduction to Mobile Communications can give you a quick start. Also, here are some templates to help you prepare your own slides on the product overview and product roadmap.

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