LTE Advanced

1. IMT-Advanced and LTE-Advanced

The development of fourth-generation (4G) mobile communication was shaped by the International Telecommunication Union (ITU) and its IMT-Advanced framework (International Mobile Telecommunications-Advanced). The framework set research targets of up to 1 Gigabit per second (Gbps) for low-mobility users and 100 Megabits per second (Mbps) for high-mobility users. Those targets gave 4G development a measurable performance direction, rather than defining a single radio implementation.

When the first LTE releases, based on 3GPP Releases 8 and 9, were launched, they brought much higher speeds and lower latency than 3G. They did not, however, meet every IMT-Advanced target. Operators and vendors still marketed many of those deployments as 4G, while the standards distinction was LTE and not yet LTE-Advanced.

LTE-Advanced addressed that gap. Starting with 3GPP Release 10 and continuing in later releases, it added technologies intended to meet the IMT-Advanced requirements, including Carrier Aggregation, enhanced multi-antenna transmission, and Coordinated Multi-Point (CoMP). The ITU accepted LTE-Advanced as an IMT-Advanced technology. It remained an evolution of LTE, allowing operators to introduce compatible software and hardware upgrades without replacing every LTE site at once.

2. Carrier Aggregation (CA)

Carrier Aggregation is one of the main LTE-Advanced mechanisms. It combines separated spectrum resources that an operator holds in distinct frequency blocks. The network can use intra-band contiguous, intra-band non-contiguous, or inter-band non-contiguous configurations when the band combination and UE capability signalling support them.

The Spectrum Problem

The radio spectrum is the finite resource that wireless communications share. Mobile operators obtain rights to specific bands, and their holdings are often split into blocks that are separated in frequency. For example, an operator might hold a 10 MHz block in the 700 MHz band and another 20 MHz block in the 2.1 GHz band. In Release 8/9 LTE, a user device generally scheduled traffic on one serving carrier at a time, so its peak rate was bounded by that carrier's bandwidth.

The Carrier Aggregation Solution

Carrier Aggregation solves this problem by allowing a capable device to use multiple component carriers in one configured serving-cell group. The scheduler treats their resources as one wider transmission opportunity, even when the carriers are separated in frequency.

Each of these frequency blocks is called a Component Carrier (CC). Release 10 LTE-Advanced allows up to five Component Carriers, and each CC can have a bandwidth of up to 20 MHz. The Release 10 CC bandwidths are 1.4, 3, 5, 10, 15, or 20 MHz. The uplink and downlink can use different numbers of component carriers, subject to UE capability. The theoretical aggregate is therefore $5 \times 20 = 100$ MHz, although the usable rate also depends on the UE category, modulation, MIMO layers, and radio conditions. The 100 MHz figure describes aggregate component-carrier bandwidth, not guaranteed application throughput.

Types of Carrier Aggregation

The technology supports three basic scenarios for combining carriers:

In a CA setup, one carrier is designated as the Primary Component Carrier (PCC) or Primary Cell (PCell). The PCell carries the main control signalling and remains configured as the anchor while the UE is connected. The other carriers are Secondary Component Carriers (SCC) or Secondary Cells (SCells), which can be activated or released as traffic and radio conditions change.

3. MIMO in LTE-Advanced

MIMO (Multiple-Input Multiple-Output) was already part of the original LTE standard. It uses multiple antenna ports and spatial layers at the base station and UE. LTE-Advanced increased the supported layer counts and added more ways to coordinate those transmissions.

Two MIMO uses

MIMO technology provides two main benefits:

• Spatial multiplexing (for throughput): It allows the transmitter to send multiple, independent data streams simultaneously over the same frequency channel, with each stream going out of a different antenna. The receiver, with its multiple antennas, can distinguish these streams thanks to the slightly different paths they take. This directly increases the peak throughput. A 2x2 MIMO system (2 transmit, 2 receive antennas) can double the data rate, and a 4x4 MIMO system can quadruple it, compared to a single antenna system.
• Transmit diversity (for reliability):In poor signal conditions, instead of sending different data, the transmitter sends the same data stream from multiple antennas. Each copy travels a different path, so it's less likely that all copies will be simultaneously affected by deep fading. The receiver can then combine these copies to reconstruct the original data more reliably. This does not increase the peak throughput, but it reduces the effect of fading on the connection.

MIMO extensions in LTE-Advanced

LTE-Advanced expanded the original MIMO framework primarily by increasing the number of supported spatial layers and improving their coordination.

• Up to 8 Downlink Layers: The LTE-Advanced downlink supports up to eight spatial layers in the relevant transmission modes. Release 10 also supports up to eight transmit antennas for downlink transmission, but the number of spatial layers remains limited by the transmission mode and UE capability. A UE and its antenna implementation must support the required ports and channel conditions, so the layer count is a capability limit rather than a rate guaranteed to every handset.
• Up to 4 Uplink Layers: LTE-Advanced added uplink spatial-multiplexing capabilities reaching four layers in supported configurations. The actual UE transmit chains, power limits, and channel quality determine whether that gain is available.
• Multi-User MIMO (MU-MIMO): This technique uses multiple base-station antennas to transmit data streams to different users simultaneously on the same time and frequency resources. The eNodeB forms beams directed at each user, limiting interference between them. It can increase the overall capacity and efficiency of the cell when users can be separated spatially.

Eight-layer operation is difficult to realize in a compact smartphone because of antenna coupling, power, and channel-estimation constraints. The broader MIMO framework remains useful in fixed wireless access points and hotspots, and the same spatial-processing principles were later extended in 5G Massive MIMO.

4. Coordinated Multi-Point (CoMP)

Cell-edge performance is a recurring problem in cellular networks. A device far from its serving base station receives a weaker signal. At the same time, a nearby neighbouring cell can produce strong interference. This combination limits throughput and connection quality at cell boundaries.

Coordinated Multi-Point (CoMP), standardized for LTE-Advanced work in later releases such as Release 11, addresses this problem by coordinating transmissions from several points. The aim is to manage neighbouring cells as cooperating radio resources instead of treating all energy from them as uncontrolled interference.

CoMP Transmission Schemes

There are several ways this cooperation can be implemented:

• Coordinated Scheduling / Coordinated Beamforming (CS/CB):

  In this scheme, the neighboring base stations share information about the radio conditions of the cell-edge user. The primary cell (the one the user is connected to) still transmits all the data. However, the neighboring cell, aware of the user's situation, will coordinate its transmissions. It might choose not to transmit to its own users on the same radio resources to avoid causing interference, or it might actively shape its antenna beams to direct its signal away from the cooperating user.

• Joint Processing (JP):

  In this variant, multiple base stations collaborate to simultaneously transmit data to or receive data from a single user.

  • Downlink JP: Multiple eNodeBs transmit the same user data to the cell-edge user at the same time. From the device's perspective, a signal from a neighbouring site can then contribute to the useful received signal. The device receives a stronger signal, which can improve cell-edge reception.
  • Uplink JP: Multiple eNodeBs simultaneously receive the signal transmitted from the user's phone. The data received at each eNodeB is then combined and processed jointly, allowing the network to recover the signal much more reliably than a single base station could.

Implementing CoMP, especially Joint Processing, is technologically demanding. It requires low-latency, high-capacity transport between the coordinating radio sites, often carried by fibre or another suitably dimensioned backhaul and inter-site link. That transport and the associated synchronization make wide deployment expensive, even though the coordination principles also appear in later 5G radio designs.

5. Additional LTE-Advanced features

Beyond carrier aggregation, MIMO, and CoMP, LTE-Advanced introduced enhancements for network efficiency and coverage. Relay Nodes extend an existing cell without a wired backhaul, while eICIC coordinates macro and small cells that reuse the same spectrum. Both features address deployment limits that carrier aggregation and MIMO alone cannot solve.