In the current wireless cellular network, spectrum efficiency is relatively low, and each user is allocated limited bandwidth, making it difficult to meet the demands of high-speed data services. As voice services become saturated, operators must focus on delivering more reliable high-speed data solutions in future wireless broadband mobile networks. Similar to how HSPA evolved from TD-SCDMA, LTE-Advanced represents a natural evolution of the LTE system. By making minor modifications to the LTE protocol, LTE-Advanced remains fully compatible with existing LTE devices while increasing the bandwidth available to advanced terminals and improving overall spectrum efficiency. This has become a key challenge for both equipment manufacturers and network operators.
At the 3GPP RAN1 #53bis meeting held in Warsaw, Poland, a proposal was accepted for the use of Carrier Aggregation (CA) technology in LTE-Advanced systems [1]. With CA, users can receive data across one or more frequency resource blocks depending on their capabilities. The main advantages of this technology include:
The base station can transmit data over a bandwidth of up to 100 MHz, allowing the downlink peak rate of LTE-Advanced to reach as high as 1 Gbit/s [2]. Additionally, terminals only need a single set of radio frequency (RF) and fast Fourier transform (FFT) components, keeping device complexity and cost manageable. Proper design of control and pilot channels can reduce the protection bandwidth, minimize signaling overhead, and ultimately improve system spectrum efficiency.
This article first introduces the fundamental principles and mainstream technical approaches of CA, followed by an overview of current research status, and finally explores the challenges associated with CA implementation.
1. Principle and Main Technical Solutions of Band Aggregation
Band aggregation technology effectively combines multiple frequency bands, enabling LTE-Advanced users to enjoy data rates beyond 20 MHz. It is generally accepted that:
To support higher peak data rates, the total bandwidth after band aggregation should exceed 20 MHz, with each carrier segment roughly matching the maximum transmission bandwidth defined in LTE Release 8.
Different carrier segments aggregated for the same user may have varying bandwidths, but there are strict limitations: the difference between them should not be too large (typically no more than twice), otherwise the benefits of CA diminish, and unnecessary signaling overhead increases. For example, 10 MHz and 20 MHz segments can be combined, but 1.4 MHz and 20 MHz cannot due to the significant gap.
These restrictions are included in the RAN4 protocol, which affects the flexibility of the RAN1 protocol.
Considering future data service characteristics, LTE-Advanced must also support asymmetric uplink and downlink bandwidths, as introduced in LTE Release 8. Uplink and downlink carrier segments can vary in size, and the number of aggregated carriers can also differ.
Newly added bandwidths proposed in WRC07, such as 450–470 MHz, 698–862 MHz, 790–862 MHz, 2300–2400 MHz, 3400–4200 MHz, and 4400–4990 MHz, should also be considered in the design.
Since different users have varying capabilities and some are restricted by RF limitations, both continuous and discrete band aggregation should be supported in LTE-Advanced.
Regarding small data packets, LTE-Advanced UEs using CA should maintain performance at least equivalent to LTE Release 8 UEs. Small packets, like TCP ACKs, paging signals, and random access signaling, are common in the system. CA requires a redesign of how these packets are transmitted to reduce unnecessary control signaling overhead.
An LTE-Advanced system utilizing CA must remain fully compatible with LTE legacy devices. This means retaining certain criteria from LTE Release 8, such as a 15 kHz subcarrier spacing and ensuring that uplink and downlink carrier segments are centered at integer multiples of 100 kHz.
1.1 Continuous Band Aggregation
At recent 3GPP meetings, discussions focused on continuous band aggregation, considering terminal capabilities and system complexity. As shown in Figure 1, continuous band aggregation refers to the combination of adjacent carrier segments in the frequency domain.
Because the spectrum is continuous, implementing band aggregation becomes easier, with lower signaling overhead and fewer frequencies to detect. Compared to discrete aggregation, continuous aggregation allows UEs to use a single set of RF and FFT components for multi-band reception, reducing terminal costs.
Current mainstream solutions for continuous band aggregation include:
Scheme 1: As shown in Figure 2 [3], the center frequency of the intermediate carrier is an integer multiple of 100 kHz, while others are not. Each carrier consists of 100 resource blocks with a total bandwidth of 18.015 MHz. Legacy LTE UEs can only access the middle carrier.
Scheme 2: As shown in Figure 3 [3], 19 subcarriers (285 kHz) are inserted between carrier segments to ensure that each carrier's center frequency is an integer multiple of 100 kHz, thus reducing the protection bandwidth at both ends of the aggregated band.
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