Research on the principle of band aggregation technology and mainstream technology solutions

In the existing wireless cellular network, the spectrum efficiency is not high, and the bandwidth occupied by each user is limited, so the transmission requirements of high-speed data services cannot be met. As voice services become more saturated, operators need to consider providing more reliable high-speed data services to users in future wireless broadband mobile networks. Similar to the relationship between High Speed ​​Packet Access (HSPA) systems and Time Division Synchronous Code Division Multiple Access (TD-SCDMA) systems, the LTE-Advanced system is a smooth evolution of the LTE system. Therefore, under the premise of modifying the LTE protocol to a small extent, it is fully compatible with LTE legacy terminals, and can increase the bandwidth occupied by LTE-Advanced terminals and improve its spectrum efficiency, which becomes a common problem faced by equipment vendors and operators.

The 3GPP RAN1 #53bis conference adopted a proposal for the use of Band Aggregation (CA) technology in LTE-Advanced systems in Warsaw, Poland [1]. Users using CA technology can simultaneously receive data on one or more frequency resource blocks according to their capabilities. Its main technical advantages include:

The base station can transmit data for one terminal in a bandwidth of about 100 MHz, and the downlink peak transmission rate of the LTE-Advanced system can reach 1 Gbit/s [2]. The terminal can use only one set of radio frequency (RF) and fast Fourier transform (FFT) devices, and the CA technology does not significantly increase the device complexity and cost of the terminal. By properly designing the control channel and the pilot channel, the protection bandwidth is reduced, signaling overhead can be reduced, and the spectrum efficiency of the system can be improved.

The article first introduces the basic principles and mainstream technical solutions of CA, then gives the research status of CA technology, and finally discusses the problems of CA.

1 Band aggregation technology principle and mainstream technical solutions

The band aggregation technology reasonably multiplexes multiple frequency bands, enabling LTE-Advanced users to simultaneously receive data with bandwidths exceeding 20 MHz. Now generally considered [3-4]:

In order to support higher data peak rates, the user bandwidth after band aggregation should exceed 20 MHz, and each carrier segment definition is approximately equal to the maximum transmission bandwidth of LTE release 8.

The bandwidth of different carrier segments that are aggregated for the same user can be different, but there should be a basic limitation: the bandwidth difference of different carrier segments cannot be too large (it is generally considered that it cannot exceed twice), otherwise the meaning of carrier aggregation is lost, and Add a lot of signaling overhead. For example, two carrier segments of 10 MHz and 20 MHz can be aggregated, but two carrier segments of 1.4 MHz and 20 MHz do not allow aggregation. Writing such restrictions directly into the RAN4 protocol has an impact on the flexibility of the RAN1 protocol.

Considering the transmission characteristics of the data service in the future, the asymmetric uplink and downlink bandwidth supported by the LTE release 8 should also be supported in the CA; the uplink and downlink carrier segments can be different in size, and the number of aggregated carrier segments can also be different.

The newly added bandwidth in the Universal Mobile Telecommunications System (UMTS) proposed in WRC07 should also be considered, for example: 450~470,698~862, 790~862, 2 300~2 400, 3 400~4 200 and 4 400 ~4 990 MHz band.

Since different users have different capability levels and some users have radio frequency restrictions and do not support discrete carrier aggregation, both continuous band aggregation and discrete band aggregation should be considered in the LTE-Advanced system.

On the support of small data packets, the LTE-Advanced user terminal (UE) using band aggregation should not be lower than the UE of LTE release 8. Because there are many small data packets in the system, such as Transmission Control Protocol (TCP), ACKs and some paging signaling and random access corresponding signaling. CA needs to redesign how to transmit such small packets, reducing unnecessary control signaling overhead.

An LTE-Advanced system using CA technology requires a UE that is fully compatible with the legacy of the LTE system. This requires retaining some of the criteria specified in LTE release 8, for example, a subcarrier bandwidth of 15 kHz, with the upstream and downstream carrier segments centered at an integer multiple of 100 kHz.

1.1 Continuous band aggregation

At the existing 3rd Generation Partnership Project (3GPP) meeting, after considering the terminal execution capability and system complexity, the proposal for continuous band aggregation is mainly discussed. As shown in FIG. 1, continuous band aggregation means that a plurality of carrier segments aggregated to serve one user are continuous in the frequency domain.

Since the spectrum of the carrier segment is continuous, it is easier for the system to implement band aggregation, the signaling overhead is small, and the frequency that the UE needs to detect is also small. Compared with discrete band aggregation, it is easier for a UE to use a set of RF and FFT devices to perform continuous reception of data in multiple frequency bands, saving terminal cost. For the discussion of the latest 3GPP conference, the mainstream solutions for continuous band aggregation include:

Scheme 1: As shown in Figure 2 [3], only the center frequency of the intermediate carrier segment is an integer multiple of 100 kHz, and the center frequency of other carrier segments is not an integer multiple of 100 kHz. Each carrier segment is composed of 100 resource blocks with a bandwidth of 18.015 MHz. The legacy UE of LTE can only access the intermediate carrier segment.

Scheme 2: As shown in Figure 3 [3], 19 subcarriers (285 kHz) are inserted between the carrier segments to ensure that the center frequency of each carrier segment is an integer multiple of 100 kHz, and protection at both ends of the band aggregation. The bandwidth will be reduced accordingly.

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