The aggregation of the carriers results in sharing of the IP (Internet Protocol) packets between the different accesses. For the downstream direction (respectively the upstream direction), the sharing is performed by the evolved node base station (eNB) (respectively the mobile) and the reassembly is provided by the mobile (respectively the entity eNB). This operation is performed exclusively in the evolved universal terrestrial radio access network (E-UTRAN).
LTE (Long-Term Evolution) aggregation operates in licensed frequency bands (Figure 3.1).
LAA (Licensed Assisted Access) aggregation is an extension of LTE aggregation. The LTE transmission, defined by the data link layer and the physical layer, takes place on LTE and Wi-Fi (Wireless Fidelity) frequency bands, between the mobile and the eNB entity, without an intermediate access point, in accordance with the 3GPP standards (Figure 3.1).
The LTE Advanced and LTE Advanced Pro evolutions respectively defined an aggregation of five channels in a licensed band and extend up to 32 channels in a non-licensed band (Figure 3.1).
LWA (LTE–Wi-Fi Aggregation) and LWIP (LTE/WLAN radio level integration with IPsec tunnel) aggregations use both LTE and Wi-Fi technologies in their respective bands. The transmission on the Wi-Fi radio channel occurs between the mobile and the access point (AP) in accordance with 802.11 standard (Figure 3.1).
LTE and Wi-Fi carrier aggregation can be implemented with collocated or remote eNB and AP entities. In the non-collocated case, the Xw interface is the point of reference between the eNB and AP entities (Figure 3.1).
The eNB entity is the anchor point for the data exchanged with the mobile, belonging to the user plane (the IP packets) and the control plane, and connects to the evolved packet core (EPC):
Carrier aggregation (CA) combines multiple component carriers (CC) and allows wider radio channel bandwidth in order to increase the throughput of the cell.
The bandwidth of the radio channel is limited to 20 MHz in the LTE release. Aggregation can be performed over five radio channels for LTE Advanced and up to 32 radio channels for LTE Advanced Pro, bringing the maximum bandwidth value to 100 MHz or 640 MHz, respectively.
The radio channels can be aggregated according to several patterns (Figure 3.2):
The notation relating to the radio channel aggregation determines the frequency band and the class of the bandwidth of the radio channel.
Class A corresponds to a bandwidth of the radio channel less than or equal to 20 MHz.
Class B corresponds to a bandwidth less than or equal to 20 MHz, obtained from the aggregation of two contiguous radio channels in the same frequency band.
Class C corresponds to a bandwidth less than or equal to 40 MHz, obtained from the aggregation of two contiguous radio channels in the same frequency band.
Class D corresponds to a bandwidth less than or equal to 60 MHz, obtained from the aggregation of three contiguous radio channels in the same frequency band.
Class E corresponds to a bandwidth less than or equal to 80 MHz, the number of concatenated radio channels in the same frequency band not being specified.
Class F corresponds to a bandwidth less than or equal to 100 MHz, the number of radio channels concatenated in the same frequency band not being specified.
One of the radio channels is the primary cell (PCell), and it has the following characteristics:
The other radio channels are the secondary cells (SCell) which only transmit IP packets from the traffic plane.
The bearer for both directions of transmission uses two paired bandwidths in the FDD (Frequency-Division Duplex) mode or a single bandwidth in the TDD (Time-Division Duplex) mode.
For the FDD mode, each direction of transmission operates simultaneously in the assigned radio channel in the frequency band.
For the TDD mode, the two directions of transmission operate in the same radio channel, each direction being assigned during a portion of the time.
The carrier aggregation can associate radio channels operating in the FDD and TDD modes.
The physical downlink control channel (PDCCH) carries the downlink control information (DCI) for one or more user equipment (UE):
For each sub-frame, the PDCCH is mapped on the first, the first two or the first three OFDM (Orthogonal Frequency-Division Multiplexing) symbols.
The number of OFDM symbols allocated to the PDCCH is indicated in the physical control format indicator channel (PCFICH).
As part of the carrier aggregation, the PDCCH can either carry the scheduling information for the radio channel where it is transmitted or allocate resources for other radio channels in the case of inter-carrier scheduling.
The inter-carrier scheduling makes it possible to avoid inter-cell interference over the PDCCH by allocating resources to the adjacent cells over different radio channels.
The inter-carrier scheduling can only be applied to the secondary channels (SCell), since the primary channel (PCell) systematically has one PDCCH allocated.
The inter-carrier scheduling can be established in accordance with two scenarios (Figure 3.3):
The medium access control (MAC) provides the management of retransmission in the case of error via the HARQ (Hybrid Automatic Repeat reQuest) mechanism, established at the level of the physical layer.
The radio channel aggregation impacts the MAC layer which must handle a HARQ mechanism for each radio channel.
The PHR (Power HeadRoom) is a MAC control element which contains the indication of the mobile power reserve, the difference between maximum power and power used for the PUSCH.
The PHR control element is periodically transmitted by the mobile, and the periodicity is indicated in the RRC ConnectionSetup or ConnectionReconfiguration message transmitted by the eNB entity.
The PHR control element is also transmitted when the variation of the attenuation due to propagation is greater than a threshold indicated in the same RRC messages.
The radio channel aggregation has introduced a new PHR control element to indicate the power reserve for each radio channel of the aggregation.
The radio channel aggregation has also introduced the ADM (Activation/Deactivation MAC) control element which concerns activation and deactivation of the SCell secondary radio channels.
The ADM control element is used when radio channels have been previously established by an RRC ConnectionSetup or ConnectionReconfiguration message.
The ADM control element makes it possible to save mobile consumption, and rapid deactivation of a secondary channel (SCell) allows the mobile to avoid processing relating to the bearer built on that channel.
The mobile categories determine the maximum data rate on the LTE-Uu radio interface, for the downlink and the uplink.
The maximum rate depends on the optimal characteristics of the radio interface (modulation, radio channel bandwidth, MIMO mechanism) and the mobile’s ability to handle the bit rate allowed by the radio conditions.
Mobile categories 1 to 5 are LTE mobiles defined in release 8 (Table 3.1).
Table 3.1. LTE mobile categories from release 8
Categories |
1 |
2 |
3 |
4 |
5 |
DL rate (Mbps) |
10 |
50 |
100 |
150 |
300 |
UL rate (Mbps) |
5 |
25 |
50 |
50 |
75 |
Bandwidth (MHz) |
20 |
20 |
20 |
20 |
20 |
Modulation DL |
64-QAM |
64-QAM |
64-QAM |
64-QAM |
64-QAM |
Modulation UL |
16-QAM |
16-QAM |
16-QAM |
16-QAM |
64-QAM |
MIMO DL |
n.a. |
2 × 2 |
2 × 2 |
2 × 2 |
4 × 4 |
MIMO UL |
n.a. |
n.a. |
n.a. |
n.a. |
n.a. |
DL: downlink; UL: uplink.
Mobile categories 6 to 12 are LTE Advanced mobiles defined in release 11 (Table 3.2).
Given the difficulty of processing the MIMO 4 × 4 for the mobile category 5, the mobile categories 6 and 7 were introduced to reach the bit rate of 300 Mbps for the downlink. Such performance is obtained by maintaining the MIMO 2 × 2 and doubling the bandwidth of the radio channel.
The mobile categories 9 and 10 (respectively 11 and 12) have a maximum bit rate of 450 Mbps (respectively 600 Mbps), obtained by the aggregation of three radio channels (respectively four radio channels), while retaining the MIMO 2 × 2.
The mobile categories 7, 10 and 12 can exceed the bit rate of 75 Mbps of the mobile category 5 for the uplink by doubling the bandwidth of the radio channel and avoiding the use of 64-QAM (Quadrature Amplitude Modulation).
Table 3.2. LTE Advanced mobile categories
Categories |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
DL rate (Mbps) |
300 |
300 |
3000 |
450 |
450 |
600 |
600 |
UL rate (Mbps) |
50 |
100 |
1500 |
50 |
100 |
50 |
100 |
Bandwidth (MHz) |
2 × 20 DL |
2 × 20 DL UL |
5 × 20 DL UL |
3 × 20 DL |
3 × 20 DL 2 × 20 UL |
4 × 20 DL |
4 × 20 DL 2 × 20 UL |
Modulation DL |
64-QAM |
64-QAM |
64-QAM |
64-QAM |
64-QAM |
64-QAM |
64-QAM |
Modulation UL |
16-QAM |
16-QAM |
64-QAM |
16-QAM |
16-QAM |
16-QAM |
16-QAM |
MIMO DL |
2 × 2 |
2 × 2 |
8 × 8 |
2 × 2 |
2 × 2 |
2 × 2 |
2 × 2 |
MIMO UL |
n.a. |
n.a. |
4 × 4 |
n.a. |
n.a. |
n.a. |
n.a. |
Release 12 introduced a separation of categories for downlink and uplink. The mobile is characterized by a combination of two categories.
Table 3.3 describes the mobile characteristics for the downlink, for 256-QAM and for the maximum bandwidth. The same bit rate can be achieved by decreasing the bandwidth and increasing the number of MIMO layers.
The mobile category 13 has the same characteristics in terms of bandwidth and MIMO as the mobile categories 5, 6 and 7, the increase in the bit rate being explained by the gain of the modulation.
The mobile category 14 has the same characteristics in terms of bandwidth and MIMO as the mobile category 8, the increase in the bit rate being explained by the gain of the modulation.
The mobile category 15 has the same characteristics in terms of bandwidth and MIMO as the mobile categories 11 and 12, the increase in the bit rate being explained by the gain of the modulation.
Table 3.3. Mobile categories for the downlink from release 12
Categories |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
20 |
Rate (Mbps) |
400 |
4000 |
800 |
1000 |
25000 |
1200 |
1600 |
2000 |
Bandwidth (MHz) |
20 or 2 × 20 |
5 × 20 |
4 × 20 |
5 × 20 |
32 × 20 |
6 × 20 |
8 × 20 |
8 × 20 |
Modulation |
256-QAM |
256-QAM |
256-QAM |
256-QAM |
256-QAM |
256-QAM |
256-QAM |
256-QAM |
MIMO |
4 × 4 or 2 × 2 |
4 × 4 |
2 × 2 |
2 × 2 |
8 × 8 |
2 × 2 |
2 × 2 |
Note |
Note: the 2-Gbps rate is achieved by 2 × 2 MIMO for six radio channels and 4 × 4 MIMO for the remaining two radio channels.
Table 3.4 describes the characteristics of mobiles for the uplink.
Table 3.4. Mobile categories for the uplink from release 12
Categories |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
20 |
21 |
Rate (Mbit/s) |
150 |
9600 |
225 |
100 |
2100 |
200 |
13500 |
300 |
300 |
Bandwidth (MHz) |
2 × 20 |
32 × 20 |
3 × 20 |
20 |
5 × 20 |
2 × 20 |
32 × 20 |
3 × 20 |
4 × 20 |
Modulation |
64-QAM |
64-QAM |
64-QAM |
256-QAM |
256-QAM |
256-QAM |
256-QAM |
256-QAM |
64-QAM |
MIMO |
n.a. |
4 × 4 |
n.a. |
n.a. |
4 × 4 |
n.a. |
4 × 4 |
n.a. |
n.a. |
The LAA mechanism exploits the U-NII (Unlicensed National Information Infrastructure) band at 5 GHz to transmit an LTE signal that is compliant with 3GPP specifications.
The radio channel operating in the licensed frequency band, used as the primary channel (PCell), supports the data of the control plane and the user plane.
The radio channel operating in the U-NII band, used as the secondary channel (SCell), only supports the data of the user plane.
The type-1 frame structure defined for the FDD mode lasts 10 ms and contains 10 sub-frames. Each sub-frame is made up of two time slots (Figure 3.4).
The type-2 frame structure defined for the TDD mode also lasts 10 ms and contains two semi-frames of 5 ms each (Figure 3.5).
Each half-frame consists of five sub-frames and the second can correspond to a special sub-frame containing three particular fields:
The sub-frames are attributed to the data for the uplink and downlink according to diverse configurations (Table 3.5):
Table 3.5. Type-2 frame configuration
Configuration |
Periodicity |
Number of the sub-frame | |||||||||
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 | ||
0 |
5 ms |
D |
S |
U |
U |
U |
D |
S |
U |
U |
U |
1 |
5 ms |
D |
S |
U |
U |
D |
D |
S |
U |
U |
D |
2 |
5 ms |
D |
S |
U |
D |
D |
D |
S |
U |
D |
D |
3 |
10 ms |
D |
S |
U |
U |
U |
D |
D |
D |
D |
D |
4 |
10 ms |
D |
S |
U |
U |
D |
D |
D |
D |
D |
D |
5 |
10 ms |
D |
S |
U |
D |
D |
D |
D |
D |
D |
D |
6 |
5 ms |
D |
S |
U |
U |
U |
D |
S |
U |
U |
D |
D (downlink) sub-frame attributed to the downlink;
U (uplink) sub-frame attributed to the uplink;
S (special) sub-frame containing the three particular fields.
The type-3 frame is applicable for LAA aggregation. It also has a duration of 10 ms. The 10 sub-frames constituting the frame are available for transmission on the downlink.
The transmission may occupy one or more consecutive sub-frames, starting anywhere in a frame and ending with the last sub-frame containing either traffic or the downstream pilot DwPTS.
The eLAA (enhanced LAA) aggregation allows the mobile to use Wi-Fi access in both directions of transmission.
The mechanism to access the radio channel is different for the LTE and Wi-Fi interfaces. For the LTE radio interface, access to the radio channel is controlled by the eNB entity. For the Wi-Fi radio interface, access to the radio channel uses the CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) mechanism.
To avoid interference with other Wi-Fi interfaces, the eNB entity or the mobile applies the LBT (Listen Before Talk) mechanism before transmitting in the U-NII radio channel. The equipment uses energy sensing to determine the presence or absence of other signals on the radio channel during the CCA (Clear Channel Assessment) observation time.
The LBT mechanism has two options: frame-based equipment (FBE) and load-based equipment (LBE).
For the FBE option, the equipment operates on the basis of a synchronization with a fixed frame period. At the end of the frame period, the equipment performs a CCA check on the radio channel. If the channel is free, then the data is transmitted immediately to the beginning of the next frame. If the channel is busy, then another CCA check is performed at the next frame period (Figure 3.6).
For the LBE option, the device performs CCA control whenever there is data to transmit. If the channel is free, then the data is transmitted immediately. If the channel is busy, then the device must wait until the timer for the backoff mechanism expires (Figure 3.7). This timer is decremented when the radio channel is free.
The LBE option is relatively similar to the backoff mechanism of Wi-Fi access. Unlike Wi-Fi access, which adopts an exponential backoff mechanism, the LBE option opts for a backoff mechanism with a fixed window.
The small cell usually deployed as a secondary cell (SCell) does not need to be permanently activated. In this case, a mobile cannot perform quality measurements on this cell.
The discovery reference signal (DRS) has therefore been introduced and includes the following signals:
The reference signal is transmitted with a period of 40, 80 or 160 ms. Only one sub-frame and only the first 12 OFDM symbols of this sub-frame are used to transmit the DRS.
The reference signal can be transmitted in any sub-frame during the discovery measurement timing configuration (DMTC) which lasts 6 ms. The DMTC is shifted by a dmtcOffset value relative to the start of the frame.
The reference signal can be incorporated in the PDSCH when there is data to be transmitted.
LWA aggregation occurs at the PDCP layer. The eNB entity carries out a switching of the bearers between, on the one hand, the S1 bearers and, on the other hand (Figures 3.8 and 3.9):
The LWA bearer is controlled by the eNB entity from measurement reports transmitted by the mobile.
The PDCP frames transmitted over the Wi-Fi access are encapsulated by an LWAAP (LWA Adaptation Protocol) header containing the logical channel identifier (LCID) of the radio bearer.
On LTE access, the LCID is carried by the MAC layer. The recipient uses the LCID to reassemble the PDCP frames of the same bearer.
The re-sequencing of the PDCP frames received by the LTE and Wi-Fi accesses is performed by the PDCP.
Frames transported on an LWA bearer are only these acknowledged, these frames corresponding to RLC frames using the acknowledged mode (AM) on the LTE interface.
The type field of the LLC header for Wi-Fi access is set to hexadecimal 9E65. The mobile uses this value to determine that the frame comes from an LWA bearer.
When the eNB and AP entities are distant, the eNB entity can be connected to multiple AP entities via the Xw interface that supports the traffic and control data (Figure 3.9).
The NAS signaling data is carried on the S1-MME interface, between the MME and eNB entities, and then on the Xw-C (Control) interface, between the eNB and AP entities.
Traffic data, corresponding to the IP stream, is transported in a GTP-U (GPRS Tunneling Protocol User) tunnel:
The WT Addition procedure is initialized by the eNB entity and is used to establish the mobile context at the access point (AP) to provide mobile resources over the Wi-Fi interface (Figure 3.10).
The WT Modification procedure can be initialized either by the eNB entity or by the access point and can be used to modify, set or release bearer contexts or to modify other properties of the mobile context.
The WT Modification procedure initiated by the eNB entity is described in Figure 3.11.
The WT Modification procedure initiated by the access point is described in Figure 3.12.
The WT Release procedure can be initialized either by the eNB entity or by the access point and is used to initiate the release of the mobile context at the access point. The recipient cannot reject the request.
The WT Release procedure initiated by the eNB entity is described in Figure 3.13.
The WT Release procedure initiated by the access point is described in Figure 3.14.
The procedure for changing the access point is initiated by the eNB entity and used to transfer the mobile context from a source AP to a target AP. This procedure is performed using the WT Release and WT Addition procedures.
The LWIP aggregation only applies to the IP packets of the S1 bearer. The RRC (Radio Resource Control) and signaling messages, which are exchanged between the mobile and the eNB entity, are carried on the LTE interface (Figure 3.15).
IP packets are transported between the eNB entity and the mobile in the LWIP tunnel. The LWIPEP (LWIP Encapsulation Protocol) header contains the LCID of the radio bearer.
The LWIP tunnel is protected between the mobile and the security gateway (SeGW) through an IP Security (IPSec) mechanism. Only one IPSec mechanism is mounted for all LWIP tunnels.
The LWIP tunnel is transmitted in a GTP-U tunnel on the Xw interface, between the eNB and the SeGW entities.
The IKE procedure for the IPSec mechanism is initialized after the association of the mobile with the Wi-Fi access point and authentication based on the EAP-AKA (Extensible Authentication Protocol–Authentication and Key Agreement) method.
Each bearer is configured so that the downstream direction, the upstream direction or both directions of transmission pass through the tunnel protected by the IPSec mechanism.
For the downstream, IP packets are transmitted either on the LTE interface only or on the Wi-Fi interface only, or simultaneously on both LTE and Wi-Fi interfaces. In the latter case, the mobile can receive IP packets out of sequence.
For the upstream, IP packets are transmitted either on the LTE interface only or on the Wi-Fi interface only.
The procedure for establishing the LWIP and IPSec tunnels is described in Figure 3.16.
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