3.2.1. Radio channels

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 radio channels can be adjacent in the same frequency bands;
• – the radio channels can be spaced out in the same frequency bands;
• – the radio channels can be localized in different frequency bands.

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 mechanisms for random access and RRC (Radio Resource Control) connection, which only occur on the primary cell;
• – the NAS (Non-Access Stratum) messages related to the mobility and the session management, which are only transmitted in the primary cell;
• – the IP packet from the user plane.

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.

3.2.2. PDCCH physical channel

The physical downlink control channel (PDCCH) carries the downlink control information (DCI) for one or more user equipment (UE):

• – resource allocation, modulation and coding schemes of the data transmitted in the physical downlink shared channel (PDSCH) and in the physical uplink shared channel (PUSCH);
• – transmit power control of the physical uplink control channel (PUCCH) and the PUSCH.

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 PDCCH is only carried by the primary radio channel (PCell);
• – the PDCCH is carried by the primary radio channel (PCell) and the secondary radio channel (SCell).

3.2.3. MAC layer

3.2.4. Mobile categories

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).

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).

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 3.4 describes the characteristics of mobiles for the uplink.

3.3.1. Frame structure

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:

• – a field for the downlink pilot time slot (DwPTS) in the downlink. This field can contain data;
• – a field for the uplink pilot time slot (UpPTS) in the uplink. This field can contain data or a preamble for the random access;
• – a time of gap period (GP) between the two preceding fields. This interval time facilitates the compensation of a time difference between different mobiles and avoids an overlap between the two transmission directions.

The sub-frames are attributed to the data for the uplink and downlink according to diverse configurations (Table 3.5):

• – sub-frames 0 and 5 are always allocated to traffic in the downlink;
• – sub-frame 1 is always allocated to the special sub-frame containing the three particular fields;
• – sub-frame 2 is always allocated to traffic in the uplink;
• – sub-frame 6 can be allocated to the special sub-frame containing three particular fields for a periodicity of 5 ms;
• – sub-frames 3, 4, 7, 8 and 9 are allocated to the downlink or uplink traffic according to the selected configuration.

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.

3.3.2. Access to the radio channel

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.

3.3.3. Discovery reference signal (DRS)

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 primary synchronization signal (PSS) ensuring the frequency synchronization and some of the time synchronization;
• – the secondary synchronization signal (SSS) completing the time synchronization. The two physical signals PSS and SSS make it possible to determine the physical-layer cell identity (PCI);
• – the cell-specific reference signal (CRS) used to calculate the transfer function of the radio channel and to make the coherent demodulation of the received signal. The CRS also makes it possible to measure the reference signal received power (RSRP) and the reference signal received quality (RSRQ).

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.

3.4.1. Protocol architecture

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 LTE bearer, for which the data only transits on the LTE access;
• – the shared LWA bearer, for which the data can pass on both LTE and Wi-Fi accesses;
• – the switched LWA bearer, for which the data only passes over the Wi-Fi access.

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:

• – on the S1-U interface, between the SGW and eNB entities;
• – on the Xw-U (User) interface, between the eNB and AP entities.

3.4.2. Procedures

3.4.2.1. WT Addition procedure

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).

1. 1) The eNB entity transmits to the access point (AP) the message Xw-AP WT Addition Request in order to allocate resources to the mobile, indicating the characteristics of the LWA bearer.
2. 2) If the access point can accept the resource request, it responds with the message Xw-AP WT Addition Request Acknowledge.
3. 3) The eNB entity sends the message RRC ConnectionReconfiguration to the mobile, indicating the configuration of the radio resource.
4. 4) The mobile applies the new configuration and responds to the eNB entity with the message RRC ConnectionReconfigurationComplete.
5. 5) The mobile associates with the access point which then transmits the message Xw-AP WT Association Confirmation to the eNB entity.

3.4.2.2. WT Modification procedure

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.

1. 1) The eNB entity sends the message Xw-AP WT Modification Request to request the access point to modify the specific bearer resources.
2. 2) If the access point accepts the request, it applies the configuration modification to the resource and responds with the message Xw-AP WT Modification Request Acknowledge.
3. 3) If the modification requires a new configuration for the mobile, the eNB entity sends the RRC ConnectionReconfiguration message, including the new configuration of the Wi-Fi radio resource.
4. 4) The mobile applies the new configuration and responds with the message RRC ConnectionReconfigurationComplete.

The WT Modification procedure initiated by the access point is described in Figure 3.12.

1. 1) The access point sends the message Xw-AP WT Modification Required to the eNB entity to modify the radio resources of the Wi-Fi access.
2. 2) The eNB responds with the message Xw-AP WT Change Confirm.
3. 3) If the modification requires a new configuration for the mobile, the eNB entity sends the message RRC ConnectionReconfiguration, including the new configuration of the Wi-Fi radio resource.
4. 4) The mobile applies the new configuration and responds with the message RRC ConnectionReconfigurationComplete.

3.4.2.3. WT Release procedure

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.

1. 1) The eNB entity sends the message Xw-AP WT Release Request to request the Wi-Fi access point to release the allocated radio resources over the Wi-Fi access.
2. 2) If necessary, the eNB entity sends the message RRC ConnectionReconfiguration to the mobile, indicating the release of the radio resources.
3. 3) The mobile responds with the message RRC ConnectionReconfigurationComplete.

The WT Release procedure initiated by the access point is described in Figure 3.14.

1. 1) The access point sends the message Xw-AP WT Release Required to the eNB entity to request the release of radio resources from the Wi-Fi access.
2. 2) The eNB entity responds with the message Xw-AP WT Release Confirm.
3. 3) If necessary, the eNB entity sends the message RRC ConnectionReconfiguration to the mobile, indicating the release of the radio resources.
4. 4) The mobile responds with the message RRC ConnectionReconfigurationComplete.

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.

3.5. LWIP aggregation

3.5.1. Protocol architecture

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.

3.5.2. Tunnel establishment

The procedure for establishing the LWIP and IPSec tunnels is described in Figure 3.16.

1. 1) The eNB entity configures the mobile with the message RRC ConnectionReconfiguration to perform measurements on Wi-Fi access in order to start the LWIP and IPSec tunnel establishment.
2. 2) The mobile applies the new configuration and responds with the message RRC ConnectionReconfigurationComplete.
3. 3) The mobile sends to the eNB entity the message RRC WLANMeasurements containing the measurements performed on the Wi-Fi access.
4. 4) The eNB entity sends the message Xw-AP LWIP Addition Request to request the security gateway (SeGW) to allocate resources for IPSec tunnel establishment.
5. 5) If the security gateway accepts the request, it responds with the message Xw-AP LWIP Addition Request Acknowledge.
6. 6) The eNB entity sends the message RRC ConnectionReconfiguration to the mobile to establish the LWIP tunnel.
7. 7) The mobile applies the new configuration and responds with the message RRC ConnectionReconfigurationComplete.
8. 8) The mobile sends the confirmation of the association with the Wi-Fi access point to the eNB entity in the message RRC WLANConnectionStatusReport.
9. 9) The eNB entity sends the message RRC ConnectionReconfiguration to the mobile to establish the IPSec tunnel and can configure the bearers that will use the IPSec tunnel.
10. 10) The mobile applies the new configuration and responds with the message RRC ConnectionReconfigurationComplete.