LTE Network Protocol Architecture

Uu – Control/User Plane

The protocol stack used on radio interface Uu is shown in Figure 1. The physical layer in this stack is represented by OFDM in the DL and SC-FDMA in the UL. Then we see the MAC protocol that is responsible for mapping the transport channels onto the physical channels, but also for such important tasks as packet scheduling and timing advance control. RLC provides reliable transport services and can be used to segment/reassemble large frames. The main purpose of PDCP is the compression of larger IP headers as well as ciphering of user plane data and integrity protection of both user plane and control plane data.

 
Figure 1: Protocol stack LTE Uu interface

On top of PDCP the stack is split into the user plane and control plane parts. On the control plane side we see RRC protocol, that is, the expression for the communication between the UE and eNB. RRC provides all the necessary functions to set up, maintain, and release a radio connection for a particular subscriber. 

RRC also serves as a transport protocol for NAS signaling messages. NAS is the expression for the communication between the UE and MME in which MME represents the core network.

On the user plane side we see IP as the transport layer for end-to-end applications. On the Uu stack the IP is always end-to-end IP, which means that all these IP packets are transparently routed, often tunneled through the mobile network. The user plane IP frames we see on Uu are the same IP frames that can be monitored at SGi reference points before or behind the PDN-GW.

The IP version can be Internet Protocol version 4 (IPv4) or Internet Protocol version 6 (IPv6). In the case of VPN (Virtual Private Network) traffic, IPsec will be used.

The applications on top of IP in the user plane stack are all protocols of the TCP/IP suite, such as the File Transfer Protocol (FTP), HTTP (web-browsing), and POP3/SMTP (for e-mail), but also Real-Time Transport Protocol (RTP) and SIP for real-time services like VoIP.

Initial UE Radio Access

Cell search is a procedure for synchronizing time and frequency to a base station sector. Additionally, cell search and synchronization include deriving basic information of the target cell.

LTE defines a hierarchical cell search as it is deployed with WCDMA UMTS. PSS and SSS provide radio frame and slot synchronization, as well as information like the duplex mode TDD or FDD and the physical layer group and c-ID.

UEs synchronizing to a new LTE cell start searching for a PSS which is a Zadoff–Chu sequence. Three sequences are defined indicating the physical cell group ID. Three physical layer c-ID groups with 168 physical layer c-IDs each are defined. After successfully detecting the PSS with its physical layer c-ID group and slot timing, the SSS is decoded which is broadcasted one OFDM symbol prior to the PSS. Now the UE retrieved DL slot, radio frame timing and frequency synchronization. With decoding successfully PSS and SSS, it obtained as well the complete 9-bit physical layer c-ID together with the radio frame type (either type 1 for FDD or type 2 for TDD) and the CP length.

Figure 1 illustrates the initial steps of cell synchronization and access. After synchronization, the UE is ready to detect and decode the PBCH in order to derive the system bandwidth, PHICH configuration, and the current System Frame Number (SFN). Other common system information now needs to be retrieved from the DL-SCH. SIBs are scheduled on regular shared channel resources by using a special C-RNTI = 0xFFFF. SIBs provide general system configuration information like UL configuration and random access configuration.

 
Figure 1: Initial cell access with level of retrieved information

Channel Mapping and Multiplexing

Besides physical and transport channels, LTE also defines logical channels. Logical channels are multiplexed to transport channels within the MAC layer. Logical channels map different content connections to transport channels, like CCCHs to multiple UEs or DCCHs to a specific UE or dedicated transport channels carrying higher layer application data.

Logical channels are addressed with a logical channel ID. The logical ID is a field within the MAC header PDU. Logical channels are multiplexed by using logical channel IDs to transport channels specifying where the information should be transmitted. Finally, transport channels are transferred with physical channels as a service provided by the physical channel.

Figures 1 and 2 show the above-described channel architecture from the basic physical channels via transport channels to logical channels bearing higher layer messages for DL and UL respectively.

 
Figure 1: Downlink channel mapping and multiplexing from logical channels via transport channels to physical channels

 
Figure 2: Uplink channel mapping and multiplexing from logical channels via transport channels to physical channels

Two basic sets of logical channels are defined:

• Control channels: CCCH and DCCHs.
• Traffic channels: CCCH and DTCHs.

The nature of common channels is such that no specific UE is addressed, but the information is either general for all cell-wide subscribers or a message from a UE which has not yet established a dedicated control/traffic channel. A typical example of a CCCH is the broadcast of SIBs.

Traffic channels carry user plane protocols like the Packet Data Convergence Protocol (PDCP) and application IP packets, while control channels carry control plane protocols as RRC and NAS.

• DL logical channels:
  • – Broadcast Control Channel (BCCH).
  • – Paging Control Channel (PCCH).
  • – Common Control Channel (CCCH).
  • – Dedicated Control Channel (DCCH).
  • – Dedicated Traffic Channel (DTCH).
• UL logical channels:
  • – Common Control Channel (CCCH).
  • – Dedicated Control Channel (DCCH).
  • – Dedicated Traffic Channel (DTCH).

Transport Channels in LTE

The physical layer provides a transport service for MAC PDUs. This service is accessed by transport channels. Most transport channels are directly mapped to physical channels. Thus, in other words, transport channels are the gateway to physical channels and a selection for MAC PDUs where they are to be transmitted.

The following DL transport channel types are defined:

• Broadcast Channel (BCH):
  • – Uses a static transport format and has the requirement that all UEs within the cell have to receive its information error-free. The reception of the BCH is mandatory for accessing any service of a cell.
• DL-SCH:
  • – Carries all semi-static broadcast information (SIB) and all UE-specific traffic channels.
  • – DL-SCH is secured with HARQ algorithms.
  • – Efficiency is realized with AMC link adaptation.
  • – Various TMs are defined to meet different environment scenarios to increase efficiency in respect of current conditions.
  • – DRX is available in order to increase handset operating time.
  • – Makes use of spatial algorithms like beamforming or MIMO.
• Paging Channel (PCH):
  • – Needs to be received in complete cell coverage area.
  • – Supports DRX in order to increase battery operating cycle.
  • – Dynamically allocated via own physical identifier (P-RNTI).
• Multicast Channel (MCH):
  • – Broadcast to entire cell coverage area.
  • – MBMS transmission with use of multiple cells.

A designated DL control channel is not defined as the PDCCH is used for physical channel control only. All the higher layer control plane is transmitted via the DL-SCH.

Defined UL transport channel types are shown in the following items:

• UL-SCH:
  • – UL-SCH is secured with HARQ algorithms.
  • – Fully dynamic and semi-static resource allocation schemes.
  • – Can make use of multi-user MIMO (UL "virtual" MIMO).
  • – Uses dynamic link adaptation like AMC.
• RACH:
  • – Accessible without UL synchronization.
  • – Collision-based and collision-free operating modes.
  • – Various modes depending on cell size and interference.

As in the DL direction, no UL control transport channel is defined as all the higher layer control plane is transmitted on the UL-SCH. The PUCCH is a control channel used by the physical layer only.

Link Adaptation in LTE

Mobile wireless reception conditions vary greatly over frequency and time as described in Section 1.8. In order to cope with these circumstances and guarantee best possible QoS, a procedure is implemented known as Adaptive Modulation and Coding (AMC). AMC controls and changes transmission parameters to achieve a defined Transport Block Error Rate (BLER) of below 10%, in order to keep retransmissions in a suitable range. This is done by adapting the modulation scheme, in LTE on the shared channels between QPSK and 64QAM, and the Forward Error Correction (FEC) coding rate.

Different modulation schemes make the bit detection more robust against noise and other distortion caused by the wireless channel. Figure 1 shows the applied modulation scheme for the LTE shared channels. Most robust transmission is achieved by mapping just 2 bits to each modulation symbol as seen with QPSK, resulting in four stages. A large distance between modulation points as seen with QPSK allows a higher probability of the correct decision at the receiver even with noisy reception conditions. Both 16QAM and 64QAM map 4 and 6 bits respectively to one modulation symbol used with better wireless channel conditions to achieve a higher data throughput. It is to find the best compromise between the modulation scheme and code rate for a given channel quality. LTE defines a list of MCS combinations and just signals an MCS index.

 
Figure 1: Different QAM schemes used with LTE and the number of bits mapped to each scheme

The data modulated with the different modulation schemes to subcarriers needs to be protected against transmission errors. LTE defines a turbo de-/encoder with trellis termination of a native code rate of one-third. The turbo coder adds redundancy bits to the data, which makes it possible to correct some bit errors. The code rate is a fraction of source data rate to resulting protected data rate; thus, a code rate of one-third encodes 1 bit into 3 bits. Other code rates are needed in order to optimize the trade-off between protection and efficiency. This is done by puncturing the native coded bit stream to a higher (less protection) code rate by deterministically leaving out coded bits, or by deterministically repeating coded bits if a smaller code rate is desired (more protection).

Additionally, one parameter being controlled is the UL transmit power. UL power control is implemented to deal with the near–far effect. Figure 2 shows an UL scenario with a near–far effect compared to a DL scenario without power differences between user signals as they are equally attenuated because the mix of the signal is transmitted from one position (eNB). This occurs when a user is close to the base station (near) and another user is far away from the base station, introducing a higher power path loss which leads to a lower receive power of the signal of the cell edge user. All UL receive signals should have equal power in order to have the same analog-to-digital converter saturation of each signal to reduce the quantization noise of users with low received signals, reducing inter-subcarrier interference between the users. This happens with imperfect UL synchronization within real-life scenarios.

 
Figure 2: Near–far effect occurring in uplink direction, compared to equal signal strength reception in downlink

UL TPC commands are sent via designated DCI formats 3 and 3A. DCI 3 and 3A are differential power control commands for PUCCH and PUSCH transmission in steps of decibels. DCI 3 is a 2bit assignment as opposed to DCI 3A which is a single bit command. These dedicated DCIs with TPC commands are only used when there is no data to be transmitted to the UE; otherwise, the TPC command is transmitted embedded in other control information on the PDCCH for this UE. An initial 3-bit TPC command is embedded in the RAR message. The different TPC commands are listed in Tables 1 and 2.

Table 1: Mapping of TPC command field in DCI format 1A/1B/1D/1/2A/2/3 to δ_PUCCH values. Reproduced with permission from © 3GPP^™ 

TPC command field in DCI format 1A/1B/1D/1/2A/2/3	δPUCCH (dB)
0	-1
1	0
2	0
3	3

Table 2: Mapping of TPC command field in DCI format 3A to δ_PUCCH values. Reproduced with permission from © 3GPP^™ 

TPC command field in DCI format 3A	OPUCCH (dB)
0	-1
1	1

UEs report their received channel quality to the eNB by transmitting a CQI value. The CQI value represents either a wideband receive quality as a scalar or a more detailed report about frequency sections (sub-bands) as a vector. CQI reports are transmitted periodically or aperiodically configured by higher layers. Sub-band CQI reports indicate the receive quality of each sub-band relative to the wideband average with four steps: worse, equal, better, and much better. The UE reports the best M sub-bands compared to the average channel quality with the best M method as depicted in Figure 3.

 
Figure 3: CQI illustration with sub-bands and best M reporting. Reproduced with permission from Nomor

The UE should derive a MCS scheme from the above measurement information, which is indexing 1 MCS out of 16 to suit the target BLER of below 10%. This enables differentiation between high- and low-cost handsets which use more or less expensive RF hardware and/or a more sophisticated IQ signal processing engine.

Recapitulating, LTE link adaptation uses various stacked link adaptation techniques, securing transmission, or in order to make it more effective using different control loop delays regarding the process's dimension. The following items summarize the LTE link adaptation functions with their responsiveness:

Adaptive frequency-selective scheduling: Assign frequency resource to UEs on a 1 ms basis, which provides each UE with the individual best reception quality. AMC: Obtain the most efficient modulation and FEC code rate in order to balance retransmissions vs. maximization of throughput. HARQ: Multiple retransmission process using prior transmission to increase the correct decoding probability. TPC: Provides UL power control in order to minimize multiple user interference.