LTE Layers Data Flow
Below is a logical digram of E-UTRAN Protocol layers with a depiction of data flow through various layers:
Packets received by a layer are called Service Data Unit (SDU) while the packet output of a layer is referred to by Protocol Data Unit (PDU). Let's see the flow of data from top to bottom:
- IP Layer submits PDCP SDUs (IP Packets) to the PDCP layer. PDCP layer does header compression and adds PDCP header to these PDCP SDUs. PDCP Layer submits PDCP PDUs (RLC SDUs) to RLC layer.PDCP Header Compression: PDCP removes IP header (Minimum 20 bytes) from PDU, and adds Token of 1-4 bytes. Which provides a tremendous savings in the amount of header that would otherwise have to go over the air.
- RLC layer does segmentation of these SDUS to make the RLC PDUs. RLC adds header based on RLC mode of operation. RLC submits these RLC PDUs (MAC SDUs) to the MAC layer.RLC Segmentation: If an RLC SDU is large, or the available radio data rate is low (resulting in small transport blocks), the RLC SDU may be split among several RLC PDUs. If the RLC SDU is small, or the available radio data rate is high, several RLC SDUs may be packed into a single PDU.
- MAC layer adds header and does padding to fit this MAC SDU in TTI. MAC layer submits MAC PDU to physical layer for transmitting it onto physical channels.
- Physical channel transmits this data into slots of sub frame.
LTE Communication Channels
The information flows between the different protocols are known as channels and signals. LTE uses several different types of logical, transport and physical channel, which are distinguished by the kind of information they carry and by the way in which the information is processed.
- Logical Channels: : Define whattype of information is transmitted over the air, e.g. traffic channels, control channels, system broadcast, etc. Data and signalling messages are carried on logical channels between the RLC and MAC protocols.
- Transport Channels: Define howis something transmitted over the air, e.g. what are encoding, interleaving options used to transmit data. Data and signalling messages are carried on transport channels between the MAC and the physical layer.
- Physical Channels: Define whereis something transmitted over the air, e.g. first N symbols in the DL frame. Data and signalling messages are carried on physical channels between the different levels of the physical layer.
Logical channels define what type of data is transferred. These channels define the data-transfer services offered by the MAC layer. Data and signalling messages are carried on logical channels between the RLC and MAC protocols.
Logical channels can be divided into control channels and traffic channels. Control Channel can be either common channel or dedicated channel. A common channel means common to all users in a cell (Point to multipoint) while dedicated channels means channels can be used only by one user (Point to Point).
Logical channels are distinguished by the information they carry and can be classified in two ways. Firstly, logical traffic channels carry data in the user plane, while logical control channels carry signalling messages in the control plane. Following table lists the logical channels that are used by LTE:
|Channel Name||Acronym||Control channel||Traffic channel|
|Broadcast Control Channel||BCCH||X|
|Paging Control Channel||PCCH||X|
|Common Control Channel||CCCH||X|
|Dedicated Control Channel||DCCH||X|
|Multicast Control Channel||MCCH||X|
|Dedicated Traffic Channel||DTCH||X|
|Multicast Traffic Channel||MTCH||X|
Transport channels define how and with what type of characteristics the data is transferred by the physical layer. Data and signalling messages are carried on transport channels between the MAC and the physical layer.
Transport Channels are distinguished by the ways in which the transport channel processor manipulates them. Following table lists the transport channels that are used by LTE:
|Downlink Shared Channel||DL-SCH||X|
|Uplink Shared Channel||UL-SCH||X|
|Random Access Channel||RACH||X|
Data and signalling messages are carried on physical channels between the different levels of the physical layer and accordingly they are divided into two parts:
- Physical Data Channels
- Physical Control Channels
PHYSICAL DATA CHANNELS
Physical data channels are distinguished by the ways in which the physical channel processor manipulates them, and by the ways in which they are mapped onto the symbols and sub-carriers used by Orthogonal frequency-division multiplexing (OFDMA). Following table lists the physical data channelsthat are used by LTE:
|Physical downlink shared channel||PDSCH||X|
|Physical broadcast channel||PBCH||X|
|Physical multicast channel||PMCH||X|
|Physical uplink shared channel||PUSCH||X|
|Physical random access channel||PRACH||X|
The transport channel processor composes several types of control information, to support the low-level operation of the physical layer. These are listed in the below table:
|Downlink control information||DCI||X|
|Control format indicator||CFI||X|
|Hybrid ARQ indicator||HI||X|
|Uplink control information||UCI||X|
PHYSICAL CONTROL CHANNELS
The transport channel processor also creates control information that supports the low-level operation of the physical layer and sends this information to the physical channel processor in the form of physical control channels.
The information travels as far as the transport channel processor in the receiver, but is completely invisible to higher layers. Similarly, the physical channel processor creates physical signals, which support the lowest-level aspects of the system.
Physical Control Channels are listed in the below table:
|Physical control format indicator channel||PCFICH||X|
|Physical hybrid ARQ indicator channel||PHICH||X|
|Physical downlink control channel||PDCCH||X|
|Relay physical downlink control channel||R-PDCCH||X|
|Physical uplink control channel||PUCCH||X|
The base station also transmits two other physical signals, which help the mobile acquire the base station after it first switches on. These are known as the primary synchronization signal (PSS) and the secondary synchronization signal (SSS).
LTE OFDM Technology
To overcome the effect of multi path fading problem available in UMTS, LTE uses Orthogonal Frequency Division Multiplexing (OFDM) for the downlink - that is, from the base station to the terminal to transmit the data over many narrow band careers of 180 KHz each instead of spreading one signal over the complete 5MHz career bandwidth ie. OFDM uses a large number of narrow sub-carriers for multi-carrier transmission to carry data.
Orthogonal frequency-division multiplexing (OFDM), is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier modulation method.
OFDM meets the LTE requirement for spectrum flexibility and enables cost-efficient solutions for very wide carriers with high peak rates. The basic LTE downlink physical resource can be seen as a time-frequency grid, as illustrated in Figure below:
The OFDM symbols are grouped into resource blocks. The resource blocks have a total size of 180kHz in the frequency domain and 0.5ms in the time domain. Each 1ms Transmission Time Interval (TTI) consists of two slots (Tslot).
Each user is allocated a number of so-called resource blocks in the time.frequency grid. The more resource blocks a user gets, and the higher the modulation used in the resource elements, the higher the bit-rate. Which resource blocks and how many the user gets at a given point in time depend on advanced scheduling mechanisms in the frequency and time dimensions.
The scheduling mechanisms in LTE are similar to those used in HSPA, and enable optimal performance for different services in different radio environments.
Advantages of OFDM
- The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions (for example, attenuation of high frequencies in a long copper wire, narrowband interference and frequency-selective fading due to multipath) without complex equalization filters.
- Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal.
- The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to eliminate inter symbol interference (ISI).
- This mechanism also facilitates the design of single frequency networks (SFNs), where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be combined constructively, rather than interfering as would typically occur in a traditional single-carrier system.
Drawbacks of OFDM
- High peak-to-average ratio
- Sensitive to frequency offset, hence to Doppler-shift as well.
LTE uses a pre-coded version of OFDM called Single Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink. This is to compensate for a drawback with normal OFDM, which has a very high Peak to Average Power Ratio (PAPR).
High PAPR requires expensive and inefficient power amplifiers with high requirements on linearity, which increases the cost of the terminal and drains the battery faster.
SC-FDMA solves this problem by grouping together the resource blocks in such a way that reduces the need for linearity, and so power consumption, in the power amplifier. A low PAPR also improves coverage and the cell-edge performance.