LTE Radio Interface

LTE is not complete without the radio interface. It has been my burning desire to understand the radio network of LTE. I did some research and this is the second post on radio side of the network.

LTE Radio Interface User Plane protocols

In downlink data from SAE will enter eNB. The data is an IP packet. The IP packet is several protocols and is passed to UE.

LTE Radio Interface Control Plane Protocols

The control has two more layers over PDCP. RRC layer is terminated at eNB, while NAS layer goes all the way to MME.

Lets take a look at each layer individually: -

NAS: Non-Access Stratum (3GPP TS 24.301)

NAS is responsible for EPS bearer management, authentication, paging and mobility handling in ECM IDLE state.

RRC: Radio Resource Control (3GPP TS 36.331)

This layer is responsible for Broadcast and paging. It also takes care of RRC connection management, radio bearer control, mobility functions and UE measurement reporting and control.

PDCP: Packet Data Control Protocol (3GPP TS 36.323)

This layer is responsible for IP header compression to avoid unnecessary overhead in the payload. This layer is also responsible for ciphering and integrity protection check.

RLC: Radio Link Control (3GPP TS 36.322)           

RLC is responsible for segmentation/concatenation, retransmission handling and in sequence delivery of messages to higher layers. RLC offers services to PDCP in form of radio bearer. These radio bearers are mapped to EPS bearers in EPC.

MAC: Media Access Control (3GPP TS 36.321)

Mac handles ARQ, uplink and downlink scheduling. The scheduling functionality is located in eNB. There is one MAC entity per cell for both uplink and downlink. The HARQ is present in both UE and eNB. MAC offers services to RLC inform of logical channels.

Physical Layer: (3GPP TS 36.201)

It handles coding/decoding, modulation/demodulation, multiple antennas etc. It offers services to MAC layer inform of transport channels.

LTE Channels Over view: (3GPP TS 36.300)

LTE Physical Channels: Downlink Channels

-       Physical Broadcast Channel: PBCH

-       Physical Control Format Indicator Format: PCFICH

o   This informs UE about number of OFDM symbols used for the PDCCH’s.

o   This is transmitted in downlink.

-       Physical Downlink Control Channel: PDCCH

o   Informs UE about resource allocation of PCH & DL-SCH and HARQ information related to DL-SCH

o   PCH: Paging channel. DL-SCH: Downlink Synchronization Channel.

-       Physical Hybrid ARQ Indicator Channel: PHICH

o   Carries Hybrid ARQ Ack/NAK’s in response to uplink transmission

-       Physical Downlink Shared Channel: PDSCH

o   Carries DL-SCH and PCH

-       Physical Multicast Channel: PMCH

o   Carries Multicast channel (MCH).

Uplink Channels

-       Physical Uplink Control Channel: PUCCH

o   Carries HARQ ACK/NAK in response to downlink transmission

o   Carries scheduling request.

-       Physical Uplink Share Channel: PUSCH

o   Carries UL-SCH

-       Physical Random Access channel: PRACH

o   Carries random access preamble.

LTE Transport Channels

The physical layer offers information transfer services to MAC and higher layers. The physical layer transport services are described by how and with characteristics data is transferred over the radio interface. (What kind of data is transferred is dealt in logical channels)

Downlink Transport Channels:

-       Broadcast channel: BCH

o   This channel is used to broadcast info in the entire cell.

o   It has fixed and pre defined Transport Format (not aware of TF’s yet)

-       Downlink Shared Channel: DL-SCH

o   This channel is used for transmitting downlink data.

o   It supports HARQ, dynamic link adaptation.

o   It I can be used to broadcast data in entire cell.

o   It supports UE discontinuous reception (DRX) to enable power saving in UE.

o   It also supports MBMS transmission.

-       Paging Channel: PCH

o   Used for transmitting paging information.

o   PCH supports DRX so that UE can sleep and wakeup to receive PCH in specific time intervals.

-       Multicast Channel: MCH

o   This channel is used to support MBMS.

Uplink Transport Channels:

-       Uplink Shared Channel: UL-SCH

o   Supports HARQ

o   Counter part of DL-SCH

-       Random Access Channel: RACH

Transport and Physical Channel Mapping

Downlink Channels:

Uplink Channels:

LTE Layer 2:

LTE layer 2 is split in MAC, RLC and PDCP.

Layer 2 Structure of downlink

Layer 2 Uplink Structure

The communication between two sub-layers is marked with circles. These are called Service Access Points (SAP). SAP between Physical layer and MAC sub-layer provides the transport channels. The SPA’s between MAC and RLC provide logical channels. Multiplexing several logical channels (i.e radio bearers) to same transport channel is preformed by MAC sub-layer.

Logical Channels:

MAC sub layer offers different kind of data services to RLC inform of logical channels. Logical channels define what type of data is transferred between UE and eNB. Logical Channels are classified into Control Channels (for control plane information transfer) and Traffic Channels (for transfer of user plane data)

Control Channels:

-       Broadcast Control Channel: BCCCH

o   This channel is used of broadcasting system control information.

o   This is downlink channel.

-       Paging Control Channel: PCCH

o   Downlink channel.

o   Transfers paging information and system information change notification.

o   This channel is used for paging when the network does not know the location cell of the UE.

-       Common Control Channel: CCCH

o   Channel of transmitting control information between UE and network.

o   This channel is used for UE’s having no RRC connection with the network.

-       Multicast Control Channel: MCCH

o   Point to Multi point downlink channel used for transmitting MBMS control information from the network to UE.

o   This channel is only used by UE’s that receive MBMS.

-       Dedicated Control Channel: DCCH

o   A point-to-point bi directional channel that transmits dedicated control information between a UE and the network.

o   Used by UE’s having an RRC connection.

Traffic Channels:

-       Dedicated Traffic Channel: DTCH

o   Uplink and downlink channel.

o   Point-to-point channel dedicated to one UE for transfer of user data.

-       Multicast Traffic Channel: MTCH

o   Point-to-Multipoint downlink channel for transmitting traffic data from network to UE.

Mapping logical and transport channels:

Uplink

Downlink

LTE: Physical Layer

I couldn't control my desire to learn the LTE physical layer, so I pushed everything aside and started reading 36 series specs. The Radio network of LTE looks fairly simple at a glance but the complexity increases as we go deep, just like any other system.

LTE Uu interface is what I am looking at. eNB behaves like a relay mapping the radio network to the IP network. The IP side consists of an interface towards MME over S1_MME and towards SGW over S1_U. The radio side communication also has two planes, user plane and control plane.

The above figure shows EUTRAN user plane. As we see we have a MAC layer, RLC and PDCP. Individual protocols shall be dealt with later. The user plane looks fairly simple as data from UE goes to eNB and eNB maps this data over GTP tunnel and sends it to SGW over S1_U. MAC, RLC and PDCP are at Layer 2 in UE and eNB.

However control plane adds few more things. L3 comes into picture for NAS signaling. This NAS signaling is carried all over to MME inside RRC signal.

Over PDCP we have RRC layer which is responsible for paging,RRC connection management, mobility functions etc etc. RRC is terminated in eNB. But NAS is terminated in MME. NAS is responsible for EPS bearer management, Authentication, security etc. Attach Request is a NAS signal which is carried all the way to MME.

Stepping few layers below we have PHY which is physical layer. This is where the actual engineering is. The whole concepts of high speeds come into picture because of sophisticated physical layer. Its no secrete what technologies are used here. OFDMA with 64 QAM and 2x2 MIMO is the most discussed combination for LTE. How does this combination give us such high speeds?

QAM : Quadrature Amplitude Modulation

Going back to engineering basics, we have a simple modulation scheme called PSK. Phase shift keying, which is analog to digital modulation scheme(transmitter side). In PSK we have 1 bit per symbol .0 and 1. Each bit is associated with a Phase shift. with 4 Phase shifts we can transmit 2 bits per symbol. As with 64 QAM we shall be able to transmit 6 bits per symbol. If we look at this scheme in the given bandwidth, by changing the modulation scheme, we are able to transmit more and more bits. This is resulting in increase of data rates.

Time to look at Shannon's theorem :

As I said above, changing the modulation scheme gives you more throughput. However hight modulation schemes can be only be used when the signal to noise ratio is high. From above theorem, channel capacity is bandwidth multiplied by logarithm of SNR. Higher the SNR higher is the channel capacity which means more throughput.

Second factor which increases channel capacity is bandwidth. Now bandwidth is directly proportional to symbol rate. Higher the symbol rate then higher is the bandwidth. But again, increasing the symbol rate doesn't increase the channel efficiency as channel bandwidth is fixed because available spectrum is finite. So there is a trade off between symbol rate and channel throughput. The basic idea is keeping on increasing the symbol rate(modulation scheme) doesn't always improve the efficiency. So considering these factors I think 64 QAM should be a suitable choice for LTE.

OFDM : Orthogonal Frequency Division Multiplexing

With above in mind lets head to OFDM. The theory behind OFDM is little confusing. Lets understand the below figure (FDMA).

Consider we have X amount of spectrum. This can be divided into channels of each Y amount of bandwidth. Each channel is separated by Guard band to avoid interference. This is basic idea in normal multiplexing schemes. I believe in CDMA we identify each channel by a code (?). So what is happening is we have equally spaced channels occupying the entire bandwidth. Note that these channels are non overlapping. Each channel has a subcarrier(?).

In OFDM: With OFDM systems, it is possible to increase throughput in a given channel without increasing channel bandwidth or the order of the modulation scheme. This is done using digital signal processing methods that enable a single channel to be created out of a series of orthogonal subcarriers. As below figure illustrates, subcarriers are orthogonal to one another such that the maximum power of each subcarrier corresponds with the minimum power (zero-crossing point) of the adjacent subcarrier. In a typical system, the bit stream for a channel is multiplexed across various subcarriers. These subcarriers are processed with an inverse Fourier transform (IFT) and combined into a single stream. As a result, multiple streams can be transmitted in parallel while preserving the relative phase and frequency relationship between them.

This way we can include more number of subcarriers in a given bandwidth thus increasing the overall system throughput.

MIMO : Multiple Input Multiple Output

The Shannon's theorem above is assumed to have 1 transmitter and 1 receiver antenna. If we consider multiple antennas then the theorem could be modified as

Thus in theory increasing the antennas will effectively increase the channel capacity without any change in available bandwidth. Now what we can do with MIMO is increase SNR by transmitting a unique bit stream using multiple antenna in the same channel. This is called Spatial Multiplexing.

With MIMO systems, the bit stream is multiplexed to multiple transmitters without changing the symbol rate of each independent transmitter. Thus, by adding more transmitters, we can increase the throughput of the system without affecting the channel bandwidth.

Thus the combination of OFDMA, MIMO and QAM will give us more bandwidth and higher data rates in LTE. The source for this post comes from various places and it would be stupid of me to post the names of the text books. Next, the above is my understanding of the system, kindly correct me if there are any mistakes. Will appreciate it.

Hope, this was helpful, more to come soon and comments are greatly welcomed.