LTE for Layman (part 3) - The Complete Picture!

In this final part of "LTE for Layman" series, we conclude the learning of first and second part to fully understand the underlying technology behind your LTE cell phone. In this entry, we try to establish complete picture of how your LTE enabled phone is able to upload in seconds! Do go through the previous parts of this series, as most of the terminologies used in this text are explained beforehand. In case you are already knowledgeable in that, do read on for understanding the complete UE life-cycle, step by step, layman style!

STEP 1: SCANNING

When you switch on your brand new LTE enabled phone for the very first time, the first step (and this is true for all generations) is scanning. Your phone scans all the available radio frequencies in the given LTE band, also known as E-UTRA Absolute Radio Frequency Channel Number or (EARFCN). This is analogous to a simple radio scan operation, which scans for all channels to check where the music is playing! For example, my Moto X play phone supports 800/900/1800/2100/2600 MHz frequency. So when my phone is switched on for the first time, it scans all these supported frequencies to see whether network is available. Suppose UE finds signals in all the band frequencies, which frequency does UE tune in? Frequency latching is decided on basis of something called RSRP or Reference Signal Received Power. This is a power measurement level of signal, analogous to which frequency is the strongest. The strongest RSRP emitting frequency is selected, and UE moves onto the next step.

STEP 2: DOWNLINK SYNCHRONIZATION

In this step, UE establishes cell level synchronization at the downlink level. This essentially means UE needs to know the physical cell id (PCI), and also the measurement units of signal, i.e slot and frame timings. This is done by acquiring Synchronization signals, which are always transmitted in center frequency of the acquired band (or center 62 sub carriers). There are two types of Synchronization signal:

• Primary Synchronization signal (PSS): Determined using DC sub carrier or center frequency, this is always transmitted at last OFDM symbol of 1st and 11th time slot. Once UE has acquired PSS, it synchronizes on slot level basis, and knows the physical layer identity (0 – 2).
• Secondary Synchronization signal (SSS): Sent at a slot just before PSS, it is responsible for frame level synchronization. UE now knows the physical layer cell identity group (0 – 167).

Using both PSS and SSS, UE can calculate PCI or Physical Cell Identity by using a formula:

PCI = 3*(Physical layer cell identity group) + physical layer identity.

If the formula or anything about synchronization seems a bit confusing, remember the basics of LTE measurement! The center frequency acts as a reference point for Y axis measurement. From there, moving a certain X axis of time slots helps UE acquire these signals to calculate the PCI and achieve complete synchronization of both X axis and Y axis

STEP 3: Decoding Broadcast Information

In this step, UE acquires key system information to establish complete downlink synchronization with the cell. There are two primary key information blocks that UE needs to know.

First key information that UE needs to know is the system bandwidth. This is acquired by something called Master Information Block (MIB). Having acquired the measurement units (slot and frame level synchronization), UE latches on the same center frequency and this time reads first 4 OFDM symbols of second slot of first sub frame. This is where MIB is transmitted on the physical broadcast channel. Since MIB is very crucial, it is transmitted every 40ms, with the copies being transmitted every 10ms. This way, UE is ensured that it can create complete MIB even from the copies, and knows when a new MIB is transmitted (and the 40ms boundary). Upon decoding MIB, UE knows the following:

• System Bandwidth: Determines how many resource blocks are functional for this cell, necessary for decoding any other physical channels.
• PHICH information: The size and duration of PHICH channel is given here, which is essential to differentiate between PCFICH and PDCCH channel (as all control channels share same region). PCFICH location is fixed and known, so by knowing PHICH location, UE can read the critical PDCCH region structure to get control level information for further decoding of broadcast information.
• System Frame Number: this is the clock of eNodeB. By knowing the SFN, UE establishes complete synchronization with the eNodeB’s system timing.

After decoding MIB, UE has enough information to understand the control region structure of the downlink channel (or PDCCH). It’s from there that UE can know the information about second most critical information block called System Information Blocks (or SIBs). SIBs are of thirteen types! But luckily, only first two SIBs are essential for UE to decode complete eNodeB information. SIB 1 carries Cell access related parameters and scheduling information of other SIBs. A new SIB 1 is transmitted every 80 ms (every eight frames), but to add redundancy and to reduce acquisition time for the UE, it is repeated every other frame. SIB Type 1 also informs the UE of the expected schedule for the remaining SIBs which can be as often as every 8 frames or as seldom as every 512 frames. For the remaining SIBs, the network operator will configure which ones are necessary.

STEP 4: OPERATOR SELECTION

For cell selection, the UE requires the PLMN ID (Public Land Mobile Network ID) of the network, cell barring status and minimum signal strength threshold from SIB type 1. 

The UE first checks the PLMN ID. PLMN ID is nothing but the operator id, and every operator is assigned one. It consists of two codes, a Mobile Country Code (MCC) and Mobile Network Code (MNC). Not going too much into the identification schemes, but if analogy is drawn, think of PLMN ID as the legal ID card for your operators (eg: Airtel) to provide telecommunication services in a country. LTE cell supports a PLMN that the UE is allowed to use (naturally, your operator), and the UE will continue on to check the cell barred status. There are several reasons a cell on the air might be barred from commercial use. 

If for example, it is a test cell, we wouldn’t want the UE to continue on this cell. If the cell barring status is okay, the UE will check the minimum signal strength threshold to learn if it passes the signal strength criteria for the cell. Some UEs at or near the cell edge can be blocked from trying to access the cell using minimum signal strength threshold. Of course, if any of these criteria fail, the UE will have to find another LTE cell. Once cell selection is successful, the UE will read the information in SIB Type 2 to get the parameters it requires for beginning uplink synchronization.

STEP 5: UPLINK SYNCHRONIZATION