All About Cell Search In LTE

Cell Search In LTE : 
- eNodeB broadcasts Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) to help UE with the Cell Search Process and Cell Id detection.
- There is total 504 Cell Ids (0 - 503) defined in LTE.
- These 504 Cell IDs are grouped in 168 Physical Layer Cell Identity Group.
- Each Physical Layer Cell Identity Group Consists of 3 Physical Layer Cell Identity.
- PSS and SSS is transmitted using central 62 sub carriers around the DC. The 5 REs above and below the Synchronization Signals are not used for transmission, i.e. they represents DTX periods.

Primary Synchronization Signal (PSS):
- PSS is allocated to Central 62 subcarriers.
- Belonging to the "Last Symbol" of slot 0 and slot 10 of every radio frame.
- So PSS is transmitted twice every 10 ms.
- Both Transmissions are Identical.
- PSS is used for :
          - Achieve SYMBOL, SLOT, and SUBFRAME synchronization.
          - Determine the Physical Layer Cell Identity (PCI) within the Physical Layer Cell Identity Group.
- There are 3 Physical Layer Cell Identity in each Group So PSS is generated using 1 of 3 different Sequences.

Secondary Synchronization Signal (SSS):
- SSS is allocated to Central 62 subcarriers.
- Belonging to the "Second Last Symbol" of slot 0 and slot 10 of every radio frame.
- So SSS is also transmitted twice every 10 ms.
- The 2 SSS transmissions within each radio frame use Different Sequences.
- This is to allow UE to differentiate between the 1st and 2nd transmission.
- This helps UE to determine the starting of each radio frame, i.e. to achive the Frame Synchronization.
- SSS is used for :
          - Achieve FRAME synchronization.
          - Determine the Physical Layer Cell Identity Group.
- There are 168 Physical Layer Cell Identity Group So SSS is generated using 1 of 168 different pairs of Sequences.

Cell Id Identification:
- Once UE read the PSS and SSS, UE will be able to get the Cell ID from the  Physical Layer Cell Identity and Physical Layer Cell Identity Group.
- Cell ID = 3 * Physical Layer Cell Identity Group + Physical Layer Cell Identity.

All About Resource Block (RB)

Resource Blocks (RB):
- Basic unit of resource for the LTE air-interface.
- eNodeB scheduler allocates RBs to UE to allow data transfer.
- Defined in both time and frequency domains.

In Time Domain:
- Occupies 0.5 ms slot in time domain.
- Consists of 7 OFDMA symbols when using Normal Cyclic Prefix.
- Consists of 6 OFDMA symbols when using Extended Cyclic Prefix.

In Frequency Domain: 
- Consists of 12 subcarriers.
- Each subcarrier is of 15 KHZ.
- Each RB occupy 12*15 = 180 KHZ in frequency domain.

- The GRID generated by One Sub-Carrier in the Frequency Domain and One Symbol in the Time Domain defines a RESOURCE ELEMENT (RE).
- RB consists of 84 (12*7) REs when using Normal Cyclic Prefix.
- RB consists of 72 (12*6) REs when using Extended Cyclic Prefix.
- A single RE can carry a Single Modulation Symbol (2 bits when using QPSK, 4 bits when using 16QAM, and 6 bits when using 64QAM).