Reading through the specification and various materials about LTE, you would have came across so many different types of numbers. For example, Zadoff Chu sequence (actually this is not a single number, but a sequence of numbers), Cell ID, RNTIs etc. If you understand the exact practical meaning of these numbers and when and for what these numbers are used, you would understand LTE low layer processing more detail and this understanding would help you a lot when you implement protocol stack, test case and troubleshoot.
Zadoff - Chu Sequence
As I briefly mentioned above, this is not a single number. It is a sequence of special numbers. You can find quite a lot of materials on this sequence from internet (try with Wikipedia).
Let's first think about how this sequence is generated. Various kinds of number sequences are used in many different kind of technologies (e.g, Walsh code in CDMA, OVSF code in WCDMA) and usually these numbers are created by a special rules or formula. Same to Zadoff-Chu sequence. The basic form of Zadoff chu sequence can be created by the formula as shown in the following spreadsheet (click on the picture to see in magnified view. Please send me an email if you want to have this spreadsheet).
Why did we chose to use these sequence ? It is because this sequence has a couple of special properties that can be very useful for LTE low layer implementation.
Followings are the special properties of the sequence :
i) This sequence has a constant amplitude. If you look into the formula, it is in the form of e^(-j theta). You may learned about this in high school math. If you convert this into Euler form, you will get e^(-j theta) = cos(theta) - j sin(theta). First, you will see this is a complex number which is made up of real and imaginary part. If you plot the numbers onto a complex plan (Real part - horizontal axis and Imaginary part on vertical axis), all the numbers will lie on the perimeter of a circle. This means the amplitude of these number is constant. See the plot above. (Column B, C is one example of Zadoff Sequence. B is the real part and C is imaginary part. The plot is the scatter plot of column B, C)
ii) Zero Autocorrelation. If you create a sequence using this formula and create another sequence just by shifting the same sequence by N (N can be 1,2,....,size of sequence -1). And if you take the correlation of the two sequence, the result become 0. Taking the spreadsheet shown above as an example, Column B,C is a sequence created by formula. and Column D,E is not the one created by the formula.. it is just shifted version of Column B, C. Cell F70 and G70 shows the correlation of Column B,C and D,E which gives almost 0. It should be 0 theoretically, but the F70,G70 is not exactly 0 because of numerical errors.. but it is almost 0. If you have two sequence of number and the correlation of the two sequence is 0, we say "the two sequences are orthogonal to each other". It means that you can create many of orthogonal sequences just by shifting a Zadoff Chu sequence. How convenient it is to create orthogonal sequences.. and you know how important to create orthogonal sequences in many wireless communication.
Any sequences that has the two properties explained above are called CAZAC sequence (constant amplitude zero autocorrelation waveform).
iii) Cross correlation of two Zadoff Chu sequence is 1/Sqrt(Nzc). If you create two sequences using the formula shown on the spreadsheet just by changing 'q' (the q value used in both sequence should be prime numbers) and take the correlation of the two sequences, the result will be 1/Sqrt(Nzc).
There are a couple of more special properties of Zadoff Chu sequences, but I don't think they are important for LTE implementation. So I would leave it to you to refer to other sources.
Where in LTE we use this Zaddoff Chu sequence. In short, the sequence are used in the following part of LTE. (I will update the details of these topics later)
i) Primary Synchronization Signal (PSS) (so called primary synchronization channel)
ii) random access preamble (PRACH)
iii) HARQ ACK/NACK responses (PUCCH)
iv) sounding reference signals(SRS).
RNTI
One of the other numbers which you would very frequently come accross is RNTI. RNTI stands for Radio Network Temporary Identifier.
As the name implies, it is a kind of Identification number. Normally we use indentification number to differntiate one thing from all other similar things. For example, your driver's license number let you identify yourself from all other drivers. Social Security number do the same thing as well.
Getting more specifically into LTE, this RNTI is used to indentify one specific radio channel from other radio channel and one user from another user. As you may recall, in WCDMA is a RNTI concept which is carried as part of MAC header to deferentiate one user to another while in communication state. and in WCDMA case it used special channelization code to deferentiate one radio channel from the other.
To my personal perception, RNTI in LTE seems to act as combined role of WCDMA RNTI and WCDMA channelization code (but RNTI has nothing to do with orthogonality.. so this is very superfical analogy. Just take this as an analogy just for understanding high level functionality).
What kind of RNTIs are there in LTE ?
The answer is A LOT -:). Followings are the brief summary of RNTIs being used in LTE. More detailed explanation will be updated continuously later.
* SI-RNTI : It stands for System Information RNTI.
* RA-RNTI : It stands for Random Access RNTI
* C-RNTI : It stands for Cell RNTI
* TC-RNTI : It stands for Temporary C-RNTI
* SPS-C-RNTI : It stands for Semi persistance Scheduling C-RNTI
* TPC-PUCCH-RNTI : It stands for Transmit Power Control-Physical Uplink Control Channel-RNTI
* TPC-PUSCH-RNTI : It stands for Transmit Power Control-Physical Uplink Shared Channel-RNTI
* M-RNTI : It stands for MBMS RNTI
Who issues these RNTI ?
Network issues RNTI.
Exactly what does RNTI do for each of those radio channel ? The detailed process differs with the types of RNTIs, but generally speaking all of these RNTI is used to scramble the CRC part of the radio channel messages. It implies that if UE does not know the exact RNTI values for each of the cases, it cannot decode the radio channel messages even though the message reaches the UE intact.
Following is the quotes from 3GPP specification showing how RNTI is used for various cases.. for the exact details, you should see the specification but this partial quote would give you a rough idea of the usage of RNTI.From 36.212 ---
5.3.3 Downlink control information
A DCI transports downlink or uplink scheduling information, or uplink power control commands for one RNTI. TheRNTI is implicitly encoded in the CRC.
5.3.3.1.3 Format 1A
Format 1A is used for random access procedure initiated by a PDCCH order only if format 1A CRC is scrambledwith C-RNTI
For distributed VRB: .. if the format 1A CRC is scrambled by RA-RNTI, P-RNTI, or SI-RNTI
5.3.3.2 CRC attachment
This section explain in detail on how CRC is scrambled by RNTI
From 36.213 ---
5.1.1.1 UE behaviour
* δ_PUSCH is a UE specific correction value, also referred to as a TPC command and is included in PDCCH withDCI format 0 or jointly coded with other TPC commands in PDCCH with DCI format 3/3A whose CRC paritybits are scrambled with TPC-PUSCH-RNTI
* if the TPC command PUSCH δ_PUSCH is included in a PDCCH with DCI format 0 where the CRC is scrambled by the Temporary C-RNTI
* The UE attempts to decode a PDCCH of DCI format 0 with the UE’s C-RNTI or SPS CRNTI and a PDCCH of DCI format 3/3A with this UE’s TPC-PUSCH-RNTI in everysubframe
5.1.2.1 UE behaviour
δ_PUCCH is a UE specific correction value, also referred to as a TPC command, included in a PDCCH with DCIformat 1A/1B/1D/1/2A/2 or sent jointly coded with other UE specific PUCCH correction values on a PDCCHwith DCI format 3/3A whose CRC parity bits are scrambled with TPC-PUCCH-RNTI.
o The UE attempts to decode a PDCCH of DCI format 3/3A with the UE’s TPC-PUCCH-RNTI and oneor several PDCCHs of DCI format 1A/1B/1D/1/2A/2 with the UE’s C-RNTI or SPS C-RNTI onevery subframe except when in DRX.
o If the UE decodes a PDCCH with DCI format 1A/1B/1D/1/2A/2 and the corresponding detectedRNTI equals the C-RNTI or SPS C-RNTI of the UE, the UE shall use the δ PUCCH provided in that PDCCH.
6.1 Physical non-synchronized random access procedure
* A preamble index, a target preamble received power (PREAMBLE_RECEIVED_TARGET_POWER), acorresponding RA-RNTI and a PRACH resource are indicated by higher layers as part of the request.
* Detection of a PDCCH with the indicated RA-RNTI is attempted during a window controlled by higher layers
7.1 UE procedure for receiving the physical downlink shared channel
* If a UE is configured by higher layers to decode PDCCH with CRC scrambled by the SI-RNTI, the UE shall decode thePDCCH and the corresponding PDSCH according to any of the combinations defined in table 7.1-1. The scramblinginitialization of PDSCH corresponding to these PDCCHs is by SI-RNTI.
* If a UE is configured by higher layers to decode PDCCH with CRC scrambled by the P-RNTI, the UE shall decode thePDCCH and the corresponding PDSCH according to any of the combinations defined in table 7.1-2. The scramblinginitialization of PDSCH corresponding to these PDCCHs is by P-RNTI.
* If a UE is configured by higher layers to decode PDCCH with CRC scrambled by the C-RNTI, the UE shall decode thePDCCH and any corresponding PDSCH according to the respective combinations defined in table 7.1-5. Thescrambling initialization of PDSCH corresponding to these PDCCHs is by C-RNTI.
* If a UE is configured by higher layers to decode PDCCH with CRC scrambled by the Temporary C-RNTI and is notconfigured to decode PDCCH with CRC scrambled by the C-RNTI, the UE shall decode the PDCCH and thecorresponding PDSCH according to the combination defined in table 7.1-7. The scrambling initialization of PDSCHcorresponding to these PDCCHs is by Temporary C-RNTI.
