Downlink Frame Structure ============
The first thing you have to be very familiar with as an engineer working on LTE is the following channel map shown above.
We can represent an LTE signal in a two dimensional map as shown above. The horizontal axis is time domain and the vertical axis is frequency domain. The minimum unit on vertical axis is a sub carrier and the minimum unit on horizontal axis is symbol. For both time domain and frequency domain, there are multiple hiarachies of the units, meaning a multiple combination of a smaller unit become a larger units.
Let's look at the frequency domain structure first.
LTE (any OFDM/OFDMA) band is made up of multiple small spaced channels and we call each of these small channels as "Sub Carrier".
Space between the chhanel and the next channel is always same regardless of the system bandwidth of the LTE band.
So if the system bandwidth of LTE channel changes, number of the channels (sub carriers) changes but the space between channels does not change.
Q> What is the space between a subcarrier and the next sub carrier ? A> 15 Khz
Q> What is the number of channels(sub carriers) for 20 Mhz LTE band ? A> 1200 sub carriers.
Q> What is the number of channels(sub carriers) for 10 Mhz LTE band ? A> 600 sub carriers.
Q> What is the number of channels(sub carriers) for 5 Mhz LTE band ? A> 300 sub carriers.
Got any feelings about sub carriers and it's relation to system bandwidth ?
Now let's look at the basic units of horizontal axis which is time domain. The minimum unit of the time domain is a Symbol, which amounts to 66.7 us. Regardless of bandwidth, the symbol length does not changes.In case of time domain, we have a couple of other structures as well. The largest unit in time domain is a frame, each of which is 10 ms in length. Each of the frame consists of 10 sub frames, each of which is 1 ms in length. Each of sub frame consists of 2 slots, each of which is 0.5 ms in length.Each of slots consists of 7 symbols, each of which is 66.7 us.
With this in mind, let's think about the scale in reverse direction.
Q> How many symbols are there in a slot ? A> 7 symbols.
Q> How many symbols in a sub frame ? A> 14 symbols.
Q> How many slots are there in a frame ? A> 20 slots.
Now let's look at the units which is made up of both time domain (horizontal axis) and frequency domain (vertical axis). Let's call this type of unit a two-dimensional unit.
The minimum two dimensional unit is resource element which is made up of one symbol in time domain and one sub carrier in frequency domain. Another two dimensional unit is resource block(RB) which is made up of one slot in time domain and 12 sub-carrier in frequency domain. Resource Block(RB) is the most important units in LTE both for protocol side and RF measurement side.
Now here goes questions.
Q> How many symbols in a resource block ? A> 7 symbols.
Q> How many sub-carriers in a resource block ? A> 12 sub-carriers.
Q> How many resource elements in a resource block ? A> 84 resource elements.
Now it's time to combine all the units we covered. The following questions are very important to read any of the LTE specification.
Q> How many resource blocks in a 20 Mhz band ? A> 100 resource blocks.
Q> How many resource blocks in a 10 Mhz band ? A> 50 resource blocks.
Q> How many resource blocks in a 5 Mhz band ? A> 25 resource blocks.
I have seen this type of mapping so many times from so many different sources, but do I really understand all the details of the map ? No not yet. It will take several years to understand every aspects of the map.
Probably what I do as the first step is to describe each part of the map in a verbal form
PBCH(Physical Broadcast Channel)
- It carries only the MIB.
- It is using QPSK.
- Mapped to 6 Resource Blocks (72 subcarriers), centered around DC subcarrier in sub frame 0.
- Mapped to Resource Elements which is not reserved for transmission of reference signals, PDCCH or PCHICH
The first L(1 or 2 or 3) Symbols
This is one of the most confusing area of the map because multiple channels are located in this area. On the first symbol is PCFICH but PCFICH takes only part of the resource blocks on the first symbol not all. PHICH is carried by this area as well. And the remaining space not occupied by PCFICH and PHICH is allocated for PDCCH.
PCFICH(Physical Control Format Indicator Channel)
- It carries the size of PDCCH
- Mapped to the first OFDM symbol in each of the downlink sub-frameThis contains the information on number of OFDM symbols for PDCCH and PHICH symbol duration received from the PBCHUE decode this channel to figure out how many OFDM symbols are assigned for PDCCH
- It is 16 data subcarriers of the first OFDM symbol of the subframe.
- The exact position of PCFICH is determined by cell ID and bandwidth.
PDCCH(Physical Downlink Control Channel)
- Mapped to the first L OFDM symbols in each of the downlink sub-frame.
- Number of the symbols (L) for PDCCH can be 1,2, or 3.
- Number of the symbols for PDCCH is specified by PCFICH
- It contains Transport format, resource allocation, H-ARQ information related to DL-SCH, UL-SCH and PCH. It is used to transmit UL and DL resource assignments defined by Data Control Informations (DCIs)
- It carries scheduling assignment (e.g, UL Grants) and other control information.
- Multiple PDCCH are supported and a UE monitors a set of control channels.
- Modulation Scheme is QPSK.
- PDCCH is like HS-SCCH for HSDPA and PDCCH for R99, E-AGCH/E-RGCH for HSUPA
- Even though PDCCH has a lot of functions, not all of them are used at the same time so PDCCH configuration should be done flexibly.
If you are interested in the detailed information mapping in this channel, refer to 6.8.1 of 36.211. Following is the initial descrition on this section.
The physical downlink control channel carries scheduling assignments and other control information. A physical controlchannel is transmitted on an aggregation of one or several consecutive control channel elements (CCEs), where acontrol channel element corresponds to 9 resource element groups. The number of resource-element groups notassigned to PCFICH or PHICH is REG N . The CCEs available in the system are numbered from 0 and N_CCE-1 , where N_CCE = floor(N_REG/9) . The PDCCH supports multiple formats as listed in Table 6.8.1-1. A PDCCH consisting of nconsecutive CCEs may only start on a CCE fulfilling imod n = 0 , where i is the CCE number.
PHICH
- Carries H-ARQ Feedback
- After UE trasmitted the data in UL, it is waiting for PHICH for the ACK.
- It is like E-HICH in HSPA
- Sometimes several PHICH constitutes a PHICH group using the same resource elements.
PDSCH(Physical Downlink Shared Channel)
- Carries user specific data (DL Payload).
- Carries Random Access Response Message.
- It is using AMC with QPSK, 16 QAM and 64 QAM
PRACH
- It carries the random access preamble
- It is occupying 72 subcarriers of bandwidth in the frequency domain. If the random access preamble is successfuly received, the random access message is transmitted on the UL-SCH.
- Within this channel is Random Access Preamble. This Random Access Preamble is generated with Zadoff-Chu sequence.
PUCCH
- It is formed by two consecutive resource blocks with frequency hopping at the slot boundary
- It carries CQI, ACK/KACK and RI/PMI (in MIMO case) and scheduling request for data transmission
P-SS(Primary Synchronization Signal)
- Mapped to 72 active sub carriers(6 resource blocks), centered around the DC subcarrier in slot 0 and slot 10.
Not a big issues until now. But when you have the following data and information, can you locate exactly which part of the channel map would carry this message ? This is one of the very tricky part of understanding LTE protocol and it would take a long time for study. (If you are an RF engineer, this may not be so important to you).
For further details of this part will be coming later.
Uplink Frame Structure ============
The Uplink slot structure looks as follows. When I was first reading LTE materials, almost every books and article says "LTE use SC(Single Carrier) FDMA for uplink signal" and because of the word 'Single Carrier' made me so confused about creating any images of Uplink slot structure. Even now I don't think I can explain clearly about 'SC FDMA'. You may ask to FPGA or DSP engineer about the details of SC FDMA mechanism.
But anyway good thing to me was that the most important factors in uplink slot is same as the one in the downlink. Just take a look at the overall uplink slot structure.
As in downlink, Frame time and slot time in Uplink is the same as in the downlink. And the resource block structure is also same both in uplink and downlink. As shown above, 7 symbols in one slot is also the same in both uplink and downlink.
A little bit of differences you would notice would be the location of the each channel. Normally in downlink case, a channel tend to lie across the whole bandwidth but the channels in the uplink slot seems to be more localized. For example, PUCCH is located only at the lowest and highest end in frequency domain and reference signals also localized in time domain or both timedomain and frequency domain.
PUCCH RS
Carries the Reference Signal that is required for demodulating PUCCH. It means if this part is not properly configured or eNodeB failed to detect this part, eNodeB cannot decode PUCCH.
PUCCH
This channel can carries a lot of information (UCI), but depending on the configuration it can carry only a few of the following information.
- ACK/NACK for the recieved PDSCH data
- CQI
- RI
- PMI
As you see in the slot structure, PUCCH is located in the either extreme ends of the uplink frequency domain in alternating fashion between the two slots within a subframe, meaning that if the PUCCH is the lowest part of frequency domain in slot 0(first slot) and it will be located in the higest part of frequency domain in slot 1 (second slot). Exactly how many resource elements is allocated to the PUCCH is determined by network and the configuration is broadcasted to UE via SIB2.
PUSCH RS
Carries the Reference Signal that is required for demodulating PUCCH. It means if this part is not properly configured or eNodeB failed to detect this part, eNodeB cannot decode PUCCH. This is always located at the center of Uplink slot.
PUSCH
Carries Uplink data that UE tries to send. and it can also carries ACK/NACK for the PDSCH the UE recieved in addition to uplink data.
