This article will explain the downlink frame structure. For the beginners, there is always a confusion about the frame structure, the physical channels etc, so to make their job easy, I am writing this article.
To start with, what does the downlink frame structure consists of? The answer is simple, it contains the downlink channels and signals. Refer article LTE Physical Downlink Channels for more details. The LTE frame is nothing but a imaginary grid of time vs frequency , where there are placeholders for different channels and signals, for Eg: the dowlink control channels are always restricted to first 3 symbols or less, of the subframe. The frame structure changes depending on the cyclic prefix type, bandwidth and duplexing modes.
Currently let us focus on a 10MHz, FDD, Normal CP type downlink frame structure, with following configuration,
- Subframe number – 0
- Duplexing mode – FDD
- System bandwidth – 10Mhz
- Number of control symbols – 2
- Number of users allocated – 1
- Number of resource blocks dedicated to the above user – 20
- Resource block start index – 0
- Number of Transmit antennas – 1
So with the above configurations, lets see, how exactly the subframe will look like in frequency domain, also since this is a subframe-0, we will have PBCH and PSS/SSS always allocated for FDD. The below diagram shows the downlink frame structure for the above configuration,
Downlink Frame Structure
Lets see the numbers here, which are fixed for given bandwidth
- Number of resource blocks(RB) across the 10MHz = 50
- Number of subcarriers/tones per RB = 12
- Number of subcarriers per sysbol = 50 x 12 = 600
- Number of symbols per subframe = 14
- Number of subcarriers per subframe = 14 x 600 = 8400
- Number of reference signals subcarriers per RB = 2
- Subcarriers occupied by PBCH = 72 (Central 6 RBs, remains unchanged for all LTE bandwidths, configurations)
- Number of symbols in which PBCH is present = 4 (First 4 symbols of second slot)
- Subcarriers occupied by PSS = 72 (Central 6 RBs, remains unchanged for all LTE bandwidths, configurations)
- Subcarriers occupied by SSS = 72 (Central 6 RBs, remains unchanged for all LTE bandwidths, configurations)
CCH – Control Channels
The CCH is group of downlink control channels, which contains PDCCH, PCFICH, PHICH. The mapping of PCFICH is determined by the system bandwidth, and hence, for a given bandwidth, the position of PCFICH remains constant. The PHICH mapping depends on several factors related to previous uplink grant configurations, for which the ACK/NACK is being currently transmitted, the determining factors are, RB start index, DMRS cyclic shift. The PDCCH mapping depends again on several factors such as the PDCCH format, aggregation level for the user etc. What so ever, the CCH occupies all 600 subcarriers always.
The symbol 0, 4, 7, 11 contains the downlink reference signals known as the cell specific reference signals, which are mapped to every sixth subcarrier in these symbols, the start index for mapping of these is determined by the cell ID of the eNodeB. These reference signals are unique to every antenna, and hence, if more than 1 antenna is used for transmission, there would be more reference signals mapped in the subframe. Similar to the cell specific reference signals, there is also UE specific reference signals which are of similar nature but are used only in case of beam forming. These reference signals are used by the UE for downlink channel estimation and equalization.
Since the number of resource blocks allocated to the user is 20RB, we can see that his data is mapped onto the first 20 resource block starting RB index 0 to RB index 19. The actual amount of the user data may be more or less than the amount of space available in the given allocation space of 20RB, so a procedure called rate matching is done to fit the user data into the available space. The number called G-Value gives the number of subcarriers/tones available for the user given his allocation. The ration of actual user data to G-value is nothing but the coding rate for that user. There are different type of resource allocation types available for downlink, such as the Type-0, Type-1 and Type-2. In the current scenario, we have used type-2 localized resource mapping.
The PBCH carriers the vital system information such as the system bandwidth , PHICH information etc. The PBCH is always transmitted in all subframe 0 of all radio frames, it occupies the central 6 RBs, which enables the UE to decode the PBCH irrespective of the system bandwidth.
The PSS and SSS are also mapped to every subframe 0 and 5 in case of FDD, the position remains same though, i.e PSS is mapped to 6th symbol and SSS to the 5th symbol of first slot. Even the PSS/SSS are always mapped to the central 6 resource blocks/72 subcarriers of the frame, again to enable the UE to decode these irrespective of the bandwidth.
So thats it! Isn’t it simple enough? I hope I have depicted it clearly. I would love to answer your query, if any or correct me, if I have put something wrong.