What is LTE Maximum Number of UE per TTI?

Hello Experts,

Speaking about LTE theoretical limits, what is LTE Maximum Number of UE per TTI?

Here you have a rough calculation for the maximum number of UEs that can be scheduled in one TTI.

Each vendor defines this value, but there’s a theoretical max limit for the number of UEs to be allocated within a single TTI.

As we mentioned before, this strictly affects the maximum achievable throughput in the cell.

Roughly speaking, PDCCH is the bottleneck when we talk about max number of UEs can be scheduled in single TTI.

Let’s say we have the following settings: one cell with bandwidth size of 20MHz, 100 PRB, and 1 antenna ports.

With this setting, we calculate the number of REG (Resource Element Group) that are available for PDCCH, when 3 OFDM symbols are dedicated to the channel.

In the first symbol we have 2RE per PRB for RS (Reference Signals), whereas the second and third symbol don’t have RS.

Thus, the first symbol has 2 REGs/RB while the 2nd and 3rd symbols each has 3REGs/RB, therefore in 100 PRB channel, there are 100 x (2+3+3) = 800 REGs for PDCCH, PCFICH and PHICH.

Considering that:
• PCFICH consumes 4 REGs
• mimimum size for PHICH in 20 MHz = 3 * ceil ( (1/6) * (100/8)) = 9 REGs
• Size of PDCCH in REGs = 800 -4 -9 = 787 REGs
• Size of PDCCH in CCEs = floor (787/9) = 87 CCEs

The number of CCEs used to transfer a DCI is called CCE aggregation level, and may be 1, 2, 4, or 8 consecutive CCEs (logical sequence), depending on the used PDCCH format. PDCCH format 0 uses 1 CCE, PDCCH format 1 uses 2 CCEs,and so on.

Three main reasons 136 LTE Theoretical Limits justify different aggregation level.

First, PDCCH format is selected according to the size of the DCI: different type of DCI are used to improve resource utilization.

Second, to accommodate different RF conditions. The ratio between the DCI size and the PDCCH size indicates the effective coding rate. With the DCI format fixed, higher aggregation levels provide more robust coding and reliability for the UEs under poor RF conditions. For a UE in good RF conditions, lower aggregation levels can save resources.

Third, to differentiate DCIs for control messages and DCIs for UE traffic. Higher aggregation levels can be used for control message resource allocations to provide more protection.

Thus, if we use only PDCCH format 0 for allocating resources within this TTI, there are possible 87 DCI allocations i.e. 87 UEs can be effectively allocated resources.

On the other extreme if we only use PDCCH format 3 (i.e. most robust scheme), then we have: floor(787 / 72) = 10 DCI allocations. Meaning 10 UEs can be allocated resources.

It is worth to be noted that that these DCI are used to allocate both DL and UL, and there are also some common messages using DCIs, such as System Info and Paging (that typically use higher formats, i.e. requires more robust PDCCH schemes, because they need to be received by users without knowing their channel conditions).

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not only depend on PDCCH CCE capacity, but also PDSCH RB capacity.
(1) From cell’s CCE capacity and minimum no of CCE for 1 user -> we can know maximum users that can receive DCI on PDCCH.
(2) From cell’s RB capacity and minimum RB required for 1 user ( RB or RBG) -> we can know maximum users can transmit/receive data on PDSCH.

And max users per TTI = min of {(1), (2)}

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