3GPP LTE : Dynamic Scheduling, Persistent Scheduling and Semi Persistent Scheduling

Dynamic Scheduling

In a subframe in physical layer the first OFDM symbol of each subframe consists of CFI information. CFI information basically tells you the number of OFDM symbols used by PDCCH. The PDCCH has DCI information that lets you decode the data from the PDSCH. In case you forgot, PDSCH has all the user data pertaining to the UE's. Now when a UE is downloading a set of files, say from the internet, each and every subframe has the PCFICH and PDCCH data in the first 3-4 OFDM symbols. This is essential when the data is robust or adaptive in nature, especially when it is web data. So it is essential that you send the control information for each subframe along with it. This kind of Scheduling is known as Dynamic Scheduling.

The advantage of Dynamic Scheduling is basically the flexibility to alter the size of data in each subframe. You can push more data in one SF, less on another. 

Persistent Scheduling 

Now consider a case where the amount of data expected is less and occurs in a fixed time interval. Yes, I'm talking about something like VoLTE (Voice over LTE). Voice data is in the form of small packets and it comes in a regular interval, which is network dependent. In such cases, sending control information in each and every subframe plays a vital role in the effective utilization of bandwidth. Thus, we use something called as Persistent Scheduling, where the control information sent across a SF is retained for every nth SF coming after it, until notified. This scheme drastically reduces the overhead.

Semi Persistent Scheduling

Now note the point that when you have a NACK for any of the DL data, the retransmission has to contain some extra information. ( Probably to indicate retransmission, SF number so on). So your retransmissions cant be pushed along with the Persistent Scheduled Time interval. In other words, you have to explicitly put the header info for retransmission SF's. Thus, Persistent Scheduling is rarely used, instead this new scheduling scheme known as Semi Persistent Scheduling is used.

Semi Persistent Scheduling Example

The time interval for SPS is informed by the RRC. The termination of SPS/alteration of time interval is also RRC triggered.

    

In VoIP services, the voice data is encoded using a codec and sent. At times, the network might have to change the codec(maybe for internal reasons, or say clarity etc). When you change the codec, the amount of data sent per Radio Frame might be different. As a result, you might have to increase the SPS interval.

In the diagram above, also note one thing. Once SPS is triggered, every nth SF is first checked for PDCCH data. This is because, PDCCH signals always have a higher priority. So just because you initiated SPS, it doesnt mean that it will continue till you tell it to stop. It will always give a higher priority for PDCCH data in that particular SF. An example for this would be downloading a webpage along with a voice call. Here, you might need PDCCH data to decode the user data.

LTE Physical Layer Simulation in Scilab. Please join to develop

Hi

I have four years of experience in working in 3GPP LTE physical layer.
Now I'm planning of making LTE Physical layer simulation in Scilab to help the students.
If you are interested in this project please contact me.

Regards
Ashok

LTE Resource Grid Visualization

Nice website to see LTE subframe structure for different configurations

http://paul.wad.homepage.dk/LTE/lte_resource_grid.html

How to Find the nearest power of 2

/* returns greatest power of 2 less than or equal to x, branch-free */

int
flp2(int x)
{
    x = x | (x>>1);
    x = x | (x>>2);
    x = x | (x>>4);
    x = x | (x>>8);
    x = x | (x>>16);
    return x - (x>>1);
}

It's entertaining to study it and see how it works. I think the only way for you to know for sure which of the solutions you see will be optimal for your situation is to use all of them in a text fixture and profile it and see which is most efficient for your purpose.

Being branch-free, this one is likely to be quite good performance-wise relative to some others, but you should test it directly to be sure.

If you want the least power of two greater than or equal to X, you can use a slightly different solution:

unsigned
clp2(unsigned x)
{
    x = x -1;
    x = x | (x >> 1);
    x = x | (x >> 2);
    x = x | (x >> 4);
    x = x | (x >> 8);
    x = x | (x >> 16);
    return x + 1;
}

DAI (Downlink Assignment Index) in LTE

The DL assignment carries a Downlink Assignment Index (DAI) indicating how many assignments the UE should have received so far within the current bundling window. If the UE detects that the DAI differs from the number of correctly received DL assignments, it does not send any HARQ feedback and the eNB can detect this. However, the eNB cannot know which of the transmissions was missed, and thus the whole bundle has to be retransmitted.

RSSI,SINR,RSRP and RSRQ in LTE

RSSI,SINR,RSRSP and RSRQ : These are  the basic measurement quantities used in LTE.
RSSI - Received Signal Strength Indicator
SINR - Signal to Interference & Noise Ratio
RSRP - Reference Signal Received Power
RSRQ - Reference Signal Received Quality

RSRP is a measure of signal strength. It is of most importance as it used by the UE for the cell selection and reselection process and is reported to the network to aid in the handover procedure. For those used to working in UMTS WCDMA it is equivalent to CPICH RSCP.

The 3GPP spec description is "The RSRP (Reference Signal Received Power) is determined for a considered cell as the linear average over the power contributions (Watts) of the resource elements that carry cell specific Reference Signals within the considered measurement frequency bandwidth."

In simple terms the Reference Signal (RS) is mapped to Resource Elements (RE). This mapping follows a specific pattern (see below). So at any point in time the UE will measure all the REs that carry the RS and average the measurements to obtain an RSRP reading.

RSRQ is a measure of signal quality. It is measured by the UE and reported back to the network to aid in the handover procedure. For those used to working in UMTS WCDMA is it equivalent to CPICH Ec/N0. Unlike UTMS WCDMA though it is not used for the process of cell selection and reselection (at least in the Rel08 version of the specs).

The 3GPP spec description is "RSRQ (Reference Signal Received Quality) is defined as the ratio: N×RSRP/(E -UTRA carrier RSSI) where N is the number of Resource Blocks of the E-UTRA carrier RSSI measurement bandwidth."

The new term that appears here is RSSI (Received Signal Strength Indicator). RSSI is effectively a measurement of all of the power contained in the applicable spectrum (1.4, 3, 5, 10, 15 or 20MHz). This could be signals, control channels, data channels, adjacent cell power, background noise, everything. As RSSI applies to the whole spectrum we need to multiple the RSRP measurement by N (the number of resource blocks) which effectively applies the RSRP measurement across the whole spectrum and allows us to compare the two.

Finally SINR is a measure of signal quality as well. Unlike RSRQ, it is not defined in the 3GPP specs but defined by the UE vendor. It is not reported to the network. SINR is used a lot by operators, and the LTE industry in general, as it better quantifies the relationship between RF conditions and throughput. UEs typically use SINR to calculate the CQI (Channel Quality Indicator) they report to the network.

The components of the SINR calculation can be defined as:

S: indicates the power of measured usable signals. Reference signals (RS) and physical downlink shared channels (PDSCHs) are mainly involved

I: indicates the power of measured signals or channel interference signals from other cells in the current system

N: indicates background noise, which is related to measurement bandwidths and receiver noise coefficients

LTE Uplink Physical Layer

Here is a brief description of LTE Uplink Physical Layer


LTE uplink Consists of

• PUSCH
• PUCCH
• PRACH
• SRS

PUSCH (Physical Uplink Shared Channel)


The physical uplink shared channel is used to transmit the uplink shared channel (UL-SCH) and L1 and L2 control information. The UL-SCH is the transport channel used for transmitting uplink data (a transport block). L1 and L2 control signalling can carry the following type of information: HARQ acknowledgements for received DL-SCH blocks, channel quality reports and scheduling requests. It uses SC-FDMA in physical layer

The processing blocks of PUSCH transmitter side is in the figure below.

Processing blocks at PUSCH receive, i.e, at eNodeB is in the figure below

More Details will be added soon based on the requirement..

Or you may refer :
http://www.steepestascent.com/content/mediaassets/html/LTE/Help/PUSCH.html

Please feel free to contact me if you need any details regarding LTE uplink. I will be happy share the knowledge I have.