08 TM51178EN03GLA1 Radio Resource Management Ppt

TM51178EN03GLA1

Radio Resource Management

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At reception of the HO request message the RAC decides in an ‘all-or-nothing’ manner on the admission / rejection of the resources used by the UE in the source cell (prior to HO). 'All-or-nothing' manner means that either both SRB AND (logical) DRB are admitted or the UE is rejected. RLT all SRB are admitted

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Deployment of shared channels offers the possibility for scheduling. In this way information on varying channel conditions can be exploited to increase the overall throughput.

Fast scheduling in time (1 ms) and frequency domain reduces latency and improves peak rate. Adaptive Modulation and Coding leads to higher data rates and optimizes spectral efficiency Hybrid ARQ leads to higher efficiency in transmission and error correction. A scheduler deploys mechanisms to determine which user(s) is(are) served in a given transmission time interval.

Dynamic assignment of radio resources to the UE is done by taking into account channel conditions and priorisation for the UE with the better channel conditions.

Benefit is the maximisation of the Node B throughput, high peak date rates for the UE and an efficient usage of the radio resources.

Furthermore OFDMA / MIMO allows scheduling decisions on the basis of three dimensions: time, frequency and space.

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As shown in the figure the channel's decorrelation in frequency and time offers the possibility to exploit the varying conditions.

Scheduling resource is the time-frequency grid. In detail, the basic scheduled resource consists of a 1 ms (sub-frame, TTI) and 12 subcarriers, 180 kHz. The efficiency of the scheduling strongly depends on the deployed algorithm.

Additionally the performance depends on the UE speed. Furthermore the gain of the scheduling may be higher the higher the number of scheduled UE's.

The scheduling functionality is provided by the MAC layer.

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Determine which PRBs are available (free) and can be allocated to Ues

Allocate PRBs needed for common channels like SIB, paging, and random access procedure (RAP)

Final allocation of UEs (bearers) onto PRB. Considering only the PRBs available after the previous steps

Pre-Scheduling: All UEs with data available for transmission based on the buffer fill levels

Time Domain Scheduling: Parameter MAX-#_UE_DL decides how many UEs are allocated in the TTI being scheduled

Frequency Domain Scheduling for Candidate Set 2 UEs: Resource allocation in Frequency Domain including number and location of allocated PRBs

The scheduling is performed on cell basis. The two main functions are to decide which UE(s) shall be scheduled, the number of resources and the MCS to be applied.

Furthermore the scheduler needs to be QoS aware. There is priority given to random access responses, control data, HARQ retransmissions.

The channel quality may be taken in consideration.

In RL10 the DL exploits CQI reports to decide on frequency and time resources. In UL the scheduling decisions in RL10 are not based on quality but a random frequency allocation is deployed.

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Time domain:

Evaluation of the #PRBs that will be assigned to UEs

Available number of PRBs per user

Multiple of 2, 3 or 5

Max. # of UEs which can be scheduled per TTI time frame is restricted by an O&M parameter. RL T and RL10 limit the number to a max. of 10 UEs per TTI

Frequency Domain:

Uses a random function to assure equal distribution of PRBs over the available frequency range ( random frequency hopping)

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The effective Eb/No and hence the spectral efficiency depend on BLER. However there are QoS requirements which also have to be considered. Taking both into account leads to a target BLER.

AMC is in use in order to tune BLER so that the target value is reached. Therefore when channel conditions change modulation and/or coding modifications might be needed.

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An initial MCS is provided by O&M (parameter INI_MCS_DL) and is set as default MCS. If DL AMC is not activated (O&M parameter ENABLE_AMC_DL) the algorithm always uses this default MCS.

If DL AMC is activated HARQ retransmissions are handled differently from initial transmissions ( For HARQ retransmission the same MCS has to be used as for the initial transmission).

An MCS based on CQI reporting from UE shall be determined for the PRBs assigned to the UE as indicated by the downlink scheduler. So the mechanism has UE scope with a frequency of several TTIs based on configurable CQI measurement intervals.

In RL T the alggorithm is based on overall signaling payload (control data volume) for all users and does not depend on the actual radio condition.

The adaptation is done on cell-basis and per TTI. In RL10 the code rate is selected for PDCCH resources (QPSK only) based on CQI reports. Thes CQI reports indicate the ‘CCE (Control Channel Elements) aggregation level’ and hence the coding rate.

The usage of PDCCH resources are based on channel condition and in addition on the availability of PDCCH resources into account. The feature may be enabled by O&M.

For AMC of the PUSCH a UE specific slow link adaptation ( 10-100ms) is applied. The decisions are based on BLER measurements.

AMC works independently of UL scheduler and UL power control. Interactions to UL PC and scheduler are result driven, i.e. to keep signaling load on eNodeB internal interfaces low, MCS is reported at the start of data transfer and only when there are changes of MCS. In case of long link adaptation updates and to avoid low and high BLER situations, the link adaptation can act based on adjustable target BLER values:

- “Emergency Downgrade” if BLER goes above a MAX BLER threshold (poor radio conditions);

- “Fast Upgrade” if BLER goes below of a MIN BLER threshold (excellent radio conditions).

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From TS 36.213 (DL example shown here)

MCS index -> from 0 to 28 -> it is decided by the scheduler which should translate a specific CQI in an MCS index

Modulation Order -> indicates the modulation type (QPSK, …) by indicating the number of bits per symbol

QPSK = 2

16QAM = 4

64QAM = 6

ITBS = TBS index

The TBS Index is mapped to a specific TBS size for a specific #RBs

Uses a different table

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Due to inaccuracies in the CQI reporting the performance may be downgraded. In order to compansate these effects CQI adaptation is applied. The algorithm is based on the comparison of the observed (averaged) ACK/NACK ratio with the target BLER. A CQI offset can be derived which is added to the actual reported CQI values and so forms the basis for the AMC algorithm.

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The transmission power is adapted in order to achieve the desired QoS (BLER/BER).

This adaptation is necessary since the propagation channel is subject to several conditions, which generally vary in space and/or time, e.g. path loss, log normal fading, short term fading , UE speed

•location (outdoor, indoor, in-car) etc.

Downlink power control determines the energy per resource element (EPRE). The term resource element energy denotes the energy prior to CP insertion. The term resource element energy also denotes the average energy taken over all constellation points for the modulation scheme applied.

Uplink power control determines the average power over a DFT-SOFDM symbol in which the physical channel is transmitted. Compared with UTRAN the UL power control is slower. The PUSCH and the PUCCH are subject to a combined open and closed loop power control algorithm, i.e. to control the transmission power for UL channels a combination of an open (input: pathloss, sysinfo and signaling) and a closed loop (TPC) method is used.

A cell wide overload indicator (OI) and a High Interference Indicator (HII) to control UL interference are exchanged over X2. An indication is given which PRBs an eNodeB scheduler allocates to cell edge UEs and hence will be most sensitive to inter-cell interference.

Power control - already being applied in 2nd and 3rd generation networks - has high potential for improvement of the performance of mobile networks.

Main benefits are:

It can bring down the interference in up- and downlink and hence enhances the capacity of the networks.

Additionally it helps to keep down the uplink-power consumption, thereby increasing the stand-by time for the UE.

Furthermore, from the EMC (Electro Magnetic Compatibility) point of view it can improve the situation considerably

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The eNodeB determines the downlink transmit energy per resource element (EPRE).

Downlink cell-specific reference-signal (RS) EPRE is constant across the downlink system bandwidth and constant across all subframes until different cell-specific RS power information is received. The downlink RS EPRE is given by the parameter Reference-signal-power provided by higher layers.

In cases 16QAM, 64QAM, spatial multiplex TRI>1 or multi-user MIMO the DL power is given by rA and rB

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Procedure for Slow UL Power control

UE controls the Tx power to keep the transmitted power spectral density (PSD) constant independent of the allocated transmit bandwidth (#PRBs)

If no feedback from eNodeB ( in the PDCCH UL PC command) the UE performs open loop PC based on path loss measurements

If feedback from eNodeB the UE corrects the PSD when receiving PC commands from eNodeB ( in the PDCCH UL PC command)

PC commands ( up and down) based on UL quality and signal level measurements

Applied separately for PUSCH, PUCCH

Scope of UL PC is UE level ( performed separately for each UL in a cell)

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Handovers (HO) can be grouped into

INTRA-SYSTEM: EUTRAN EUTRAN (1):

Intra-frequency -(example 1a):

Inter-frequency -(example 1b):

INTER-SYSTEM / INTER-RAT:

EUTRAN UTRAN, GERAN .. (example 2)

GERAN, UTRAN … EUTRAN HO’s. (example 3),

Not discussed further, since this HO type is triggered by GERAN.

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Handover Algorithm

A Handover will be initiated by a measurement report, which is sent via the Radio Resource Control (RRC) protocol. Upon the reception of this measurement report, the handover algorithm will decide whether a handover should take place.

In response to the handover decision, the handover execution will be carried out using the corresponding procedures. After the handover execution, the handover algorithm will be informed, whether the handover was successful or not.

The Handover procedure is composed of a number of single functions:

•Measurements

•Filtering of measurements

•Reporting of measurement results

•Hard handover algorithm

•Execution of handover

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1. The source eNB configures the UE measurement procedures with MEASUREMENT CONTROL

2. UE is triggered to send MEASUREMENT REPORT to the source eNB. It can be event triggered or periodic

3. Source eNB makes handover decision based on UE report + load and service information

4. The source eNB issues a HANDOVER REQUEST to the target eNB

5. Target eNB performs admission control

6. Target eNB sends the HANDOVER REQUEST ACKNOWLEDGE to the source eNB

When the UE is in LTE_ACTIVE state, mobility handling takes place via network controlled handovers with UE assistance. UE assistance here simply means that the UE does measurements and reports them to the eNB to assist in the handover decision. Currently it is planed that neighbor cell measurements are based on the UE’s cell detection capabilities rather than on a network supplied neighbor cell list.

When the source (current serving) eNB decides to start a handover of an UE to a neighbor cell in a new (target) eNB it will contact this target eNB. This is done via the X2-AP message HANDOVER REQUEST. The message will contain the target cell for the UE, the current serving MME and SAE GW. It is task of the target eNB to allocate virtual capacity in the target cell via its admission control function.

If this is done the target eNB returns part of the handover message for the UE within the X2-AP message HANDOVER REQUEST ACKNOWLEDGE. In this message also a data forwarding tunnel (TEID from target eNB) is indicated. It allows the source eNB to forward still buffered or still arriving downlink packets to the target eNB.

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7. Source eNB generates the HANDOVER COMMAND towards UE, Source eNB starts forwarding packets to target eNB

8. Source eNB sends status information to target eNB

9. UE performs the final synchronisation to target eNB and accesses the cell via RACH procedure DL pre-synchronisation is obtained during cell identification and measurements

10. Target eNB gives the uplink allocation and timing advance information

11. UE sends HANDOVER CONFIRM to target eNB, Target eNB can begin to send data to UE

The source eNB can now give the HANDOVER COMMAND (RRC) to the UE. The command contains the configuration for the UE in the new cell and possibly already an UL/DL resource allocation. The UE will detach from the old cell and synchronize itself to the new cell. In the mean time the source eNB can start downlink packet forwarding via X2 interface.

Once synchronization between UE and the new cell is achieved, the UE confirms the handover with RRC message HANDOVER CONFIRM. This will trigger a HANDOVER COMPLETE message of S1-AP to be sent to the MME. It simply informs the MME that now a new eNB is responsible for the UE. Thus this message will contain the IP addresses and TEIDs of the target eNB for the S1 tunnels.

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12. Target eNB sends a PATH SWITCH message to MME to inform that the UE has changed cell

13. MME sends a USER PLANE UPDATE REQUEST message to Serving Gateway

14. Serving Gateway switches the downlink data path to the target side

15. Serving Gateway sends a USER PLANE UPDATE RESPONSE message to MME

16. MME confirms the PATH SWITCH message with the PATH SWITCH ACK message

17. By sending RELEASE RESOURCE the target eNB informs success of handover to source eNB and triggers the release of resources

18. Upon reception of the RELEASE RESOURCE message, the source eNB can release radio and C-plane related resources associated to the UE context

The MME’s task is to send this information via GTP-C UPDATE BEARER REQUEST to the SAE GW. This will switch the traffic path now completely from SAE GW to target eNB.

When the path is switched, the old eNB will get the S1-AP message RELEASE RESOURCE which will clear down all allocated resources for the UE that is already in the new eNB.

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08 TM51178EN03GLA1 Radio Resource Management Ppt