Section: A.5 Monitor of cell level QoS and radio resource utilisation
Content: In an E-UTRAN cell the quality of service achieved is directly influenced by a number of factors, including: - Loading of users on the cell - Traffic loading and characteristics - UE locations and mobility - RRM policies - Scheduling - congestion control - admission control - layer 2 protocol configuration - Mapping of traffic to QCI - Setting of QoS parameters other than the QCI. It is very important to be able to monitor the QoS to determine whether the combined effect of these policies, algorithms and external factors is satisfactory. Unsatisfactory QoS may be rectified by adjusting policies and RRM settings, for instance.In case the rectification is not leading to desired QoS some other policies (ACB, rejected RRC connection setup) based on restricting the UEs for network access to the cell may be applied Cell bit-rate A fundamental measure of QoS is the throughput (data rate) of the cell. The total cell throughput measured across all radio bearers gives an indication of the loading and activity in the cell. Adding a per QCI counter allows the loading on the different QCIs to be measured. For example, if QCI 1 is used exclusively for VoIP then the loading of conversational speech can be directly determined. Finally, the maximum throughput can indicate to the operator whether there is enough capacity in the network; for example, is the backhaul sufficient. Separate counters should be configured on the downlink and uplink. Complexity may be reduced by performing the counters at layer 3, giving the ingress bit-rate to the eNB on the downlink and the egress bit-rate from the eNB on the uplink. Cell throughput includes both User Plane data and Control Plane data. To support the User Plane data, necessary Control Plane data also need to be transmitted. This Control Plane data although required, will not be perceived (felt) by the User. The total cell throughput helps to evaluate the usage of bandwidth and radio resource. Operators ideally want to see the Control Plane data as small as possible when compared to the User Plane data without compromising on the service. Hence it is important to monitor the total cell throughput as well as how much is occupied by Control Plane Data. Number of actives UEs It is also of interest to determine how many users are enjoying the throughput numbers determined for each QCI. Therefore, we may count the number of users that are active for each QCI – here active users have data queued pending transmission. A simple division of the throughput (data rate) of a QCI by the number of active users on the QCI indicates the throughput per user on the QCI. For example, taking QCI 1 this metric could indicate the typical codec rate being employed in the cell. Alternatively, for QCI 9 supporting low priority TCP-based traffic it can indicate the typical bandwidth pipe size for a user when he has data to send / receive. DL packet delay Latency is of prime concern for some services, particularly conversational services like speech and instant messaging. A counter is added to measure the mean delay for IP packets incurred within the eNodeB. Separate counters are provided per QCI which are particularly useful when the QCI is used by very few services and the packet sizes vary little. It is only practical to measure packet delays on the downlink. In case of the eNodeB serving one or more RN, the packet delay includes both the internal processing delay at eNB) and UE/RN as well as the packet transferring delay on the radio link. As RN UE’s packets need to be transferred via between E-UTRAN snfRN while packets for UEs directly connected to eNB need to pass through only Uu interface, packet delay optimization mechanism may be different for RN UEs and eNB UEs. Therefore it is beneficial to have measurements on packet delay separately for packets transmitted between the eNodeB and UEs and for packets transmitted between the E-UTRAN and RNs. DL packet drop rate When a cell is heavily loaded congestion can take place. When congestion is not severe. the impact is typically the incurrence of additional delay for non-GBR radio bearers. However, when congestion is severe the eNodeB may be forced to discard packets. It is important for the operator to have visibility of packet discard so that corrective action can be instigated (for example, by adjusting admission control settings in the network). It is only practical to measure packet discards on the downlink. Packet discards on handover should not be included in the count. PRB Usage The resource utilisation, measured in terms of physical resource blocks (PRBs), is a useful measure of whether a cell is lightly loaded or not. Loading is a key input to network capacity planning and load balancing. Furthermore, when resource utilisation per QCI is reported the distribution of resources between different services can be estimated. The PRB usage distribution could provide operators the load distribution information of the network during the collecting time period.According to the PRB usage distribution information, the scenarios where a cell may experience high load in certain short times (e.g. in a second) and recover to normal very quickly can be recognized.The PRB usage distribution is a useful measure for operator to be aware of whether a cell has ever experienced high load or not in the monitoring duration. This distribution information could also help in the root cause analysis in case when the application problems are caused by load bursts. The distribution information could show the median usage and the peak usage. The median usage is within the range of the bin that got the highest number of samples and the peak usage is within the range of the “highest” bin that got >0 samples. The distribution information is a key input to network capacity planning and load balancing. For a RN that requires subframe configuration, the RN can only be scheduled by the eNodeB within subframes configured for RN, while macro UE can be scheduled in any subframe. Therefore, in certain scenario it may be possible that the PRB usage is different for the subframe configured for the RN and for any other subframe. For example, in case there are many RNs in the network and only a few UEs connected directly to the eNodeB, it may happen that the PRB usage in the subframe configured for RN is quite high, while the subframes used for UEs is low. Therefore, it is beneficial to measure the PRB usage separately for RN and the total PRB usage. The total PRB usage includes the PRB usage for RN traffic and UE traffic, while the RN PRB usage includes only PRB usage for RN traffic. Resource Full Utilisation Resource congestion is a critical situation in the network that needs to be monitored closely in order to be assessed and addressed immediately (e.g. by expanding related resources). Measurements reflecting the time during which the resources are fully used are needed to properly assess the congestion. Congestion affects PRB so appropriate congestion measurements are needed. Transport Resource Usage Transport resource utilisation provides information on the overall traffic of an eNodeB with the rest of the network, together with information on the traffic of the individual cells it gives the ability to localise bottlenecks. On the other hand it is a key input for planning of timely expansion of transport (backhaul) resources in the network provides . Measurements enabling to track the transport resource utilisation are therefore useful. Hardware Resource Usage Hardware resource utilisation provides information on the overall load of an eNodeB, together with the information on the individual loads it gives the ability to localise bottlenecks. On the other hand it is a key input for planning of timely expansion of hardware resources in the network. Measurements enabling to track the hardware resource utilisation are therefore useful. Downlink Air interface packet loss rate The downlink air interface packet loss can be directly compared with the PELR value of a QCI to see if the packet loss (over the air interface) aspect of quality of service is being met within the cell (see [12] for more details on PELR). On the downlink this measurement can be added to the congestion losses (see DL packet drop rate) to determine the total packet loss rate at the eNodeB. Consequently, the downlink useful bit-rate can be estimated by scaling the measurement of the downlink PDCP ingress bit-rate by (1 – DL packet drop rate) (1 – air interface packet loss rate). In case or RN deployment the communicstion between the E-UTRAN and RN is going through the air interface. It is possible that the radio conditions are bad, which can lead also to high packet loss rate, which can contribute also whether the packet loss aspect of the quality of service is met or not. Therefore it is beneficial to have separate packet loss rate measurement for RN traffic and for the traffic connected directly to the eNodeB. Uplink packet loss rate The uplink air interface packet loss rate (per QCI) can be compared directly with the PELR defined for that QCI. An estimate of the uplink air interface packet loss may be provided by the “Uplink PDCP SDU loss rate”. This uplink measurement is based on PDCP sequence numbers and cannot precisely measure the air interface losses. Any packets discarded by the UE within the protocol stack (i.e. at layer 2) are also counted since they will have been given a PDCP sequence number. Discards at layer 3 are not counted. In case or RN deployment the communicstion between the E-UTRAN and RN is going through the air interface. It is possible that the radio conditions are bad, which can lead also to high packet loss rate, which can contribute also whether the packet loss aspect of the quality of service is met or not. Therefore it is beneficial to have separate packet loss rate measurement for RN traffic and for the traffic connected directly to the eNodeB. RACH Usage The RACH plays a vital role in the following procedures: - Initial access from RRC_IDLE; - Initial access after radio link failure; - Handover requiring random access procedure; - DL data arrival during RRC_CONNECTED requiring random access procedure; - UL data arrival during RRC_CONNECTED requiring random access procedure; Furthermore, the random access procedure takes two distinct forms: - Contention based using a randomly selected preamble (applicable to all five events); - Non-contention based using a dedicated preamble (applicable to only handover and DL data arrival). In the use-case of RACH configuration optimization, received Random Access Preambles and a contention indicator are signalled across an OAM interface. Monitoring of the preamble usage in a cell allows the operator to determine if the resources allocated to the RACH by the eNodeB are appropriate for the number of random access attempts. If the resources are underutilised then the operator may reconfigure the eNodeB (via CM) to allocate less resource to RACH thereby freeing up resource for other uplink transmissions. Alternatively, if the resources are heavily utilised then this is indicative of RACH congestion leading to increased latency for the procedures listed above. To this effect, measurements directly reflecting RACH congestion experienced by the eNodeB and by the UEs are useful. The eNodeB can partition the RACH resource between dedicated preambles, randomly selected preambles in group A and randomly selected preambles in group B. This partitioning can be evaluated when usage measurements are made on each set separately. To further monitor and analyze the latency of the procedures listed above created at the RACH level, the measurement of the number of RACH preambles sent per RACH attempt is useful. RACH optimisation function optimises RACH related configurations to achieve minimizing of access delays and random access collision probability for all the UEs. eNB may request the UEs to report the number of attempts to access the network (numberOfPreamblesSent in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [8]). Based on the attempting number of random access of all the UEs, the eNB can know the distribution of RACH preambles sent by UEs and calculate the access probability (AP) of a cell accurately. The eNB may also request the UEs to report contention detected while attempting to access the network (contentionDetected in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [8]). Based on the estimated time per random access and the estimated contention level in the network, the eNB can estimate the access delay probability (ADP) of a cell. Time period the requested quality of service cannot be achieved An overload situation in the cell can be related also to other conditions leading to insufficient QoS. That means UE is allowed for network access but later one due to some other reason either the session is dropped, or there is big PDCP SDU delay or low IP scheduled throughput, etc. leading to degradation of quality of provided services. Let’s consider a UE which is having an GBR service ongoing. The radio conditions in the cell are bad and thus despite there is no RLF detected for the UE because the QoS characteristics for the UE like throughput and delay cannot be achieved the UE is either abnormally released or quality of provided services has significantly degraded. Another example could be for the UE having an nonGBR service ongoing and RLF has been detected which leads to abnormal UE release or significant degradation of quality of provided services as well. In both cases despite the cell serving the UEs did its best from the point of view the most robust MCSs and thus maximum possible resources used, maximum UE transmit power and using other possible techniques (like HO, redirection….) it was not possible to keep the UEs active in the cell with guaranteed QoS simply because the cell did not have the needed capacity (features) to do so. For the operator it is therefore very important to have measurements to monitor duration the requested quality of service cannot be achieved.