RACH Optimization

The configuration of the random access procedure has a critical impact on end-user experience and overall network performance. A poorly configured Random Access CHannel (RACH) may increase access setup time and accesses failures, impacting both call setup and handover performance. With optimal random access parameter setting, maximum end-user experience can be obtained. This is achieved by reaching the desired balance in the radio resource allocation between random accesses and services while at the same time avoiding creating excessive interference. To keep the RACH optimized for all cells during varying conditions, the optimization can be repeated periodically or run continuously.

In LTE, RACH (Random Access Channel) is an uplink unsynchronized channel, used for initial access or uplink synchronization. The triggers for Random Access procedure include:

• Connection setup
• Radio Link Failure
• Downlink data transmission in uplink unsynchronized state
• Uplink data transmission in uplink unsynchronized state
• Handover

So the Random Access procedure performance influences the call setup delay, handover delay, data resuming delay, call setup success rate and handover success rate. Besides, physical resources for RACH are reserved for its special use. So the configuration for RACH influences the capacity of the whole network.

An optimized RACH configuration enables end-user benefits and network performance gains through:

• Reduced connection time
• Higher throughput
• Better cell coverage.

By automating the optimization of RACH, maximum performance is achieved with no operator intervention or effort. The network will dynamically adapt to network changes and end-user behavior to always deliver best possible performance and resource utilization.

Necessity for RACH optimization
The performance of Random Access performance is evaluated by its delay and success rate. The performance depends on following factors:

• Population under the cell coverage;
• Call arrival rate;
• Incoming handover rate;
• Whether the cell is at the edge of a tracking area;
• Traffic pattern, as it affects the DRX (Discontinuous Reception) and uplink synchronization states, and hence the need to use RACH.

These factors are affected by network configurations, such as antenna tilt, transmission power and handover threshold, and also by the load of network. If network configurations or load is changed, the performance of Random Access procedure may change greatly, which influences the performance of other procedures, such as call setup, data resuming and handover. Therefore the automatic optimization of RACH would be beneficial.

Possible RACH optimization algorithm The configurations of RACH include:

• RACH physical resources
• RACH preamble allocation for different sets (dedicated, random-low and randomhigh)
• RACH persistence level and backoff control
• RACH transmission power control

Measurements are done in eNB, recording random access delay, random access success rate and random access load. The random access load can be indicated by the number of received preambles in a cell in a time interval. It is measured per preamble range (dedicated, random-low and random-high), and averaged over the PRACHs configured in a cell.

Thresholds are set separately for random access delay and success rate. If either of the thresholds is reached, RACH optimization is triggered. First, Random access load is analyzed to check if the random access is overload in any of the three preamble ranges. If one of them is overload, RACH preambles are reallocated among these three preamble ranges. If all of them are overload, more physical resources need to be reserved for RACH. If none of them is overload, other parameters need to be adjusted, such as increasing the transmission power step and distributing the backoff time in a wider range.

Description

A User Equipment (UE) in idle state is unknown to an LTE network. In order to start establishing a relation to the LTE network, the UE searches for the most suitable cell and reads its broadcast system information. The broadcast system information (SIB2) from the cell provides the UE with cell-specific random access format and procedure details. These details determine essential parameters, such as preamble format and initial power setting. The UE randomly selects one of the preambles in an attempt to establish a relation to the cell. As long as no other UE is using the selected preamble at the same time instant, the access attempt can succeed. The success also relies on that the preamble can be heard and identified by the eNB. If there is no response or rejection from the eNB, the UE needs to retry until it succeed. The impact on user experience during the random access procedure is mainly delayed access and interrupted transmission during a handover. The automatic RACH optimization function will balance between optimal access performance and least resource utilization needed to meet set quality target such as, an acceptable level of access success rate. By selecting the most suitable preamble format, based on say traffic type, and dynamically adjusting the broadcast power control parameter (P0), the access success rate can be optimized while still maintaining a low interference level. This results in best user experience with fast access and best possible throughput.

The automatic RACH optimization function can be executed continuously resulting in RACH related parameters being modified automatically. The changes could be based on inputs from both the connected UEs and each cell’s neighboring cells. By collecting the recently introduced RACH report from the UE, the actual access delay can be determined. The RACH report is included in the message UEInformationResponse (3GPP Rel-9 spec TS 36.331). It contains two new parameters: numberOfPreamblesSent and contentionDetected. Based on this new information, the automatic RACH optimization function can adjust the power control parameter (P0) or change the preamble format to reach the set target access delay. By optimizing P0, the probability for an eNB to read the preamble increases. However, as a result, the interference on neighboring cells may also increase. The second option adjusts the preamble format to use, under the premise that a correct preamble format assures successful preamble detection for the traffic type in a cell.

The automatic RACH optimization function automates the required continuous adaptation of the RACH. The automatic RACH optimization function can automatically react when receiving notification over X2 of a RACH parameter change in a neighboring cell. The information is transmitted via the eNB configuration update X2 message. For instance, allocation of same root sequence index in neighboring cells should be avoided to reduce interference. Automatic RACH optimization, based on the optimization on real traffic data and neighbor cell information, makes it well suitable for eNB function localization. The automatic RACH optimization function can execute autonomously with no operator intervention or effort. The operator controls the function with a few policies that set the outer limits for the function. These limits assure that the function will not move to an extreme setting when finding the best tradeoff between access delay and resource utilization.

Energy Savings

Mobile network operators are increasingly aiming at decreasing power consumption in telecom networks to lower their OPEX and reduce greenhouse emissions with network energy saving solutions for long term sustainable development. With the expected deployment of large numbers of mobile network radio equipment, in the form of Home NB/eNBs, OPEX reduction becomes even more crucial.

Energy consumption is a significant part of an operator’s OPEX. OPEX reduction can be accomplished by designing network elements with lower power consumption and temporarily shutting down unused capacity when not needed. Power amplifiers consume a significant portion of the total energy consumption in a wireless network.

When a cell is switched off, there may be a need for the neighboring cells to pick up the load. However switching off a cell should not cause coverage holes or create undue load on the surrounding cells. A switched off cell is not considered a cell outage or a fault condition. All traffic on that cell is expected to be moved to the underlying umbrella cells before any switch off occurs.

When a NE is "switched off" for energy savings purposes, no alarms should be raised to the OAM manager for a condition that is a consequence of a "switched off" NE. The operator should have the capability to prevent the network from automatically compensating based on the cell that is in energy savings mode in order to prevent unnecessary disruption in the network.

OAM of mobile networks can contribute to energy saving by allowing the operator to set policies to minimize consumption of energy, while maintaining coverage, capacity and quality of service. The permitted impact on coverage, capacity and quality of service is determined by an operator’s policy.

3GPP Rel-11 has defined two energy saving states for a cell with respect to energy saving namely: notEnergySaving state and energySaving state.Based on the above energy saving states, a full energy saving solution includes two elementary procedures: energy saving activation (change from notEnergySaving to energySaving state) and energy saving deactivation (change from energySaving to notEnergySaving state).

When a cell is in an energy saving state it may need neighboring cells to pick up the load. However, a cell in energySaving state cannot cause coverage holes or create undue load on the surrounding cells. All traffic on that cell is expected to be drained to other overlaid/umbrella cells before any cell moves to energySaving state.

A cell in energySaving state is not considered a cell outage or a fault condition. No alarms should be raised for any condition that is a consequence of a network element moving into energySaving state.
Criteria for the energySaving state is defined in 3GPP namely: degree of energy saving effect, controllability from the network, and service availability.

The various Energy Savings Management (ESM) concepts can apply to different RATs, for example UMTS and LTE. However, 3GPP has specified that some of these ESM concepts may be limited to specific RATs and network elements, and specific solutions may be required for them.

In Rel-11, three general architectures that are candidates to offer energy savings functionalities are described, namely: distributed, network management centralized, and element management centralized. Energy savings management use cases such as the cell overlay use case and the capacity limited network use case, are described in detail. Requirements for element management centralized energy savings and distributed energy saving are specified. Coordination between energy saving and cell outage is addressed.

ICIC - Fractional Frequency Reuse

Interference management techniques are critical to the performance of heterogeneous cellular networks, which will have dense and overlapping coverage areas, and experience high levels of interference. Fractional frequency reuse (FFR) is an attractive interference management technique due to its low complexity and overhead, and significant coverage improvement for low-percentile (cell-edge) users. FFR sets restrictions on RB allocation between the different UEs in each cell.

FFR method separates the frequency bands allocated to the areas near a base station where no signal interference from adjacent base stations occurs from the frequency bands allocated to the areas far from the base station where signal interference from an adjacent base station can occur. The transmit power is reduced and the frequency-reuse factor is set to 1 for the frequency band allocated to the cell area where no signal interference from adjacent base stations occurs, and conversely, the transmit power is increased and the frequency-reuse factor is set to 3 for the frequency band allocated to the cell area where signal interference from an adjacent base station can occur. This improves the signal to interference plus noise ratio (SINR) and throughput for users located at the cell edge without degrading spectral efficiency.

To implement FFR, the frequency band allocated to a user at a cell edge must be different from that allocated to another user in the adjacent cell, as shown in Figure above . The LTE standard specifies an inter-base-station interface that enables adjacent base stations to exchange information on bands generating large interference in other cells and on bands that are affected by large interference from other cells.

The following describes interference-coordination signals that can be used for implementing ICIC in the downlink and uplink.

1) Downlink
The signal used for interference coordination in the downlink is called relative narrowband transmit power (RNTP). This signal can take a value of 0 or 1 and is sent to multiple base stations serving adjacent cells for each resource block (RB).note 2) Specifically, this value is set to 0 if the ratio between the transmit power of the downlink signal allocated to the RB and the average transmit power of the system frequency band is guaranteed to be under a certain threshold and to 1 otherwise.3) This scheme enables a base station to learn about an RB that may be transmitting at high power in an adjacent cell and to reduce interference by avoiding allocating that RB to a user experiencing poor reception. Increasing the transmit power above the system average for a user experiencing poor reception should also improve the quality of that users reception.

2) Uplink
There are two types of signals for interference coordination in the uplink: high interference indicator (HII) and interference overload indicator (OI). The HII signal is used by a base station to notify to multiple base stations serving adjacent cells of the uplink RB it has allocated to a cell-edge user. This enables cell-edge users in adjacent cells to be allocated different bands, the same as in the downlink approach, which means that improved throughput can be expected for these cell-edge users. The OI signal, on the other hand, is used by a base station to notify to multiple base stations serving adjacent cells the results of measuring interference power for each RB and classifying those results into multiple levels. Thus, the base station of a cell that receives notification of high interference power from an adjacent cell can reduce the transmit power of its users and thereby reduce the amount of interference created in the adjacent cell.

Reference 

Inter-Cell Interference Coordination (ICIC)

LTE uses OFDMA in downlink and SCFDMA in uplink. The inherent orthogonality of these transmission schemes reduces the intracell interference. The problem of intercell interference impacts the UEs at cell edges because as the frequency is reused across the cells, the edge UEs may be allocated the same subcarriers. As these UEs operate on high power to reach the eNBs, the signals interfere strongly. The edge UEs receive equally strong/weak signals from the adjacent cells, and so the strong interference causes difficulty for the UE when receiving downlink transmission.

Interference is caused because cells only know what radio resources their own UEs are using, and not what other UEs in the neighbor cells are using. For example, in the figure above, Cell A knows what resources A1 is using, but not about what B1 is using, and vice versa. And the cells independently schedule radio resources for their own UEs. So, to the UEs at cell edges (A1 in Cell A and B1 in Cell B), same frequency resource can be allocated.

As seen in the figure below, if the two UEs are located in cell centers like A2 and B2, no interference is caused because they use low power to communicate. However, if they are at cell edges like A1 and B1, their signals cause interference for each other because the two use high power to communicate.

ICIC reduces inter-cell interference by having UEs, at the same cell edge but belonging to different cells, use different frequency resources. Base stations that support this feature can generate interference information for each frequency resource (RB), and exchange the information with neighbor base stations through X2 messages. Then, from the messages, the neighbor stations can learn the interference status of their neighbors, and allocate radio resources (frequency, Tx power, etc.) to their UEs in a way that would avoid inter-cell interference.

For instance, let's say a UE belonging to Cell A is using high Tx power on frequency resouce (f3) at the cell edge. With ICIC, Cell B then allocates a different frequency resource (f2) to its UE at the cell edge, and f3 to its other UE at the cell center, having the one at the center use low Tx power in communicating.

Inter Macro Cell Interference

The interference problem gets even more complicated in small cells, wherein the small cell overlaps the macro cell. As the small cell eNBs operate at a lower power level, the problem extends to ensuring that a UE actually attaches to small cell eNB in proximity, rather that attaching to a macro eNB operating at high power. In this case, the cell edge is the point where the UE receives similar strength signals from both the macro and small cell.

Inter Macro-Small Cell Interference

Smallcell-eNBs have much lower transmit power than macro eNBs. In conventional cellular systems a UE connects to the BS providing the best downlink Signal to Interference-plus-Noise Ratio (SINR), and since the eNB has low transmit power the coverage of small cells will be small. But UEs receiving signals with larger SINR from a macro eNB might have much lower path loss to a small cell eNB and hence cause significant uplink interference to the small cell eNB.

If not placed specifically in a hot-spot, only a small number of UEs will be connected to the small cell-eNB which will limit the gain from offloading the traffic from the macro cells. One way to improve this is to expand the small-cell, for example, by introducing a bias in the cell selection based on reference signal received power (RSRP) or perform UE association determined by minimal path loss. This is often referred to as Cell Range Extention (CRE). In this case UEs at the cell-edge of the pico cell will also experience severe downlink interference from the macro eNB.

Macro-Femto Cell Interference
HeNBs or femtocells use low transmission power, usually lower than the power transmitted by a user terminal. The deployment of HeNBs is usually uncoordinated as opposed to deployment of macro and pico cells which are usually planned. A HeNB is typically operated in closed mode meaning that only certain users that are part of a closeds group (CSG) are allowed to connect to the HeNB.

The CSG restriction introduces complex high-interference scenarios in both the uplink and downlink direction. Consider a UE located close to the HeNB, possibly even in the same room as the HeNB, that are not part of the HeNB’s CSG. In this case the UE will experience severe downlink interference from the HeNB. In fact, the HeNB will often create a coverage hole in the macro cell. Also, the HeNB will experience severe uplink interference from the UE.