Channel Element at R99/HSDPA/HSUPA

Channel Element

Channel Element (CE) describes the SW licensed capacity resources required for a dedicated channel (DCH or E-DCH). The number of channel elements required in a RBS is based on the traffic type, and is dependent on the radio bearers to be used, as well as on the number of simultaneous users for each specific radio bearer.

Hardware

Each time a dedicated channel is allocated, channel elements will be consumed in UL and DL. The consumption can be described as a cost, and depends on the type of radio bearer for the dedicated channel. The channel element cost can be expressed with weighting factors for the various radio bearers.

If there is an insufficient amount of channel elements available, blocking might occur. If possible, to avoid blocking, a best effort user will be switched down to a lower rate to make channel elements available for the requested dedicated channel.

Channel Element (CE) Resource

CE resources are a type of NodeB hardware resource. The number of CEs supported by single NodeB indicates the channel demodulation capabilities resource of the NodeB. The more CEs a NodeB supports, the more powerful the channel demodulation and service processing capabilities for serving the customers. Services at different rates require different numbers of CEs to ensure proper channel demodulation.

In a RAN, CE resources are managed by both the RNC and NodeB. The NodeB reports its CE capacity to the RNC. The RNC determines whether to admit a new service based on the number of CEs that need to be consumed and controls CE resources during CE congestion. This ensures the proper use of CE resources. The NodeB dynamically manages CE resources and rapidly adjusts the number of CEs that can be consumed based on the actual service rate. This increases CE resource usage.

Basic Channel Element Concepts

CE is a basic unit that measures the channel demodulation capabilities of a NodeB. CEs are classified into uplink (UL) CEs and downlink (DL) CEs.

• One UL CE needs to be consumed by a UL 12.2 kbit/s voice service (SF = 64) plus 3.4 kbit/s signaling.
• One DL CE needs to be consumed by a DL 12.2 kbit/s voice service (SF = 128) plus 3.4 kbit/s signaling.

If only 3.4 kbit/s signaling traffic is carried on a DCH or HSPA channel, one CE still needs to be consumed. The number of CEs that need to be consumed by services of other types can be calculated by analogy.

The number of UL and DL CEs supported by a NodeB is determined by the NodeB hardware capabilities and the licensed CE capacity. The number of UL and DL CEs supported by the NodeB hardware is called the physical CE capacity. The licensed CE capacity may differ from the physical CE capacity. The smaller determines the number of CEs that can be used by an operator.

CE is a concept of the NodeB side. On the RNC side, it is called NodeB credit. The RNC performs admission and congestion control based on the NodeB credit. In the UL, the number of Node credit resources is twice that of CEs. In the DL, the number of NodeB credit resources equals that of CEs.

Rules for Calculating CE Consumption

The RNC determines the number of CEs required for a service based on the SF that matches the service rate. When an RAB connection is set up or released for a service, CE resources must be allocated or taken back and the number of CEs must be deducted or added accordingly. Different rules for calculating CE resource consumption apply to channels or services of different types.

• CE resources reserved by the NodeB for common and HSDPA channels are shown in gray. 
• CE resources that need to be consumed by R99 and HSUPA services are shown in pink.

Common Channels CE Consumption

CE resources required on the UL and DL common channels are reserved by the NodeB. Therefore, they do not occupy the licensed CE capacity. These CEs do not need to be considered in the calculation of CE consumption.

HSDPA Channels CE Consumption

Similarly, the NodeB reserves CE resources for the high-speed downlink shared channel (HS-DSCH) and the related control channels if HSDPA is used. These CEs also do not need to be considered in the calculation of CE consumption.

Note that the signaling of an HSDPA UE that is not performing an R99 service occupies one DCH and needs to consume one DL CE. If the SRB over HSDPA function is enabled, the signaling of an HSDPA service does not consume additional CE resources. For an HSDPA UE that is performing an R99 service, its signaling and the R99 service occupy the same DCH. Therefore, only the CEs consumed on R99 traffic channels need to be calculated.

Traffic on the HS-DSCH will not consume channel elements. However, every HSDPA connection will require A-DCHs for UL/DL signaling and UL traffic. A?DCHs are dedicated channels, which consume channel elements. Dimensioning of DL resources for A-DCH is not needed since DL A-DCHs do not consume resources from the normal pool of SW licensed channel elements. However, resources for UL A-DCH have to be considered since they consume channels elements from the normal SW licensed pool.

R99 Service CE Consumption

For an R99 service, the RNC determines the number of CEs and NodeB credit resources that need to be consumed based on the SF that matches the maximum bit rate (MBR) of the service.

Enhanced Uplink (EUL)

EUL can only be used for connections that use HS-DSCH in the DL. Each admitted EUL user will carry traffic and signaling on an E-DCH. Similar to HSDPA, the E-DCH user has one serving cell. The E-DCH serving cell is always equal to the HS-DSCH serving cell for the connection. If the terminal is in soft or softer handover there is always at least one non-serving E-DCH cell.

E-DCH users consume channel elements from the SW licensed CE pool, but the resources for E-DCH are handled in a different way compared to A-DCH and DCH. The resources for E-DCH are controlled by the EUL scheduler located in the RBS and resources for A-DCH and DCH are controlled by the RNC.

Each admitted E-DCH user will have a minimum amount of channel elements allocated. The minimum amount of channel elements depends on licensing.Those channel elements are reserved to the E-DCH user as long as the user is active in the cell. The minimum channel element allocation can be used for non-scheduled data (signaling) and scheduled data to a certain rate (user data). The channel elements represented by the minimum CE allocation are guaranteed and are not affected by Enhanced Soft Congestion or re-scheduling in the RBS. As a result, an admitted E-DCH user has the possibility to reach a minimum rate supported by the minimum CE allocation.

If an E-DCH user requests rates higher than the rates supported by the minimum CE allocation, additional channel elements will be needed. These are handled by the EUL scheduler only and are not seen by the Admission Control function. The EUL scheduler will grant an E-DCH user a higher rate if it is possible to allocate more CEs in addition to the other two required UL resource types (air interface interference and Iub bandwidth [2]). The additional CEs for an E-DCH user might be affected by Enhanced Soft Congestion or re-scheduling in the RBS.

If the minimum amount of channel elements is not available at the E-DCH establishment attempt it will not be possible to admit a new E-DCH user.

Notes

It is not necessary to dimension for common channels, since common channels do not consume channel elements. 

Allocation of channel elements due to compressed mode has not been considered, since there is no additional channel element cost for connections in compressed mode.

This article is based on search at different documents for different vendors and 3GPP TS 36.213.

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