E-DCH Channel Structure

For the E-DCH concept a number of additional channels were introduced in both the uplink and downlink directions as shown in Figures 3.47 and 3.48. These are used in addition to existing channels, which are also shown in the figure below. For further explanation of these channels, see Section 3.4.3 for Release 99 channels and Section 3.10.1 for HSDPA.

As shown on the left side in Figure 3.47, HSUPA introduces a new transport channel which is called the enhanced-DCH (E-DCH). While still being a dedicated channel for a single user, the dedicated concept was adapted to use a number of features, which were already introduced with HSDPA for the downlink direction. Therefore, the following overview just

gives a short introduction to the feature and the changes required to address the needs of a dedicated channel:

• Node-B scheduling: while standard dedicated channels are managed by the RNC, E-DCH channels are managed by the Node-B. This allows a much quicker reaction to transmis- sion errors which in turn decreases the overall round-trip delay time of the connection. Furthermore, the Node-B is able to react much more quickly to changing conditions of the radio environment and variations of user demands for uplink resources, which help to better utilize the limited bandwidth of the air interface.

• HARQ: instead of leaving the error detection and correction to the RLC layer, the E-DCH concept uses the hybrid automatic retransmission request (HARQ) scheme which is also used by HSDPA in the downlink direction. This way, errors can be detected on a per MAC-frame basis by the Node-B. For further details see Section 3.10.2 which describes the HARQ functionality of HSDPA in the downlink direction. While the principle of HARQ in the uplink direction is generally the same, it should be noted that the signaling of acknowledgments is done in a slightly different way due to the nature of the dedicated channel approach.

• Chase combining and incremental redundancy are used in a similar way for E-DCH as described in Section 3.10.2 for HSDPA in order to retransmit a frame when the HARQ mechanism reports a transmission error.

On the physical layer the E-DCH is split into two channels: the enhanced dedicated physical data channel (E-DPDCH) is the main transport channel and is used for user data (IP frames carried for RLC/MAC frames) and layer 3 RRC signaling between the terminal on the one side and the RNC on the other. As will be further shown below the spreading factor used for this channel is quite flexible and can be dynamically adapted from 64 to 2 depending on the current signal conditions and the amount of data the terminal wants to send. It is even possible to use several channelization codes at the same time to increase the overall speed. This concept is called a multi-code channel and is similar to the HSDPA concept of assigning frames on several downlink shared channels to a single terminal. As will be shown in more detail below the maximum number of simultaneous code channels has been limited to four per terminal with two channels being used with SF = 2 and the other two with SF= 4. In terms of frame length, 10 milliseconds are used for the E-DPDCH by default with 2 millisecond frames having been standardized as an optional feature for the terminal.

The enhanced dedicated physical control channel (E-DPCCH) on the other hand is used for physical layer control information. For each E-DPDCH frame a control frame is sent on the E-DPCCH to the Node-B which most importantly contains the seven-bit traffic format combination ID (TFCI). Only by analyzing the TFCI is the Node-B able to decode the MAC frame on the E-DPDCH as the terminal can choose the spreading factor and coding of the frame from a set given to it by the Node-B to adapt to the current signal conditions and uplink user data buffer state. Furthermore, each frame on the E-DPCCH contains a two-bit retransmission sequence number (RSN) to signal HARQ retransmissions and the redundancy version (see Section 3.10.2) of the frame. Finally, the control frame contains a so-called ‘happy’ bit to indicate to the network if the maximum bandwidth currently allocated to the terminal is sufficient or if the terminal would like the network to increase it. While the spreading factor of the physical data channel is variable, a constant spreading factor of 256 is used for the E-DPCCH.

A number of existing channels, which might also be used together with an E-DCH, is shown in the middle and on the right of Figure 3.47. Most of the time, an E-DCH is used together with HSDPA high-speed downlink shared channels which require a separate dedicated physical control channel (DPCCH) to send control information for downlink HARQ processes. In order to enable applications like voice and video telephony during an E-DCH session a mobile must also support simultaneous Release 99 dedicated data and control channels in the uplink. This is necessary because these applications require a fixed and constant bandwidth of 12.2 and 64 kbit/s, respectively. In total, an E-DCH capable terminal must therefore be able to simultaneously encode the data streams of at least five uplink channels. If multi-code operation for the E-DPDCH is used, up to eight code channels are used in uplink direction at once.

In the downlink direction, HSUPA additionally introduces two mandatory and one optional channel to the other already numerous channels that have to be monitored in downlink direction. Figure 3.48 shows all channels that a mobile station has to decode while having an E-DCH assigned in the uplink direction, HSDPA channels in the downlink direction and an additional dedicated channel for a simultaneous voice or video session via a circuit-switched bearer.

While HSUPA only carries user data in the uplink direction, a number of control channels in the downlink direction are nevertheless necessary. For the network to be able to return acknowledgments for received uplink data frames to the terminal, the enhanced HARQ information channel (E-HICH) is introduced. The E-HICH is a dedicated channel, which means that the network needs to assign a separate E-HICH to each terminal currently in E-DCH state.

In order to dynamically assign and remove bandwidth to and from individual users quickly, a shared channel called the enhanced access grant channel (E-AGCH) is used by the network

Figure 3.48 Simultaneous downlink channels for simultaneous HSUPA, HSDPA and dedicated channel use

that must be monitored by all terminals in a cell. A fixed spreading factor of 256 is used for this channel. Further details about how this channel is used to issue grants (bandwidth) to the individual terminals are given below in Section 3.11.3.

Finally, the network can also assign an enhanced relative grant channel (E-RGCH) to individual terminals to increase or decrease an initial grant which was given on the E-AGCH. The E-RGCH is again a dedicated channel which means that the network has to assign a separate E-RGCH to every active E-DCH terminal. The E-RGCH is optional, however, and depending on the solutions of the different network vendors there might be networks in which this channel is not used. If not used, only the E-AGCH is used to control uplink access to the network. Note that although all channels are called ‘enhanced’, none of these channels has a Release 99 predecessor.

Besides these three control channels, an E-DCH terminal must also be able to decode a number of additional downlink channels simultaneously. As HSUPA will normally be used together with HSDPA, the terminal also needs to be able to simultaneously decode the HS-DSCHs as well as up to four HS-SCCHs. If a voice or video call is established besides the high-speed packet session, the network will add another two channels in the downlink direction as shown in Figure 3.48 on the right-hand side. In total, an E-DCH mobile must therefore be capable of decoding 10–15 downlink channels at the same time. If the mobile is put into soft handover state by the network (see Section 3.7.1) the number of simultaneous channels increases even further as some of these channels are then broadcast via different cells of the terminal’s active set.