IP-based multimedia services and seamless networking will soon be with us and for mobile networks, the vital requirements are high performance and low cost solutions. And that's where High Speed Downlink Packet Access (HSDPA, standardized by 3PGG Release 5) comes in, as a key technology enabling mobile operators to optimize their provision of emerging multimedia services, mobile business VPN and multiplayer gaming. The NEC HSDPA solution delivers higher bit rates, reduced roundtrip times and the opportunity to expand the system capacity two to three times beyond current configurations


The High Speed Downlink Shared Channel (HS-DSCH) is designed to increase system throughput and deliver an improved user experience. Node B schedules shared a ”fat pipe” in HS-DSCH according to the radio environment of each UE; however in Release '99 of W-CDMA, individual DCH UEs have their own dedicated radio resource channels, whether they have downlink data or not. In the case of HSDPA, a wide band downlink channel can be shared among all HSDPA-capable UEs, and Node B provides scheduling packet transmissions every 2ms.



According to changing radio environment needs, the modulation scheme and coding rate can be quickly and flexibly modified. Both Quadrature Phase-Shift Keying (QPSK), which is used in Release 99, and 16-Quadrature Amplitude Modulation (16QAM), can be supported in HSDPA. With improved radio conditions, 16QAM can offer twice the capability of QPSK.


Figure 2 Comparisons of QPSK and HSDPA


A fast-scheduling algorithm always provides the UE with the best channel radio conditions. Node B monitors the radio condition of UEs using channel quality indication provided by those same UEs. The purpose of monitoring is to determine which user to transmit in a given transmission time interval. In ideal conditions, a UE is dynamically assigned for receiving packet data at specific time intervals. As a result, Node B can make the best use of channel code resources and provide higher user and system throughput.


Figure 3 Channel quality variation among two UEs


To reduce retransmission data, HSDPA makes use of two advanced automatic repeat request methods. When a retransmitted packet is received, the UE synthesizes it with the original packets: this "soft" combining technique can improve the SIR. Node B controls the set of coded bits used for retransmission, taking into account available memory in the terminal.
The scheduler for HSDPA is located in the MAC-hs protocol layer in Node B. The scheduler's functional entity, unique to each cell, controls the use of the resources available for HSDPA. For each TTI, the scheduler assigns the available channelisation codes to the UE. Optionally, it may decide which modulation and transport block size the UE shall use, or control the Tx power of the physical channels of the UE. UE Capabilities, QoS Requirements for pending data, buffer states and estimated channel quality are all important inputs for reaching a scheduling decision.




Both are designed for evolution towards HSDPA. The upgrade from Release ’99 to HSDPA is basically a SW upgrade: the SW is remotely downloaded and activated via NEC’s OMC-R. With the introduction of the HSDPA-capable Channel Coding Card (CHC), the upgrade procedure will involve a simple software download resulting in a CAPEX and OPEX saving to the mobile operator.
The HSDPA-capable CHC provides the additional functionality necessary to support HSDPA. Where one CHC is assigned per cell, the throughput capacity is up to 14.0 Mbit/s: this is the peak throughput per cell/user using 16QAM and 15 channelisation codes. If three cells are allocated to one CHC, 5 codes and 16QAM may be supported, providing a maximum of up to 4.8 Mbit/s per cell.


The NEC HSDPA solution allows any combination of Release ’99 and HSDPA on separate or common carriers in a single/multi-cell mode configuration. Our products do not limit mobile operators to any particular deployment scenario. On the contrary, mobile operators have complete flexibility over how to deploy HSDPA in the way that best supports their business and operational strategy. The NEC HSDPA solution supports low-cost entry by enabling users to start by simply adding a single channel card to each Node B.

Figure 4 TX power utilization in operating Release ’99 and HSDPA
It is not necessary to add another carrier just for HSDPA. By sharing single channel card' resources among three cells, HSDPA coverage can be deployed quickly and economically. It is possible to share TX power for the channels of Release '99 and those of HSDPA on a RF carrier, as shown on Figure 4. This is a particularly attractive option in areas where HSDPA coverage is required, but where the initial take-up in HSDPA handsets/subscribers is expected to be gradual. Additional channel cards and/or RF carriers can be added later, based on traffic demand.


The main advantage for a 3G operator with a W-CDMA network is the ability to move up to HSDPA with relatively insignificant additional infrastructure cost but via a simple software upgrade of the existing W-CDMA network. HSDPA is fully backwards-compatible with W-CDMA, and any application developed for W-CDMA will also work with HSDPA.
HSDPA maximizes the ability of W-CDMA to provide broadband services. In the same way that EDGE increases spectral efficiency compared to GPRS, so HSDPA increases spectral efficiency compared to W-CDMA. The higher spectral efficiency and higher speeds not only enable new classes of applications, but also support a greater number of users accessing the network, with HSDPA providing over twice the capacity of W-CDMA in Release '99.