29-08-2011, 03:57 PM
High Speed Downlink Packet Access (HSDPA) – A Means of Increasing Downlink Capacity in WCDMA Cellular Networks
Abstract:
The increasing demand for capacity in order to provide high data rate multimedia services in wireless environments necessitates enhanced radio transmission techniques and network protocol functionality. Such techniques have to be added to already existing mobile cellular networks, e.g. as provided by EDGE. For 3rd generation UMTS networks based on WCDMA, the High Speed Downlink Packet Access (HSDPA) is being introduced to meet this demand and improve spectral efficiency. This technique is based on well-known Link Adaptation (LA) by using various modulation and coding schemes and with this realizing several different data rates for downlink transmission. Moreover, fast scheduling techniques are introduced to enable efficient and flexible sharing of the radio resources among different users and services. This paper gives a detailed overview of the integration of HSDPA in UMTS networks and provides a performance evaluation with respect to a multi-cellular environment with various types of applications. The question whether LA mechanisms are beneficial in a WCDMA system is discussed and advice to the network dimensioning is given.
1. Introduction
The success of 3rd generation wireless cellular networks is mainly based on an efficient provisioning of the expected wide variety of services requiring different Quality of Service (QoS) with respect to data rate, delay and error rate. In order to improve support for high data rate packet switched services, 3GPP is currently developing an evolution of UMTS based on WCDMA known as High Speed Downlink Packet Access (HSDPA) which is included in the Release 5 specifications. HSDPA is targeting increased capacity, reduced round trip delay, and higher peak data rates up to 8–10 Mbps [8]. To achieve these goals, a new shared downlink channel, called the High Speed Downlink Shared Channel (HS-DSCH) is being introduced. In addition, three fundamental technologies are foreseen, which are tightly coupled and rely on rapid adaptation of the transmission parameters to the instantaneous radio conditions. Fast page link adaptation techniques enable the use of spectrally efficient higher order modulation when channel conditions permit, and revert to robust Quaternary Phase Shift Keying (QPSK) modulation for less favorable channel conditions. Fast Hybrid Automatic Repeat Request (HARQ) algorithms rapidly request the retransmission of missing data entities and combines the soft information from the original transmission and any subsequent retransmissions before any attempts are made to decode a message. Fast scheduling shares the HS-DSCH among the users. This technique, which exploits multi-user diversity, strives to transmit to users with favorable radio conditions. Moreover, the time interval considered for scheduling is no longer based on radio frames of 10 ms but shortened to the WCDMA time slots of 667