UMTS network has been introduced as a third generation mobile communication system. 3GPP organization [20] is in charge of its specifications. It has specified different technologies for UMTS networks: for example, Frequency Division Duplex (FDD), Time Division Duplex (TDD), and HSPA. In the majority of 3GPP specification documents, the name UTRAN is used to stand for the UMTS radio access network. The transmission rate capability of UMTS provides at least 144 kbps for full-mobility applications in all environments, 384 kbps for limited-mobility applications in the macro- and microcellular environments, and 2 Mbps for low-mobility applications particularly in the micro or pico-cellular environments. In 3GPP release'5, the transmission rate capability is enhanced for the downlink to reach 10 Mbps.
The UMTS system offers different types of quality of service (QoS) for different types of customers and their applications. A key QoS attribute includes priority access for different types of users. For example, real time priority access typically applies to voice services and reliable data transfer is applied to interactive data services. There are four different QoS classes: conversational, streaming, interactive and background class [21]. The main distinguishing factor between these QoS classes is how delay sensitive the traffic is.
Whilst the UMTS radio interface is completely new with respect to any 2G system, the core network (CN) infrastructure is based on an evolution of the current GSM/GPRS one. Figure 2.3 shows UMTS network architecture as it stands in Release'5 [19]. It consists of a set of Radio Network Subsystems (RNSs) connected to the CN via the Iu interface.
The CN primarily consists of a circuit-switched (CS) domain and a packet-switched (PS) domain. These two domains differ in how they handle user data. The CS domain offers dedicated circuit- switched paths for user traffic and is typically used for real-time and conversational services, such as voice and video conferencing. The PS domain, on the other hand, is intended for end-to- end packet data applications, such as file transfers, Internet browsing, and e-mail.
The RNS consists of a controller (the Radio Network Controller, or RNC) and one or more entities called Nodes B, which are connected to the RNC through the Iub interface. A Node B superintends a set of cells which may be FDD, TDD, or mixed. In UMTS, Different RNCs can be connected to each other through the Iur interface. RNC is the boundary between the radio domain and the rest of the network. The protocols opened in the user terminal to manage the radio link are terminated in the RNC. Above the RNC are the protocols that permit interconnection with the CN and which depend on it.
Node B UTRAN Iu Iur Iub Node B Node B Node B RNC VLR HLR GSN+ MSC+
UMTS Core Network
PCM ATM/ AAL2 IP/ GTP RNC Packed Service CN =GPRS + Connection Service CN Node B Node B UTRAN Iu Iur Iub Node B Node B Node B Node B Node B Node B RNC RNC VLRVLR HLRHLR GSN+ MSC+ MSC+
UMTS Core Network
PCM ATM/ AAL2 IP/ GTP RNC RNC Packed Service CN =GPRS + Connection Service CN
Figure 2.3. UMTS architecture.
UMTS networks uses in the air interface WCDMA (Wide-band Code Division Multiple Access) access technology. The concept of WCDMA inherits from the spread spectrum CDMA. CDMA uses a form of direct sequence which is, in essence, multiplication of a more conventional communication waveform by a pseudo-noise binary sequence in the transmitter. Spreading takes place prior to any modulation, entirely in the binary domain. The noise and interference, being uncorrelated with the pseudo-noise sequence, become noise-like and increase in bandwidth when they reach the detector. Filtering mechanism that rejects most of the interference power can enhance the Signal-to-Interference plus Noise Ratio (SINR). It is often said [19], that the SNR is enhanced by the processing gain W/R, where W is the chip rate and R is the data rate. WCDMA uses a chip rate equal to 3.84 Mcps which leads to a carrier bandwidth of approximately 5 MHz. The inherently wide carrier bandwidth of WCDMA supports high user data rates and also has certain performance benefits, such as increased multi-path diversity. WCDMA uses variable spreading factor and multi-code connections. In addition to the basic radio access capabilities, UMTS architecture provides several other advantages, including higher bandwidth over the radio interface and better handoff mechanisms, such as soft handover for circuit-switched bearer channels. Soft handover refers to the ability to maintain an ongoing connection between the mobile terminal and the network through more than one base station; this capability is particularly important in 3G systems.
UMTS system underwent a considerable evolution by the introduction of the HSDPA (High Speed Downlink Packet Access) in 3GPP release'5 [22]. HSDPA specification was released in 2002 and it was considered the most significant radio related update since release'99. HSDPA is based on a distributed architecture where the processing is closer to the air interface at the Node B for low delay link adaptation.
To achieve a high-speed downlink transmission, HSDPA implements a scheduling for the downlink packet data operation, higher order modulation, adaptive modulation and coding,
Hybrid Automatic Repeat Request (HARQ) and link adaptation according to the momentary channel conditions.
The HSDPA concept offers over 100% higher peak user bit rates than Release’99 in practical deployments. It is comparable to Digital Subscriber Line (DSL) modem bit rates in wireline communication. It extends the UMTS bit rates up to 10 Mbps. This higher bit rates are obtained with higher order modulation, 16-QAM, and with adaptive coding and modulation schemes. HSDPA is able to support not only non real time UMTS QoS classes but also real time UMTS QoS classes with guaranteed bit rates.
A new improvement in the UMTS radio interface was specified in release'6 with the introduction of HSUPA (High Speed Uplink Packed Access) [23]. HSUPA uses an uplink enhanced dedicated channel (E-DCH) with dynamic link adaptation methods as already enabled in HSDPA, i.e. shorter transmission time intervals, thereby enabling faster link adaptation, and also a hybrid HARQ with incremental redundancy, thereby making retransmissions more effective. HSUPA offers peak data rates up to 5.5 Mbps.
The last evolution of UMTS networks is observed in release'7 with the introduction of a new system which has been at the 70% of its specifications while writing this dissertation. This evolution is referred to the 3GPP Long Term Evolution (LTE) systems [97] [98]. LTE system, called sometimes "super 3G" or "4G", is expected to offer a spectral efficiency between 2 to 3 times bigger than 3GPP release'6. It will provide up to 100Mbit/s for 20 MHz of spectral bandwidth. Both the radio and the core network parts of the LTE technology are impacted: The system architecture is more decentralized; The RNC present in the 3G systems is removed; and RRM functionalities are moved to an "upgraded" base station called Evolved Node B (eNB). Further description of LTE system is given in chapter 5.