3G Cellular Systems

There is some debate over which cellular systems are so-called 3G systems, especially with regard to EDGE and some cdma2000 systems. Because GPRS and EDGE are GSM based, it is fairly intuitive to put them into the 2.5G category. In the CDMA camp, one cdma2000 system, called cdma2000 1x RTT, has been arguably considered a 3G system. Generally, all cdma2000 and UMTS/WCDMA systems may be considered 3G systems.

The International Telecommunication Union (ITU) made a request for proposal (RFP) in 1997 for cellular technologies for the International Mobile Telecommunication (IMT)-2000 program. A proposal for a UMTS was submitted by the European Telecommunication Standards Institute (ETSI) to ITU. Its radio interface is universal terrestrial radio access (UTRA). Other 3G radio access technologies are listed as

• IMT-2000 TDMA single carrier, originally promoted by the Universal Wireless Communications Consortium (UWCC); EDGE is one of the IMT-2000 TDMA SC technologies.

• IMT-2000 FDMA/TDMA, also known as DECT, the enhanced version of the cordless phone standard.

UMTS/WCDMA Versus cdma2000
A UMTS system works in two modes: Its frequency-division duplex (FDD) mode is the wellknown wideband CDMA (WCDMA), whereas its time-division duplex (TDD) mode seems to remain unnoticed by the public. The cdma2000 is the evolution of cdmaOne, the current CDMA system in the United States. In fact, strictly speaking, WCDMA only refers to the radio interface aspect of the entire UMTS system. The same radio interface technology is used by NTT DoCoMo and J-Phone (a subsidiary of Vodafone) as well. As an FDD system, WCDMA does not require time synchronization among base stations. It allows a bit rate up to 384 Kbps, compared with the maximum rate of 2 Mbps in TDD UMTS systems. In particular, China has proposed a TDD UMTS system, called TD-SCDMA, and is vigorously promoting this technology among Chinese telecommunications device manufacturers and wireless operators.

The cdma2000 is a general term representing technical specifi cations such as cdma2000 1x RTT, cdma200 1x EV-DO, cdma2000 1x EV-DV, and cdma2000 3x RTT. RTT stands for radio transmission technology, EV-DO for evolution-data optimized, and EV-DV for evolution-data and voice. 1x RTT can provide a peak rate of 153.6 Kbps, while 3x RTT may theoretically offer a peak rate of 3.09 Mbps. The fi rst commercial system of cdma2000 1x EVDO was launched in South Korea in January 2002. The cdma2000 is backward compatible with existing IS95/cdmaOne systems, whereas WCDMA requires an overhaul of existing base stations. There is no synchronization in WCDMA systems, thus sophisticated protocol designs and handoff mechanisms are not required. On the other hand, cdma2000 requires base-station synchronization. In some sense, WCDMA can be seen as an opportunity for operators to challenge Qualcomm’s CDMA technology monopoly. Readers interested in the evolution of mobile networks are encouraged to refer to Vriendt et al.

A UMTS system comprises three components and two interfaces. The components are the user environment (UE), the UMTS terrestrial radio access network (UTRAN), and the core network (CN). The interface between UE and UTRAN is referred to as Uu. The interface between UTRAN and a Node B is Iub. UMTS introduces Node Bs as base stations (BTSs in GSM) and radio network controllers (RNCs) as BSCs in GSM. Similar to GSM and GPRS, MSCs and SGSNs control RNCs through the Iu interface. In particular, an MSC connects to an RNC through an Iu – CS (circuit-switching) interface, whereas an SGSN connects to an RNC through an Iu – PS (packet-switching) interface. In UMTS, GMSCs and GGSNs connect to PSTN and PDNs. Other components such as HLR and VLR are the same as in GSM but with enhanced functionality for UMTS.

UMTS uses a pair of 5-MHz channels, one in the 1900-MHz range for uplink and one in the 2100-MHz range for downlink. In contrast, cdma2000 uses one or more arbitrary 1.25-MHz channels for each direction of transmission. UMTS is expected to deliver a user data rate of 1920 Kbps, although in reality 384 Kbps is probably what the system can really offer. A future version of UMTS/WCDMA, high-speed downlink packet access (HSDPA), will offer data speeds up to 8 – 10 Mbps and 20 Mbps MIMO antenna systems. The data modulation scheme is QPSK for uplink and BPSK for downlink. The chip rate is 3 M chips per second.

As a spread spectrum radio interface, WCDMA uses soft handoff just as cdmaOne does for the same reason: It is quite diffi cult to control power beyond the hysteresis if hard handoff is employed because in CDMA systems forcing a mobile station to operate over some hysteresis level will cause large interference.

The first UMTS network went into operation in the United Kingdom in 2003. AT & T
Wireless in the United States deployed UMTS in selected cities in late 2004. Japan’s largest telecommunication service provider NTT DoCoMo launched the fi rst WCDMA-based 3G network, dubbed Freedom of Mobile Multimedia Access (FOMA), in 2001.

cdma2000 is another standard under the ITU-2000 program. It comes in two stages: 1x and 3x. Using the existing cdmaOne infrastructure, cdma2000 1x can supply a maximum user data rate of 207 Kbps and a typical data rate of 144 Kbps in general. It doubles the voice capacity of cdmaOne systems and offers six times the capacity of GSM or TDMA systems. cdma2000 3x further improves the user data rate to 2 Mbps.

In cdma2000, three major components exist in the overall network architecture: mobile station, radio access network, and core network. The interface between a mobile station and radio access network is called Um, and the interface between radio access network and core network is called A. In addition, the core network can be further decomposed into two portions: One portion, the packet core network (PCN), connects to external IP networks via a Pi interface, whereas the other connects to PSTN via an Ai interface. Similar to a UMTS network; the core network of cdma2000 also has MSCs, HLRs, and VLRs. The principal difference between the core network of cdma2000 and those of other cellular systems is the PCN that provides IP network access to mobile stations. A component in PCN, the packet data service node (PDSN), performs roughly the same task as an SGSN in UMTS or GPRS; however, in cdma2000, two IP access methods are provided: simple IP access and mobile IP access. Simple IP access is the traditional way to obtain and retain an IP address within a geographically located subnet. When the mobile station moves to another subnet, it has to redo the DHCP procedure and obtain a new IP address. This is the case when a mobile worker uses a laptop computer to connect to an enterprise network across several buildings. Mobile IP access enables a mobile station to use the same IP address across different regions. In this case, a home agent of the mobile station will assume the responsibility of maintaining the same IP address for the mobile station. A foreign agent that is part of the PDSN is used to assign a temporary address to the mobile station that just moved in, and tunnels packets from the home agent to the mobile station. Note that GPRS has only a single IP access method: the simple IP access. It has to be emphasized that cdma2000 has better IP support. This is indeed a tremendous advantage of cdma2000 over UMTS, as in the long run the core cellular network will be interoperable with other wired or wireless networks with IP as the underlying network protocol.

Another major task of the PCN is authentication, authorization, and accounting (AAA). Three parties are involved: home AAA (HAAA), broker AAA (BAAA), and visited AAA (VAAA). HAAA stores a subscriber’s profi le information. Once requested by a VAAA, it will authenticate and authorize a subscriber and send the response back to the VAAA. For accounting, VAAA is able to receive accounting information from HAAA and provides the subscriber’s profi le to the PDSN. BAAA is used as an intermediate server when VAAA and HAAA are not directly associated with each other.

Source of Information :  Elsevier Wireless Networking Complete 2010


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