Introduction [Under UMTS]
UMTS stands for Universal Mobile Telecommunication Service.
Even before 2G wireless systems came into existence and became popular, research programmes like RACE I & II, ACTS/FRAMES were started to look for 3G technologies in Europe. Standard and research organisations had estimated well in advance that mobile and wireless communications will be required to provide various types of bearer services in addition to voice call and SMS. These new bearer services will require higher, variable or constant, on-demand bandwidth from radio access. The (maximum) data rates which we are talking about is in the range of hundreds of kpbs to the level of Mbps. In addition, radio access will have to be highly efficient to support growing number of users. 3G systems were supposed meet these requirements.
Needless to say that support of high speed data connections (broadband technologies) in wireline market will lead to introduction of better and better communication applications. And this will push wireless market to come out with technologies that will make better and better use of air interface.
Let us see how UMTS achieves its goals.
1) First step would be to find appropriate radio access technology.
2) Second step would be to make both access and core network support required connectivity to external networks.
3) Another goal or step not directly visible here is to design interfaces between access and core networks and within both of the networks scalable, open and generic. Generic meaning it should be as much independent of radio access technology being used as possible. Today we use technology X for radio access; with the progress in engineering and research field, if we change access technology to Y, impact to nodes/interfaces away from radio access should be minimal or limited to like software upgrade. This works other way round.
First step - Air interface
After decade's research (done between 1988 and 1998), ETSI selected WCDMA (for FDD operation) as radio access technology for UMTS. The other alternatives were based on TDMA, CDMA, OFDMA, and combinations of it. The main reasons for selection of WCDMA is inherent characteristics of CDMA like diversity techniques, support of various types of traffic simultaneously etc. Other regional standard organisations doing research on 3G technologies also preferred WCDMA.
WCDMA is Direct Sequence CDMA (refer CDMA article for more on CDMA) technique. It uses chip rate of 3.84 Mcps, giving bandwidth of around 5 Mhz and so is the name Wideband CDMA; IS-95(cdmaOne) uses chiprate of 1.2288 Mcps and has bandwidth of 1.25 Mhz. Target deployment frequencies are the range of 1.9 GHz to 2.1 GHz.
Second and third steps - Network interfaces
Like GSM/GPRS, UMTS consisted of radio network having Node B (like BTS) and RNC (like BSC) connected to MSC/VLR and SGSN.
ATM network was chosen as backbone network for both transmission of both signaling and user data traffic in radio access network and between radio access and core network. ATM is cell switched technology that can support various types of services and high speed (refer ATM article).
The network interfaces consisted of independent network layer and transport layer. For transport layer, both signaling data and user data coming from network layer is user data, only differing in the characteristics of data. Transport layer will have its own signaling and user planes independent of network layer. This way impact due to change in radio access technology is minimised and at the same time, it gave freedom to change transport mechanism (ATM in this case) with no impact on radio bearer services (of course transport should provide required type of services).
Another important consideration that UMTS took care is: interoperation with GSM/GPRS/EDGE systems (mainly service continuity during handover to and from UMTS). This will happen most often during migration phase from GGE to UMTS or GGE being retained from mobile users point of view.
In next article, we will look at UMTS network architecture.
References: UMTS by Sanchez and Thioune, WCDMA for UMTS by Holma and Toskala, and 3gpp website.
| Copyright © Samir Amberkar 2010 | § |
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