Flying the standard

Flying the standardEuropean firms are gearing up for the implementation of UMTS the third generation mobile phone standard by the year 2002. Roy Rubenstein looks at what is involved
Europe’s proposal for the third generation mobile phone standard, UMTS, promises many things to many people.
For a start it will boost the data rates supported by mobile handsets, from 384kbit/s up to 2Mbit/s.
And even though UMTS services will begin in Europe after 2002, equipment makers such as Nokia and Ericsson are already investing huge amounts in developing the technology.
In turn DSP vendors are starting to unveil the architectures that will power such terminals. One example is the 1,800Mips StarCore SC140, the jointly developed DSPcore from Lucent Technologies and Motorola. CDMA at a glance
Several standards have been proposed for the International Telecommunications Union’ third generation
IMT-2000 standard, of which UMTS is Europe’s offering. All make use of CDMA. Why?
“With access modes you achieve a certain efficiency [amount of information across a given spectrum]. The belief is that wideband CDMA is the most efficient of the schemes,” explained Gregory Luxford of Symbionics.
CDMA, sometimes referred to as spread-spectrum multiple access, uses a spreading code to disperse the dataover the wider bandwidth. In the FDD mode, the spreading code varies from 4 to 256-bits long.
The spreading is achieved by multiplying the data with a unique chip code, the result being the chip rate. For UMTS the chip rate is 4.096Mchip/s.
The spreading codes used are also orthogonal. This ensures that there is minimal interference between the signals at the detection stage, which uses a correlator to extract the signal of interest.
UMTSuses 10ms-long slots. “This is the basic form of currency in CDMA. When you first establish a call, synchronisation is achieved on a slot basis,” said Luxford. Sixteen slots make up a frame.
Having synchronised on a slot, the next stage is to determine where the slot resides within the frame. Once the start of the frame is determined, the particular code group is identified to descramble the data.  
 
So what are the key technical characteristics of UMTS, a standard which promises to do for mobile data what GSM has already done to untethered voice communications?
The first thing to stress is that UMTS, in drawing upon two access schemes, has a dual personality. It uses both wideband code division multiple access (W-CDMA) and the hybrid time division multiple access/ CDMA (TD-CDMA).
Time division multiple access – the scheme GSM adopts – restricts a user to a particular time slot on a shared frequency band. CDMA, in contrast, sends transmissions at the same time and across the same frequency band. For this to work, unique chip codes are used. These codes are orthogonal to enable the unscrambling of individual transmissions without undue interference from other users sharing the same band.
UMTS uses a wideband flavour of CDMA, having a channel size of at least 5MHz. This contrasts to narrowband CDMA schemes using a 1.25MHz band.
According to Gregory Luxford, a third generation baseband principal engineer at Cadence’ company, Symbionics, 1.25MHz was selected for no better reason than that was all the bandwidth available at the time. In addition to squeezing more information across the wider bandwidth, another advantage of W-CDMAis that it is less susceptible to frequency selective fades, said Luxford.
UMTS’ dual character results in two features or modes of operation (as shown by the table).
The first uses W-CDMAfor a paired band of frequencies – referred to as frequency division duplex (FDD). Here separate parts of the spectrum are used for the uplink and the downlink
The second mode uses the hybrid TD-CDMA in an unpaired band – or time division duplex (TDD). In TDD one frequency is used for both transmit and receive although the data rates are typically asymmetric.
“FDD mode is likely to be used in a macro-cellular environment, where UMTS is one cell island in a sea of GSM,” said Nick Hallam-Baker, a principal engineer also at Cambridge-based Symbionics.
With this wide area coverage, the data rate is limited to 384kbit/s. This is due to the band being shared by a large number of users, with not all the spectrum being allocated to particular users.
TDD mode, in contrast, will be deployed in pico-cellular environments like the insides of buildings – ‘localised hot spots’ as Luxford calls them. “TDD can be seen as a replacement for the digital cordless standard, DECT,” said Luxford.
The data rate achieved in this local area environment is up to 2Mbit/s. “You are not sharing the 5MHz band with as many people,” said Hallam-Baker.
Both modes of operation offer high service flexibility. This refers to the ease in which the services provided can be changed from one 10ms frame to another (see CDMA box). “You don’t have this flexibility in GSM to rapidly change the data rates,” said Luxford.
UMTS has several noteworthy technical features. One is the support for both packet and circuit switched services. With packet switching the user only pays for the time it takes to download data rather than the overall connection time as is currently the case with a circuit switched approach.
Another feature is soft (and softer) handover. As a user moves between GSMcells one connection is dropped as a new cell connection is picked up. This is a ‘hard’ handover. Moving from one UMTS cell to another, a second connection is made and only when it is in place is the first dropped. “This prevents drop outs and the losing of calls,” said Luxford.
Softer handover refines the concept by allowing the user to talk to two or three basestations simultaneously. This promises an improved quality of service especially if a user is physically far from the current cell’s basestation. “This combining of data brings what is called ‘macro diversity’,” said Luxford.
Joint detection refers to the basestation’s ability to see all the calls occupying the frequency spectrum and subtract them from the signal of interest as a way of reducing interference noise. The issue here is that although the basestation can send out all the scrambled signals perfectly aligned in time, the received signals from the handsets are not.
Finally, both FDD and TDD modes will support the hand-over to each other and to GSM. Since UMTS is likely to start off as ‘an island in a sea of GSM’, the first handsets will be dual mode: supporting FDD/GSM and maybe TDD/GSM.
Two error control schemes are proposed for UMTS. Convolution coding, the less powerful of the two, is described by Hallam-Baker as an adequate scheme for encoding speech. The second is turbo coding to protect the data transmissions.
“Turbo coding is claimed to offer the best level of error correction and error protection of any coding system at the moment,” said Hallam-Baker.
There is nothing trivial when it comes to the UMTS specification. Feature or Parameter FDD mode TDD mode Radio access technology Wideband CDMA (W-CDMA) Hybrid TDMA/ CDMA (TD-CDMA) Typical deployment scenario Wide area coverage Local area coverage Maximum data rate
(in typical deployment scenario 384kbit/s 2Mbit/s Service flexibility High High Technical features & service Support efficient packet access
Soft and softer handover
Adaptive antennas, advanced receiver
structures and transmitter diversity
Handover to GSM and TDD Aggregate asymmetric services
Joint detection
Handover to GSM and FDD CDMA chip rates 4.096Mchip/s
(expandable to 8.192Mchip/s
and 16.384Mchip/s) 4.096 Mchip/s Duplex operation Frequency Division Duplex (FDD)
Variable duplex spacing 130 – 190MHz Time Division Duplex (TDD) Frame duration 10ms 10ms Slots per frame 16 16 Code spreading Orthogonal variable spreading factor codes (4, 8, 16, 32, 64, 128, 256) Up to 10 spreading codes per slot. Variable spreading factor (2, 4, 8 & 16) Error control method 1/2 and 1/3 rate convolutional coding
Turbo coding 1/2 and 1/3 rate convolutional coding
Turbo coding


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