Will cdmaOne Be The Third Choice?
By Stanley Chia
For many industry observers, its clear that CDMA will offer the best platform for third generation systems. But there is more than one CDMA path available. For many established operators already using cdmaOne technology, the ideal third generation approach would involve an evolution from the existing standard. Can cdmaOne deliver?
CDMA has long been recognized by cellular radio development engineers as a viable approach for delivering feature-rich mobile services. For instance, back in the mid 1980s when GSM first selected its radio interface, two out of the five candidate system proposals, CD900 and MATS-D, were based on some form of code division spectrum spreading. However, due to factors linked with technology maturity and implementation cost, the CDMA elements of these proposals were not incorporated in the final GSM Specification.
While the Europeans decided to proceed down the TDMA path, the momentum of CDMA continued to build rapidly in North America in the early 1990s with Qualcomm leading the way to create a new cellular system based on the IS-95 standard. As cdmaOne, this standard is rapidly gaining popularity worldwide. Recognizing the importance of CDMA, the European Commission and various industries soon responded by launching a project to develop a CDMA based test-bed for third generation mobile systems under the European-wide RACE II (Research into Advanced Communi-cations for Europe - Phase 2) Program. This has established Europes interest in adopting some form of CDMA for evolving to a third generation mobile system. Likewise, in Japan, the conclusion was reached by NTT DoCoMo that its third generation mobile system should also be CDMA-based. Thus, CDMA has firmly emerged as a global focus of the third generation mobile system standardization efforts.
BENEFITS TO CUSTOMERS
Third generation mobile systems are expected to provide high
data throughput and coverage as shown schematically in Fig
1. In the nearer term, it is envisaged that a data rate of
no more than 2 Mbit/s will be required to support the above
services and, in practice, an average of 144 kbit/s should
satisfy most of the perceived needs. Support of 2 Mbit/s data
rates refers more to the peak data rate. Clearly, the data
rate which can be supported in different environments at mobile
speeds will be different. Specifically, for vehicular speeds,
the peak data rate will be up to 144 kbit/s. For pedestrian
speeds, the corresponding data rate will be 384 kbit/s. For
an indoor office environment, however, the transmission can
be up to 2 Mbit/s.
In summary, third generation mobile systems have two broad service requirements: multimedia and high-speed data for a high end service offering and low-cost voice for a general consumer application at the low end. Fixed wireless can, indeed, be viewed as a service category for the developing world. However, high service quality and low cost are common to both developed and developing countries.
There is some uncertainty about the penetration rate. For example, the UMTS Forum forecast that more than 45% of mobile users will have high-speed data uptake by 2010 whereas other, low end forecasts have shown that this could be less than 10%. On the other hand, the voice market is also expected to grow and could be perceived as an opportunity for third generation technology provided it can offer sufficient cost and performance advantages.
The highest growth region will be in the Asia Pacific rim with some 40% of the worlds future subscribers. The second highest is North America and Latin America, which collectively constitute an additional 33%. Europe is expected to have 25% of the subscribers, with Africa and Middle East at 1% to 2%.
Japan. With limited overseas markets for their second generation PDC (Personal Digital Communications) system so far, Japans Ministry of Post and Telecommunications (MPT) wanted the new third generation standard to be more widely adopted across the world. The MPTs timetable for third generation system introduction suggests a call for operators in 1999. In the early part of the year, the first issue of the standard will be made available and license awards will begin during the latter part of the year. Commercial deployment is expected to be round 2001. In recent months, NTT DoCoMo has invited infrastructure and handset vendors to participate in its wideband CDMA development. An experimental system is expected to be operational by early 1998.
As shown in Table 1, NTT DoCoMo proposes to use different modulation and coding from cdmaOne (IS-95) with different Layer 2 and 3 signaling, different handoff algorithm and asynchronous base stations. NTT DoCoMo has not established a requirement for backward compatibility with any existing second generation system or analog standard. In addition to the adoption of INAP (Intelligent Network Application Part, Signaling System Number 7) protocols and ISDN (Integrated Services Digital Network) call models, new protocols will be established for services and mobility. Interoperability with existing second generation systems is proposed to be provided through gateway functions.
Europe In Europe, five contending concept groups for access
technology have been proposed as candidates for standardisation.
EVOLUTION FROM cdmaOne
Proposed by Motorola under the banner of "Wideband 95", the main objective of this scheme is to improve the forward link capacity and the reverse link range. The technology can be deployed in any band and is, therefore, fully backward compatible with existing deployments. It is designed for spreading over a bandwidth of 1.25 MHz, 5 MHz, and10 MHz and may also be 20 MHz to support 144 kbit/s, 384 kbit/s, 512 kbit/s and 2.0 Mbit/s peak data rates, respectively. In theory, the wideband spreading technique can continue to be used to increase the spreading bandwidth with increasing receiver complexity. The limit will be the point at which there are too many multipaths of which some are too low in amplitude to be fully tracked by a rake receiver.
A number of potential technical challenges could result from
this approach to which solutions have to be provided. For
the reverse link, the linearity of the power amplifier may
be an issue. By contrast, for the forward link, the number
of rake fingers has yet to be optimized. It has been proposed
that a pilot-assisted coherent reverse link be used to improve
range and a fast forward link power control algorithm be incorporated
to improve capacity.
An alternative which is not explored in the standards today is to apply some form of time division multiplexing to partition the spectrum utilisation of the wideband and narrowband channels to ensure orthogonality (see Figure 5). Instead of using a fixed assignment for all channels, the spectrum can be used on a time-shared or statistical basis. Depending on the demand and priority, the wideband transmission is suppressed during the narrowband transmission and vice-versa. This could permit simpler coexistence and ease the migration path.
A multicarrier approach. By contrast, the multicarrier approach
is to directly concatenate three 1.2288 Mbit/s channels together
to provide a total bit rate of 3.6864 Mbit/s for the forward
link. This is shown in Figure 4. With a three-carrier configuration,
a data rate of up to 153 kbit/s using QPSK modulation can
be achieved. This app-roach is designed to reuse the multiple
carrier architecture of the existing base station infrastructure.
The efficiency is expected to be 5% to 10% below a wideband
spreading scheme. For the reverse link, a wideband carrier
spanning a 5 MHz spectrum is proposed. This is designed to
ensure that handsets are not required to transmit multiple
carriers and to take into account linearity issues.
It should be noted that handsets for the multicarrier approach would be more complex than those used in the wideband approach as the former have to demodulate three carriers simultaneously. In addition, synchronised power control could also be an issue.
While the worldwide third generation mobile system standardization effort is addressing specific greenfield frequency bands, it is important to realize that there is no fundamental restriction to the evolution from any frequency bands. For example, the migration from the existing cdmaOne to W-95 can be achieved with a similar procedure to that involved in migration from AMPS to cdmaOne. Of the two key approaches discussed above, the direct sequence approach will be better for greenfield implementation, while the multicarrier approach is expected to be better for migration with backward compatibility. The exact magnitude of this difference will be studied extensively before any final decisions are made. However, if an operator has a 5 MHz allocation and if at least 1 x 1.25 MHz is already in use and is busy, the implementation of either approach could still be challenging.
The trade-off between the two is in system efficiency and handset complexity. However, at this point, not enough performance differences between the two approaches can be noted. For the direct sequence approach, it is better to have the 1.25 MHz channel residing outside the wideband channel as the transmitter radio front end for the two channels will be different and will perform better if the two channels are on separate carriers. If an operator is spectrum constrained, derating the voice channel capacity and overlaying the 3.6 MHz data channel on top could be considered. In this case, more base sites for the cdmaOne users may be required. If the band has to be cleared prior to wideband services implementation, the technology has to be very cheap to encourage their introduction.
Based on the economics of the two approaches, operators with
a larger amount of spectrum allocation would benefit from
the wideband approach, while cellular band operators would
benefit from the multi-carrier solution. A summary of the
comparison is shown in Table 3. The above discussion would
support arguments on where new spectrum is to be made available
for third generation mobile systems. An allocation to existing
operators would be beneficial to foster the delivery of these
new services to customers.
This platform will lead to a departure from todays
direct association of the switch cost with performance and
provide a more competitive platform for operator differentiation
through the use of in-house developed or third-party service
products. With this platform, operators can capitalise on
common and commercialized technologies to reduce service development
costs and enable a faster time to market.
Other key issues which are receiving close attention from researchers and manufacturers are radio resource management techniques, the round trip delay of codecs, fast wide dynamic range A/D and D/A converters, radio frequency integrated circuits, spectrally efficient linear power amplifiers with up to 75 dB or 80 dB dynamic range, and spatial processing using smart antennas.
In general, the amount of radio frequency processing continues to reduce at the expense of increased digital signal processing. With the maturity of high speed analog-to-digital conversion with 14-16 bit resolution, 70 dB dynamic range and a sampling rate of 30 MHz being available at a reasonably low cost, the development of wideband software radio systems becomes more feasible.
The crossover point between discrete channel approach and block down-conversion approach is getting closer. In the past, the crossover point was at 20-30 TDMA channels; this is now coming down to four to five channels.
While VLSI baseband circuitry can be available at nearly zero cost, leading manufacturers have also pointed out that the radio frequency (RF) component cost has been a challenge in the past and will continue to be so in the future. By contrast, although linear power amplifiers have been expensive and challenging to implement up to now, breakthrough techniques are already emerging which could lead to significantly more cost effective implementation.
Terminals. Third generation system handsets will typically have dual bandwidth, dual frequency with dual resonant antenna and integrated RF front end.
While software downloadable vocoders are feasible, the use of customized as opposed to multi-mode modems is seen by the industry to be the option which makes more economic sense.
As for the handset itself, in the future the handset will
simply consist of ASICs, a keyboard, and an antenna connector
in an enclosure. Terminal equipment can, however, have many
options ranging from being a virtual full client to split
client, very thin client, partial client and defeatured client.
Battery technology and power management are expected to be further key elements. The current talk time is for supporting a data rate of 10 kbit/s for a few hours. With a data rate of the order of 100 kbit/s, the same battery will not be able to support long data sessions. Thus, thin client software and non-memory intensive equipment are also key parts of the technology development.
In conclusion then, efforts under way in worldwide third generation system development suggest that CDMA has many advantages that make it attractive in meeting the requirements for high quality broadband service features and characteristics. In particular, we have seen that evolution from cdmaOne is a feasible approach, with technical proposals emerging from leading infrastructure vendors. Differences in frequency bands and details of the vendor proposals suggest that global intersystem roaming may still be as challenging with third generation as it is with second generation mobile systems.
With the parallel effort in Europe and Japan on third generation system standardisation, operators and suppliers should work together toward the ultimate goal of ensuring that customers can benefit from better quality and higher service flexibility of next generation systems whether they are at home or travelling.