More on chip rates:
CDMA Development Group White Paper:
Third Generation Systems
Objective | Background | Evolution of cdmaOne and Development of cdma2000
Convergence and Harmonization | Conclusion
Objective
The objective of this paper is to present the CDMA Development Group's (CDG) view on key topics with respect to third generation (3G) and address some of the more technical aspects of the cdma2000 3G proposal. The paper also documents the ongoing activities within the CDG and standards bodies that are taking place toward cdmaOne ™ evolution and 3G standards.
Background
cdmaOne has clearly demonstrated its superiority in the second generation wireless marketplace. In September 1998, only three years after the first commercial deployment, there were 16 million subscribers on cdmaOne systems worldwide. Over 35 countries have either commercial or trial activity ongoing. The CDG has over 100 members of whom 40% are companies based outside of North America, testimony to the truly international reach of CDMA.
The CDG established the Advanced Systems Initiative to provide a growth path for cdmaOne to next generation systems. Primary goals of the initiative include development of a worldwide standard that meets IMT-2000 requirements and other services identified as critical to operator members, and graceful evolution to next generation cdmaOne systems. The Advanced Systems Initiative is a means for CDG members to define the requirements and priorities for cdmaOne and to collaborate with regional and international standards organizations to meet industry objectives. CDG members have been involved with IMT-2000 since its inception.
In addition to the work of the Advanced Systems Initiative, the CDG leadership is actively engaged in industry-wide efforts on 3G. The CDG is ensuring the rapid evolution of cdmaOne and the development of cdma2000 to meet the needs of operators worldwide, enabling the availability of 3G products and services beginning in 1999.
Evolution of cdmaOne and Development of cdma2000
The path to 3G
A great deal of attention has been focused on 3G harmonization and convergence. While the CDG believes in the ITU's vision of a global standard, we are quickly building on the technical foundation of cdmaOne to deliver many advanced services in the near future in a way that allows operators the flexibility to offer these services as the market demands. The CDG efforts are focused around an evolution strategy so that capabilities can be introduced in phases during the next few years, based on and leading to the complete capabilities of cdma2000. The bottom line: The CDG is working aggressively to enable fast-track development of the cdma2000 standard.
cdmaOne is the only technology with a clear evolution to 3G because it builds on the design and framework of today's cdmaOne system. Looking at 3G from an operator's perspective, preservation of investments made in infrastructure and spectrum are significant issues in defining requirements for technology migration. Services designated as "3G"will be available with cdmaOne in existing as well as new spectrum bands. This point is important in considering the position of established operators who may not choose, or be able, to get new spectrum. This point is also vitally important in developing regions considering the allocation of PCS spectrum for 2G. With cdmaOne, operators and subscribers in these regions can reap the benefits of today's advanced digital technology while assured their investments are protected. Evolution from technologies such as GSM to WCDMA, however, will require significant change out of equipment and costly upgrades.
Capabilities of cdmaOne evolution have already been defined in standards. IS-95B provides ISDN rates up to 64 kbps. The next phase of cdmaOne is a standard knows as 1XRTT and enables 144 kbps packet data in a mobile environment. Other features available when the standard is published in 1Q99 are a two-fold increase in both standby time and voice capacity. All of these capabilities will be available in an existing cdmaOne 1.25 MHz channel.
The next phase of cdmaOne evolution will incorporate the capabilities of 1XRTT, support all channel sizes (5 MHz, 10 MHz, etc.), provide circuit and packet data rates up to 2 Mbps, incorporate advanced multimedia capabilities, and include a framework for advanced 3G voice services and vocoders, including voice over packet and circuit data. This phase of the standard will be complete by 4Q99.
In addition to the capabilities of the cdmaOne air interface, evolution of the ANSI-41 core network will enable subscribers to continue to benefit from advanced services offered by the cdmaOne platform. Investment in costly infrastructure and network upgrades are not necessary.
The myths and the facts about chip rate
The debate about cdma2000 and WCDMA convergence has been based on the fact that these CDMA-based proposals have certain parameter definitions that present an opportunity for compromise. The most discussed and debated parameter is the system chip rate. WCDMA uses a chip rate value of 4.096 Mbps. cdma200 uses 3.6864 Mbps. WCDMA proponents liken the higher rate to more horse power and claim the lower cdma2000 rate degrades performance. This falsity requires clarification.
Deployment scenarios in various bands
First, WCDMA proponents claim that the WCDMA chip rate provides as much as a 10% capacity improvement over that of cdma2000. This should be examined under a realistic scenario of how the technology will be deployed, and must include all factors affecting system performance. While some operators will deploy 3G in as little as 5 MHz of spectrum many will use allocations of 10, 15, or 20 MHz. This is important since it is the usable spectrum, in conjunction with chip rate, which affects capacity. Figures 1-3 illustrate the deployment scenarios for cdma2000 and WCDMA in 10, 15, and 20 MHz bands respectively. Even with the required guard bands as verified in today's operational cdmaOne systems, greater overall capacity is achieved with a mixture of cdma2000 1X and 3X channels as compared with using WCDMA channels. With that configuration it can be shown that up to 13% capacity improvement is achievable in a 20 MHz deployment ( 1 ).
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Figure 1 Deployment scenario for cdma2000 and WCDMA in a 2x 10 MHz operation
Figure 2. Deployment scenario for cdma2000 and WCDMA in a 2x 15 MHz operation
Figure 3. Deployment scenario for cdma2000 and WCDMA in a 2x 20 MHz operation
Examining chip rate in context with other characteristics
Second, chip rate alone does not determine overall system capacity. To build on the automobile analogy referenced earlier, assuming chip rate is the only factor affecting capacity is like assuming tire pressure is the only thing affecting gas mileage. One of the main parameters in determining the capacity of a CDMA system is the ratio of energy per information bit to noise power spectrum density (Eb/No) required to achieve certain QoS (Quality of Service) requirements such as frame or bit error rate. The required Eb/No value depends on frame structure, coding and modulation characteristics, diversity techniques and channel model. The small difference in chip rate between 3.6864 Mcps and 4.096 Mcps has negligible impact on the Eb/No requirement. Instead, other system designs such as channel structure (including pilot structure), power control mechanisms, diversity techniques, handoff efficiency, and base station synchronization have a much greater impact on system capacity. The impact of system design on capacity is illustrated in Table 1, where the normalized spectrum efficiency in Erlangs/MHz/cell for voice services in a vehicular environment is shown, taken from the cdma2000 and the UTRA (WCDMA) RTT ( 2 ). Table 1 also contains the simulation results from the RTT evaluation report submitted by the Chinese evaluation group. We can see that a larger chip rate does NOT translate into higher spectrum efficiency.
Table 1. Spectrum efficiency for voice in a vehicular environment: cdma2000 & WCDMA
System
Chip Rate (Mcps) Spectrum Efficiency for Forward Link/Reverse Link *
(Erlangs/MHz/ Omni cell)
Self evaluation
Chinese evaluation
cdma2000
3.6864
36.7/29
26.4/27.2
UTRA WCDMA
4.096
17.8/22.4
18.4/22
*Higher Erlangs/MHz/Omni Cell equates to greater efficiency
Consideration of power emissions
Finally, what proponents of the WCDMA chip rate often overlook are the negative effects on spectrum use and power emissions by using the higher value chip rate. The CDMA air interface signal of IMT-2000 needs to fit into a 5 MHz spectrum to comply with different frequency plans around the world. For example, if deployed in a 5 MHz spectrum such as in the D, E, F North American PCS blocks, the WCDMA system as specified currently cannot meet the FCC out-of-band emission requirements. All major wireless technologies use guard bands to separate their signal spectra from those of services in adjacent bands. It is unreasonable to assume that WCDMA can operate without such guard band protection. For instance, the guard band used to separate IS-95 CDMA from TDMA/AMPS is 270 KHz on each side; the guard band used to separate DECT from adjacent service bands is 2.396 MHz to the lower band, and 1.052 MHz to the upper band. This issue is particularly significant for the PDC systems in Japan, as well as anywhere there is another service operating in the band adjacent to the IMT-2000 band.
WCDMA advocates propose using more complex filters to address this. While in theory such an approach can be conceived, the required filter is hard to realize within a 5 MHz bandwidth ( 3 ). Essentially, the purported 10% capacity gain is not realizable in practical deployments that in many markets need to consider adjacent channel interference or FCC power emission requirements – not a realistic solution for operators.
In summary, chip rate is not a simple issue with a direct cause and effect relationship. More is not necessarily better. cdma2000 enables 3G services without the deployment risks and cost of WCDMA.
Convergence and Harmonization
The CDG has been actively trying to achieve the ITU's goal of a global standard for IMT-2000. To that extent, the CDG and its members have been active on cdma2000/WCDMA harmonization in regional standards bodies (ARIB, ETSI, TIA, TTA, T1P1), discussions with worldwide operators, and meetings with government entities. Convergence can enable a number of benefits for consumers, operators, and manufacturers. ARIB (Japan) recognized this early on and has been instrumental in reducing the number of differences between cdma2000 and WCDMA to a handful. However, some WCDMA proponents have not been receptive to these efforts. The CDG believes in the benefits of convergence, but will not be able to achieve it alone. In any case, cdmaOne evolution proceeds on a fast track, ensuring that operators can deliver 3G services as the market demands.
Conclusion
The growth of cdmaOne technology is certain. Whether new capabilities are labeled 3G or not is not of material importance since the real challenge is having advanced services ready for market when customers demand them, and delivering these services cost effectively. Whatever results from the 3G standards process, cdmaOne operators will have standard solutions that enable 3G services with a
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