Table of Contents
SUMMARY................................................................................................................................ 1
Table of Contents....................................................................................................................... 2
Introduction................................................................................................................................ 3
Major Conceptual Similarities..................................................................................................... 3
Major Differences...................................................................................................................... 3
Chip Rate................................................................................................................................ 4
Migration from existing technologies................................................................................... 4
Numerology........................................................................................................................ 4
Eb/No Requirement............................................................................................................. 4
Overall Capacity.................................................................................................................. 5
Out of Band Emissions........................................................................................................ 5
Summary of Chip Rate Comparison.................................................................................... 6
Forward Link Pilot Structure................................................................................................... 6
Basic Fundamental Approaches.......................................................................................... 6
Dedicated Time Multiplexed Pilot (DTMP).......................................................................... 6
Common Code Multiplexed Pilot (CCMP)........................................................................... 7
Base Station Synchronization.................................................................................................. 8
Base Station Acquisition and Detection............................................................................... 9
Terminal Battery Life Characteristics...................................................................................... 9
Dedicated Mode................................................................................................................ 10
Idle Mode.......................................................................................................................... 10
Inter-Frequency Hard Handoff and Hierarchical Cell Structure (HCS)................................. 11
Frame Length........................................................................................................................ 11
Differences in Deployment Possibilities................................................................................... 12
Benefits of 1.25 MHz Deployment Option........................................................................... 13
Deployment in 2x 5 MHz.................................................................................................. 13
Band Packing and Band Planning...................................................................................... 13
Benefits of Multi-Carrier Spreading Option.......................................................................... 14
Appendix 1 – Numerology....................................................................................................... 16
Appendix 2 - Overall Capacity................................................................................................. 18
Appendix 3 - Emission Limits.................................................................................................. 21
Filter Characteristics.............................................................................................................. 21
Output Power Level.............................................................................................................. 21
Impact on Forward Link Capacity..................................................................................... 23
Appendix 4 – cdma2000 Inter-frequency Hard Handoff procedures....................................... 26
Inter-frequency Search Procedure......................................................................................... 26
Minimizing Impact of Search Excursion on Current Frame................................................... 27
Off-Line Pilot Searching.................................................................................................... 27
Increasing Symbol Energy of Current Frame..................................................................... 27
Forward Link Power Control during Search Visit.............................................................. 27
Reverse Link Power Control during Search Visit............................................................... 28
Search excursion position within a frame............................................................................. 29
Hard Handoff Failure Recovery........................................................................................ 29
References................................................................................................................................ 30
IntroductionDevelopment of the 3G CDMA systems for IMT-2000 is oriented towards a wider set of services than provided in second generation cellular systems. Both WCDMA and cdma2000 are to provide these new services. Since currently deployed CDMA systems have been the source of many of the new ideas behind using CDMA for cellular and PCS applications, there are naturally similarities between the two RTTs. Furthermore, harmonization activities have brought the RTT contributions even closer. So, first we list similarities and then discuss more extensively the residual differences, which have not yet been converged by the continuing harmonization efforts. The similarities are mainly at a functional and conceptual level. Many of the technical details such as nomenclature, timing, and precise parameters are different, due to parallel on-going standardization efforts. This is true for both the physical layer and lower layers.
Both cdma2000 and WCDMA were designed to meet the ITU IMT-2000 Requirements in terms of data rates, enhanced spectrum efficiency, and range of operating environments. New frequency bands have been allocated in many parts of the world to deploy IMT-2000 systems. The WCDMA system is mainly targeted for deployment in new (green field) spectrum. By contrast, cdma2000 was designed to be deployed both in existing bands and new spectrum. This lead to some of the conceptual differences in terms of spreading method and chip rates discussed further in this document.
Major Conceptual SimilaritiesThis list pertains to functional and conceptual similarities between the two CDMA IMT-2000 proposals. It should be noted that while the concepts are similar, the details of the physical layer are different. This is mainly due to separate standardization efforts. However, since many of the underlying physical layer concepts are identical, many of the details can be harmonized without impacting the system performance or integrity.
Similarities in both systems include:
· Coherent forward and reverse link
· Fast power control on forward link as well as reverse link.
· Variable length orthogonal Walsh sequences used for forward link channelization
· Complex QPSK spreading on both forward and reverse link
· Convolutional codes used as baseline (identical polynomials)
· Parallel Turbo codes for higher data rates
· Orthogonal Walsh functions separating the users parallel code channels on the reverse link
· Variable spreading factor to achieve higher data rates
· Non orthogonal reverse link based on PN scrambling codes
· Mobile Assisted inter-frequency hard handoff procedures
· Variable rate operation with blind rate estimation for simple services (e.g. voice)
· Continuous reverse link operation
Major DifferencesThe following items are major physical layer differences being addressed for future convergence. There are many other differences in the details of the physical layer parameters and procedures. The list below addresses some of the major differences in the physical layer concepts and parameters affecting harmonization of the two systems.
Table 1: Major Physical Layer Differences between WCDMA and cdma2000
| | | Chip rate
| | | Forward link pilot for channel estimation
| | | Antenna beam forming and spot beams
| | | Base station synchronization
| | | Base station acquisition and detection
| 3 step parallel code search for base station detection and slot/frame synchronization | Time-shifted PN correlation |
Chip Rate Considerations for chip rate selection are:
· Migration from existing technologies
· Numerology
· Capacity
- requirement
- Overall capacity
· Out of band emissions
Migration from existing technologies In the ITU REVAL, A3.4.2.4.3, Annex 3, ease of migration from existing technologies is given the G1 rating of Most Important. Of course, if ease of migration extracts a penalty in performance or implementation, then a tradeoff must be weighed. The use of 3.6864 Mcps, however, provides smoother migration from existing CDMA systems without any such penalties. This is elaborated on below.
NumerologyA well-engineered and technically sound system design will utilize parameters that simplify implementation and provide maximum performance with minimum complexity. Under these assumptions, acceptable chip rates for a 5 MHz DS-CDMA system (based on accommodating different frame lengths, power control update rates, and numbers of modulation symbols per power control group and chips per modulation symbol) are limited to a few possible chip rates. It is shown in Appendix 1 that the two chip rates proposed by WCDMA and cdma2000 meet those numerology criteria. For the selection of a particular chip rate among the candidates, one must look at the relative merits of each of these two parameters under all other systems requirements, for further optimization.
Eb/No RequirementSummary: Compared to other factors such as modulation characteristics, power control performance, mobile speed, number of transmit / receive antennas, and radio channel environment, a 4.096 Mcps vs. 3.6864 Mcps chip rate has a negligible impact on Eb/No requirement.
One of the main parameters driving the capacity of a CDMA system is the energy per information bit to noise ratio (Eb/No) required to achieve a given link quality (e.g. required frame or bit error rate). The Eb/No requirement depends on the modulation characteristics, demodulator and decoder performance, mobile speed, number of receiving antennas, number of transmitting antennas, multi-path environment (time diversity), and total signal bandwidth (frequency diversity). All those determining factors being identical, a small change in signal bandwidth (chip rate) has a negligible impact on Eb/No performance. Other factors such as usage of transmit diversity, coding scheme, or pilot structure and energy overwhelm the impact of a change in chip rate. Thus, a particular chip rate between 3.6864 Mcps and 4.096 Mcps has virtually no impact on Eb/No requirements.
Overall CapacityThe overall capacity of a CDMA system is the minimum between the forward and reverse link capacities.
In theory, theforward link capacity is limited by the pole capacity resulting from interference generated by non-orthogonal users of the same cell (multi-path) and users of neighboring cells.
In practice, other aspects limit the forward link capacity befo............. |
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发表于 2007-3-6 19:51:43
