Reverse Channels in CDMA 2000

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teamques10 ★ 68k

Reverse channels carry information from MS to BS. These channels are of two types, Dedicated channels and Control channels. Reverse link physical channels include dedicated channels to carry information from a single mobile to the base station and common channels to carry information from multiple mobiles to the base station. Below table lists the various reverse link physical channels.

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Reverse Link Power Control

The primary objective of power control in the reverse link is to resolve the near-far problem, The CDMA 2000 reverse power control scheme is a generalization of the IS-95 version. The system supports the same open and closed loops, but it integrates the functionality of the power control of the fundamental and supplemental channels in a simple scheme - easy to expand. The key factor in the simplification is the introduction of the R - PICH, which is used as a reference for measurement and scaling in the open and inner loops, and then translates them to corrections that apply to F- FCH. The scaling is performed per channel and per data rate, so that rate equalization can be performed easily. This simplifies the design of the outer loop of the R - FCH, since all rates can be treated equally.

Comparison of WCDMA/UMTS and CDMA2000

UMTS and CDMA2000 were developed separately and are two separate ITU approved 3G standards. UMTS was developed mainly for countries with GSM networks because these countries have agreed to free new ranges of UMTS networks. UMTS uses new technology, new frequency band, and new RAN. UMTS phones are dual mode and are compatible with GSM systems. CDMA2000 1xRTT, CDMA2000 1xEV-DO, and CDMA2000 3x were developed to be backward compatible with CDMA1. Both 1x types have the same bandwidth, chip-rate and it can be used in any existing CDMA1 frequency band and network. 3G WCDMA and CDMA2000 Standards are presented in below table.

UMTS WCDMA	CDMA 2000
No Backward Compatibility	Backward Compatibility with CDMA 2000
Cell site not Synchronised	Cell site Synchronised through GPS timing
Each cell site with different scrambling code for spreading	Adjacent cell sites use different time offset of same scrambling code for spreading
Complex code 38,400 chips, frame of 10 msec.	Pseudo Random (PN) sequence of length $2^{15}-1$ chips; period of 26.67 msec; different site offset of 64 chips.
OVSF Codes	Walsh Codes

Below table shows the comparative differences between the different technologies and it shows the clear difference between them. The CDMA2000 is widely used for the fixed Internet and data services but the WCDMA is the new technology designed for both the data and voice services and it performs better operations. The comparison between WCDMA and CDMA2000 technologies is given in below table.

Parameters	WCDMA/UMTS	CDMA 2000
Multiple Access Method	DS-CDMa	Uplink: DS CDMA Downlink: MC CDMA/DS CDMA
Carrier spacing/Channel Bandwidth	5 MHz	3.75 MHz (1.25 × N MHz Initially, N may be 1, 2, or 3, but later could be 6, 9, or 12).
Spectrum Allocation	FDD mode: 1920–1980 MHz uplink 2110–2170 MHz downlink TDD mode: 1900–1920 MHz 2010–2025 MHz	1850–1910 MHz uplink 1930–1990 MHz downlink
Chip rate	3.84 Mcps	1.2288 Mcps
Data modulation	Downlink – QPSK Uplink - QPSK	Downlink – QPSK; Uplink – BPSK
Frame Length	10ms with 15 slots	5,10,20,40,80 ms
Pilot structure	Dedicated pilots on uplink.Dedicated or common pilots on downlink	Code divided continuous dedicated pilot on uplink Code divided continuous common pilot, dedicated or common auxiliary pilots
Spreading Modulation	QPSK both direction	Uplink: M-ary PSK Downlink : QPSK
Detection	Coherent detection pilot aided	Coherent detection pilot aided
Power control frequency	1500 Hz	800 Hz
Variable data rate implement.	Variable SF; multi code	Repeat., puncturing, multi code
Scrambling Code	Uplink: short code (256 chips) Long code (38400 chips Gold code) Downlink: Gold Code	Long Code : ($2^{42} -1$ chips) Short Code :($2^{15} -1$ chips)
Channelization	Code OVSF Code	Uplink : Walsh code Downlink: Walsh or quasi orthogonal code
Base Stations synchronized/ intercell site operation	Asynchronous	Synchronous
Base Station acquisition/detect	Three step; slot, frame, code	Time shifted PN correlation

The main advantages of CDMA 2000 are as follows,

Superior Voice Quality
High speed broad band connectivity
Low end to end latency
Increased voice and data throughput capacity
Long term, robust and evolutionary migration path with forward and backward compatibility
Differentiated value added services such as VoIP multicasting, position location etc.
Flexible network architecture with connectivity to all types of networks
Application, user and flow based QoS
Flexible spectrum allocation with excellent propagation characteristics
Multi band, multi mode and global roaming feature
Improved security and privacy
Lower total cost of ownership

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