The cdma1x system is the evolution of the 2G IS-95 CDMA system and is supported by organization in the United States and Korea. In order to be backward-compatible with IS-95 networks, cdma1x retains many of the attributes of the IS-95 air interface design.

Many carriers that deployed cdmaOne will choose cdma1x because the upgrade is quicker and less costly than UMTS. For example, in Korea, carriers have already rolled out IS-95C at 144 Kbps.

Adopting a synchronous mode of operation, transmission and reception timings of cell sites are synchronized by a single common timing source such as the Global Positioning System (GPS). Variable length Walsh codes (from 4 to 1,024 bits) are used for the spreading on supplemental channels to support various information rates.

In the forward link, Walsh codes are unique within channels of the same user as well as across different users in the same cell. Cell separation is performed by two PN sequences of length 215– 1 chips, one for the in-phase channel and one for the in-quadrature channel. Shifted versions (in multiples of 64 chips) of the same sequences are used in different cells.

Each cell uses a unique PN offset to distinguish its transmission from its neighboring cells. On the reverse link, cdma1x also uses Walsh codes to differentiate between channels from the same user. User separation is achieved by user-specific long PN codes.

All the users in all cells use the same long code. The transmission from different users, however, is offset by a different number of bits. This offset is achieved by using the electronic serial number (ESN), which is unique to each mobile station.

cdma1x introduces dedicated and common control channels to provide efficient packet data services. Fast power control with an update rate of 800 Hz is applied only in the reverse link of IS-95. Its implementation in the forward link of cdma1x provides significant performance improvements in low mobility environment where most of the high data rate applications will occur.

Both IS-95 and cdma1x employ a common pilot channel shared by all mobiles to provide a reference signal to receivers thus helping the coherent demodulation. In cdma1x this common pilot channel is a code-multiplexed channel using Walsh codes for orthogonal spreading. The incorporation of a pilot channel also on the reverse link offers significant performance gain by providing a coherent phase reference for coherent demodulation at the BS.

It also reduces the power control loop delay. When beam-forming is applied to cover a smaller portion of a cell, the receivers would require an additional dedicated pilot for reliable channel estimation. Channel estimations will not be accurate if the reference pilot traverses a different path compared to the data signal. Thus, dedicated and common auxiliary channels are introduced to take advantage of smart antennas.

Systems can provide IS-95B and cdma1x services simultaneously to the MS. A new burst mode capability is defined that addresses the technical issuesthat arise when direct sequence spread spectrum has to support higher-data rate packet services.

Turbo codes are also adopted in the cdma1x standard. cdma1x also includes a sophisticated MAC feature to support effectively very high data rate services (up to 2 Mbps) and multiple concurrent data and voice services. Enhancements over IS-95B include the introduction of the suspended and control hold states for packet data MAC. Additional enhanced features can be found in.

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