Advantages of SCDMA | Disadvantages of S-CDMA in DOCSIS
This page covers advantages and disadvantages of SCDMA in DOCSIS standard. It mentions benefits or advantages of SCDMA and drawbacks or disadvantages of S-CDMA. SCDMA stands for Synchronous Code Division Multiple Access.
What is SCDMA?
CDMA : The CDMA is the short form of Code Division Multiple Access. In this scheme, entire allocated bandwidth is shared among multiple subscribers or users by assigning unique codes. Here entire bandwidth is used by users at the same frequency channel and at the same time instant. The figure depicts concept of CDMA access type by assigning different codes for all the channels such as traffic channels or control channels.
Figure-1 : CDMA
SCDMA stands for Synchronous Code Division Multiple Access. It has been introduced in DOCSIS 2.0 standard along with ATDMA. SCDMA uses 128 orthogonal codes in order to transmit 128 symbols simultaneously. SCDMA allows multiple cable modems to transmit in the same time slot. SCDMA increases data throughput of the system like ATDMA. Refer architecture of DOCSIS network >>.
Benefits or advantages of SCDMA
Following are the benefits or advantages of SCDMA:
➨It spreads transmissions over time period. Hence it has burst noise advantage over ATDMA.
➨SCDMA scheme uses longer symbol time than ATDMA. This reduces number of errored symbols.
➨It uses framing and subframing similar to byte interleaving used in ATDMA. This spreads bytes over multiple Reed Solomon code words to help combat burst and impulse noise.
Drawbacks or disadvantages of SCDMA
Following are the drawbacks or disadvantages of SCDMA:
➨Interoperability issues have been observed between different cable modem vendors.
➨SCDMA provides throughput benefits only when 60% upstream traffic is utilizing SCDMA scheme.
➨It requires periodic ranging for all the cable modems every second.
➨To achieve higher throughput all the cable modems must be time aligned within fraction of symbol rate.