Terminology » Data Communication » SCDMA (Synchronous CDMA) Advantages and Disadvantages in DOCSIS

SCDMA (Synchronous CDMA) Advantages and Disadvantages in DOCSIS

scdma docsis cdma atdma cable modem

This page explores the advantages and disadvantages of Synchronous Code Division Multiple Access (SCDMA) within the DOCSIS standard. We’ll cover the benefits and drawbacks of S-CDMA.

What is SCDMA?

CDMA stands for Code Division Multiple Access. In CDMA, the entire allocated bandwidth is shared among multiple subscribers or users by assigning unique codes. Essentially, all users utilize the entire bandwidth at the same frequency channel and at the same time.

The figure below illustrates the concept of CDMA, showing how different codes are assigned for various channels, such as traffic channels or control channels.

CDMA Figure-1

SCDMA, or Synchronous Code Division Multiple Access, was introduced in the DOCSIS 2.0 standard alongside ATDMA. SCDMA uses 128 orthogonal codes to transmit 128 symbols simultaneously, allowing multiple cable modems to transmit within the same time slot. Like ATDMA, SCDMA aims to increase the data throughput of the system.

Benefits or Advantages of SCDMA

Here are some key benefits of SCDMA:

  • Burst Noise Immunity: SCDMA spreads transmissions over a longer time period, making it more resistant to burst noise compared to ATDMA.
  • Reduced Errored Symbols: SCDMA utilizes a longer symbol time than ATDMA, which reduces the number of errored symbols.
  • Byte Interleaving: Similar to byte interleaving in ATDMA, SCDMA employs framing and subframing. This spreads bytes across multiple Reed-Solomon code words, helping to combat burst and impulse noise.

Drawbacks or Disadvantages of SCDMA

However, SCDMA also has some limitations:

  • Interoperability Issues: Problems with interoperability have been observed between cable modems from different vendors.
  • Throughput Dependency: SCDMA only provides noticeable throughput benefits when approximately 60% of the upstream traffic is actively utilizing the SCDMA scheme.
  • Frequent Ranging: All cable modems require periodic ranging every second.
  • Strict Time Alignment: To achieve higher throughput, all cable modems must be precisely time-aligned within a fraction of the symbol rate.