CDMA Network
Understanding CDMA Network
Cellular Architecture and Frequency Reuse
A CDMA network is built on a cellular topology composed of base stations (BTS), base station controllers (BSC), and a mobile switching center (MSC). Unlike TDMA and GSM networks, which use a frequency reuse pattern (e.g., reuse factor of 4 or 7) to prevent adjacent cells from interfering with each other, CDMA networks operate with a frequency reuse factor of 1. This means that every cell sector in the network transmits and receives on the exact same carrier frequency.
This universal frequency reuse simplifies frequency planning, but it means that the capacity of a cell is determined by the interference generated by neighboring cells. The network boundary between cells is soft: a mobile device located at the boundary receives signals from multiple base stations on the same frequency, enabling the network to implement soft handoff.
Soft Handoff and Macro-Diversity
In GSM networks, handoff is hard: the mobile device must disconnect from the serving base station before connecting to the target base station, which can cause call drops. In a CDMA network, a mobile device near the boundary of two cells establishes a connection with both base stations simultaneously. Both base stations receive the mobile's uplink signal and forward the decoded frames to the BSC, which selects the best frame.
On the downlink, both base stations transmit the same user data to the mobile device on different Walsh codes. The mobile's RAKE receiver combines the signals coherently, yielding a macro-diversity gain of 2 to 3 dB. This soft handoff mechanism eliminates call drops at cell boundaries and increases the capacity of the network by reducing the transmit power required to maintain the link.
Key Mathematical Relations
Technical Specifications Comparison
| Network Standard | Generational Class | Channel Bandwidth | Peak Downlink Rate | Coding Architecture | Key Architectural Feature |
|---|---|---|---|---|---|
| IS-95A (cdmaOne) | 2G Voice | 1.25 MHz | 14.4 kbps | Convolutional (viterbi) | First commercial CDMA; soft handoff |
| IS-95B | 2.5G Data | 1.25 MHz | 115.2 kbps | Convolutional | Channel bundling (aggregates up to 8 codes) |
| cdma2000 1xRTT | 3G Voice/Data | 1.25 MHz | 153.6 kbps | Turbo Coding for data | Doubled voice capacity; added packet core |
| cdma2000 1xEV-DO | 3G Broadband | 1.25 MHz | 3.1 Mbps (Rev A) | Turbo Coding | Data-only carrier; time-division scheduling |
| WCDMA (UMTS) | 3G 3GPP Standard | 5.0 MHz | 384 kbps (DL) | Turbo Coding | Wider bandwidth; asynchronous cell operation |
Frequently Asked Questions
What is the role of the Base Station Controller (BSC) in a CDMA network?
The BSC manages the radio resources of multiple base stations. It controls cell configuration, allocates Walsh codes for downlink channels, manages handoff decisions, and performs frame selection for calls in soft handoff.
How does voice activity detection increase the capacity of a CDMA network?
In a voice conversation, each person speaks about 40% of the time. CDMA networks use variable-rate vocoders that reduce the data rate (and therefore the transmit power) during silent intervals. This reduces the average interference in the cell, allowing the network to support more users.
What was the purpose of the EV-DO standard?
EV-DO (Evolution-Data Optimized) was a 3G standard that separated data traffic from voice traffic. It allocated a dedicated 1.25 MHz carrier exclusively to high-speed packet data, using time-division multiplexing and link adaptation to achieve peak data rates of 3.1 Mbps.