Mobile Wireless Communications Today (cont.)
by Puneet Gupta
Code Division Multiple Access (CDMA) Technology (IS-95) (cdmaOne)
The CDMA technology used in North America is based on the IS-95 protocol standard first developed by QUALCOMM. CDMA differs from the other two technologies by its use of spread spectrum techniques for transmitting voice or data over the air. Rather than dividing RF spectrum into separate user channels by frequency slices or time slots, spread spectrum technology separates users by assigning them digital codes within the same broad spectrum. Advantages of CDMA technology include high user capacity and immunity from interference by other signals. Like TDMA IS-136, CDMA operates in the 1900-MHz band as well as the 800 band.
Work on developing the CDMA standard is conducted mainly by the CDMA Development Group (CDG), a consortium of the main CDMA manufacturers and operators formed to standardize and promote CDMA technology. Whilst work to develop CDMA as a third-generation technology has attracted a great deal of attention over recent months, the CDG has also been working to improve the current performance of CDMA as a second-generation technology. The CDMA Development Group (CDG) has formally adopted the cdmaOne name and logo as a technology designator for all IS-95-based CDMA systems. The term represents the end-to-end wireless system and the necessary specifications that govern its operation. cdmaOne incorporates the IS-95 CDMA air interface, the ANSI-41 network standard for switch interconnection and many other standards that make up a complete wireless system.
The CDMA technology, used in the Interim Standard IS-95, maximizes spectrum efficiency and enables more calls to be carried over a single 1.25 MHz channel. In a CDMA system each digitized voice is assigned a binary sequence that directs the proper response signal to the corresponding user. The receiver demodulates the signal using the appropriate code. The resulting audio signal will contain only the intended conversation, eliminating any background noise. This allows more calls to occupy the same space in the communication channel, thereby increasing capacity. As a simple, example let us assume a user is talking into a mobile phone on a CDMA network. The transmitted portion of a voice signal has frequency components from approximately 300~3400 Hz. This analog signal is digitally encoded, using QPSK (Quadrature Phase Shift Keying), at 9600 bps. The signal is then spread to approximately 1.23 Mbps using special codes that add redundancy. Some of these codes include a device ID that is unique to the phone (like a serial number). Next the signal is broadcast over the channel. When broadcast, the signal is added to the signals of the other users in the channel. On the receiving end, the same code is used to decode the incoming signal. The 9600 bps signal is obtained and the original analog signal is reconstructed. When the same code is used on another user's signal, the redundancy is not removed and the signal remains at 1.23 Mbps.
The problems are the quality of reception and voice squeakiness. To address this major PCS carriers are using 13 kbps vocoders instead of 10 kbps. This improves quality but at the cost of capacity. The technology has been widely adopted by major cellular and PCS carriers in the United States and also internationally. CDMA networks provide operators with reliable digital systems that offer higher capacity, large coverage area and improved voice quality and above all a good 3G upgrade path, CDMA 2000 (I'll discuss this later). It also offers simplified system planning -- through the use of the same frequency in every sector of every cell.
Factors contributing to CDMA's capacity gains are:
- Frequency reuse
- Soft handoffs
- Power control,
- Variable rate vocoders
Some of the benefits of using cdmaOne are:
- Capacity gains of eight to ten times that of AMPS analog systems
- Improved call quality, with better and more consistent sound as compared to AMPS systems
- Simplified system planning through the use of the same frequency in every sector of every cell
- Enhanced privacy through the spreading of voice signals
- Improved coverage characteristics, allowing for fewer cell sites
- Increased talk-time for portables
cdmaOne technology improves quality of service through the use of soft handoffs, which greatly reduce the number of dropped calls and ensure a smooth transition between cells. In soft handoff, a connection is made to the new cell while maintaining the connection with the original cell. This transition between cells is one that is almost undetectable to the subscriber. cdmaOne technology also takes advantage of multipath fading to enhance communications and voice quality. Using a rake receiver and other improved signal-processing techniques, each mobile station selects the three strongest multipath signals and coherently combines them to produce an enhanced signal.
The cdmaOne data capabilities are based on IS-95A, which can provide data speeds of 14.4kbit/s. IS-95B and IS-95C are designed to enhance CDMA's data capability. IS-95B can provide data speeds of up to 64kbit/s by aggregating existing channels. IS 95-B can provide these enhanced data rates through software upgrades only. IS-95C aims to offer a minimum of 24.4kbit/s per channel and aggregated data speeds of more than 115kbit/s. It is expected that IS-95C will define CDMA's capability as a third-generation system. CDMA already supports asynchronous data and faxing (IS-99) and has standardized packet data (IS-657).
The major development initiatives being taken by the CDG for 2G CDMA systems enhancements include Enhanced roaming enables transparent roaming across cellular and PCS networks, with selection of networks and location services. Enhanced roaming will provide roaming between CDMA systems similar to that on GSM: registration, authentication and credit-checking are automatically carried out between the networks without users having to do anything more than switch on their mobiles. Roaming agreements will still be needed between operators.
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