Engineers Inn: April 2008

CDMA in Wireless Communications:-

A problem facing the wireless communications industry is the limited available frequency spectrum. A conventional system might assign a channel or set of channels to service a geographical region. This region is typically chosen so that the coverage area is maximized. The capacity of the region is then limited to the number of channels allocated. This pattern of assigned channels is then repeated over an entire geographic region. While resolving the problem of spectral congestion, the reuse of channels results in co-channel interference. Care must be taken to limit the interference that will occur between these clusters of cells as this interference dominates the capacity of the system [1]. The current analog cellular standard, Advanced Mobile Phone System (AMPS), is based upon frequency modulation (FM). While analog FM works well for voice, it lacks the flexibility in configuration for handling digital video and data sources. The integration of voice, video, and data services has pushed the demand for digital systems significantly. Digital modulation techniques are indifferent to the source of the transmission and can be designed to sufficiently handle the requirements for all types of service. Powerful forward error correcting (FEC) codes and advances in speech coding further enhance the capabilities of digital systems. While the transit to digital technology is undeniable, questions still remain about the choice of modulation and access technique.

The AMPS system uses frequency division multiple access (FDMA) to separate the duplex channels on which each user communicates. Each user is assigned a duplex channel in FDMA. Typically, a seven cell cluster is used to implement an AMPS system. FDMA performs poorly in a cellular environment and a lead to low capacity limits [2]. In an effort to increase capacity, a digital standard for the United States was proposed. The United States Digital Cellular (USDC) or Electronic Industry Association Interim Standard-54 (IS-54) uses time division multiple access (TDMA) to share each frequency channel between three users, but maintains the same 30 kHz channel bandwidth as the AMPS system [3]. Service providers may allocate some of their current AMPS channels to an IS-54 system while still supporting existing AMPS users. In TDMA, multiple users share a forward and reverse frequency assignment and are reserved a time slot during which only they are allowed to transmit. The user is inactive during other time slots. IS-54, may support 3 full-rate or 6 half-rate users in each channel, when the data rates are compressed by speech coding techniques. IS-54 has been revised to include all digital control channels and standardized as IS-136.

While IS-54 (IS-136) may triple the capacity of current AMPS service, there is still a need for a solution that will increase capacity by a greater factor, in expectation of a coming boom in personal communications. A CDMA system (IS-95) has been recently standardized for this reason [4]. For cellular, IS-95 would replace 40 analog channels (1.25 MHz of spectrum) but in a PCS application the entire band might be dedicated to a CDMA layout. It is important to note that in the cellular case, IS-95 does not overlay existing narrowband channels within the same cell. Very broadband signals that might overlay a cellular band have been suggested but not yet implemented. Resistance to narrowband jamming is of less concern in a cellular arena since the FCC strictly regulates transmissions in the cellular band. CDMA appears capable of greatly increasing the capacity of a cellular system; some claim by a factor of 10-14 [5]. These claims are contingent upon the performance of the power control algorithm and the duty cycle of the voice activity detection.

Another TDMA technology has recently been adopted for use in the PCS band. The Global System for Mobile (GSM) was developed in Europe as a second generation cellular system and has now become the world's most popular standard for new personal communications equipment. The GSM system will be frequency up shifted to 1.9 GHz and renamed DCS 1900 for PCS use. The primary competition for any CDMA system for PCS will most likely come from GSM systems that will be setup in the United States.