IS-95, or cdmaOne, and GSM (Global System for Mobile Communication) are two fundamental technologies enabling today's cellular phone networks. IS-95 and GSM are 2G technologies implemented using CDMA (code division multiple access) and TDMA (time division multiple access) respectively. Approximately 80% of the mobile communications industry uses GSM technology, with IS-95 following at approximately 10-15%. As the market shifts to 3G technologies, GSM is moving towards the implementation of Wideband CDMA (W-CDMA), while the successor of IS-95 is CDMA 2000. The following document gives a technical overview of all these technologies.
2. Strengths and Weaknesses of IS-95(CDMA) and GSM (TDMA)
Both IS-95 and GSM are the most commonly used mobile communication technologies for cellular networks. Given the derivation from different multiple access schemes, both technologies persist with strengths and weaknesses. A summary of these qualities can be seen below in Figure 1. More detail about how CDMA and TDMA are structured and differ will be discussed in the following sections.
Figure 1: IS-95(CDMA) and GSM(TDMA) Summary
3. Technical Specifications
The technologies behind IS-95 and GSM are CDMA and TDMA, respectively. While both CDMA and TDMA use the same overall frequency range, they differ in how that range is divided up. TDMA uses time to divide the spectrum, allowing each communication the entire spectrum over a discrete time period. CDMA however allows every communication to have the entire spectrum all of the time. See Figure 2 below.
Figure 2. TDMA and CDMA Spectrum Division
4. GSM (TDMA)
There are three OSI layers involved with GSM: the physical Layer, the data layer, and signaling protocol layer. The physical layer is implemented with TDMA, using Gaussian Frequency Shift Keying (GFSK) as the modulation scheme. The common frequency range of GSM is in the 850-900 MHz range with channel spacing of 200 kHz. Peak data rates are around 14.4 kbps.
For TDMA, the signal is divided by time using the fundamental unit of a burst period. This burst period lasts approximately 15/26 ms and is grouped together by 8 bursts into a frame. A single traffic channel is defined by grouping 26 frames together; giving a total timeframe of 120 ms. These traffic channels are used to transfer speech and data.
Additional control channels also exist for the following purposes:
- Broadcast Control Channel (BCCH) serves for BS identification, broadcasts, and frequency allocations.
- Frequency Correction Channel (FCCH) and Synchronization Channel (SCH) – used for synchronization, and physical layer definition (time slots, burst time…)
- Random Access Channel (RACH) used by mobile to request access to the network.
- Paging Channel (PCH) used for locating the mobile user
- Access Grant Channel (AGCH) used to obtain a dedicated channel.
The data layer consists of the same protocol used in Integrated Services Digital Network or ISDN. The signaling protocol layer is further subdivided into three categories:
- Radio Resource Management
- Mobility Management
- Connection Management
5. IS-95 (CDMA)
IS-95 consists of a physical layer implemented by CDMA. The modulation scheme commonly used is quadrature-phase shift keying (QPSK). The common frequency bands in the US and Korea are 825-849 MHz with channel spacing of 1.23 MHz. The data transfer rate is 9.6-115 kbps depending on which revision of the technology is being used.
Instead of segregating the spectrum by time, CDMA uses a unique spreading code technique to differentiate the channels. The signal is transmitted below noise level and is received through a correlator for dispreading of the wanted signal. This wanted signal is then processed through a narrow bandpass filter to reject unwanted signals. The codes, one/zero sequences, used to differentiate signals are designed and generated at a much higher rate than the baseband information. This rate is referred to as a chip rate rather than a bit rate. The relationship between chip rate and bit rate can be seen in Figure 3. The Spread Factor used in CDMA is defined as equation 1.
6. Applications and future Generations (3G)
Applications for IS-95 (CDMA) and GSM (TDMA) include popular cellular phone services. GSM controls a majority of the worldwide services; however CDMA is popular in certain regions such as the US.
Future 3G versions of the technology include IS-2000 which uses CDMA2000 as its physical layer. This incorporates much higher transfer rates than the previous 2G versions. GSM 3G/EDGE technology is being implemented using CDMA, and a special form of W-CDMA. These new physical layers will be competitive with each other and lesser known 3G solutions in regards to data transfer rates and spectral efficiency.
IS-2000 (CDMA 2000)
There are several varieties of CDMA 2000 ranging from hybrid 2.5G technologies to full on 3G solutions. These variations include but are not limited to 1xRTT and EVDO. It is defined to operate at 450, 700, 800, 900, 1700, 1800, 1900, and 2100 MHz.
The 1xRTT variant of CDMA 2000 is defined to operate within the same radio bandwidth of the 2G IS-95. It improves upon the traditional IS-95 technology by adding 64 additional traffic channels. These new channels are orthogonal to the original 64 channels, doubling the capacity of the technology. A variation of 1xRTT is 3xRTT which triples the channel width from 1.25 MHz to 3.75 MHz in addition to the 64 new channels.
EVDO, or Evolution-Data Optimized, is an enhanced version of CDMA 2000 designed to provide higher data transfer rates. The EVDO channel is exactly the same spectral width as traditional IS-95 or IS-2000 channels. The difference lies in the time-based multiplexing of the tower to mobile device link. This forward link is split up into 1.667 ms long divisions, allowing each mobile device full access to the spectrum over the given time with independent modulation. These modulation schemes are determined by the quality of the mobile device’s current RF environment. The schemes can range from using QPSK, 8-PSK, and 16-QAM given multi-path and fade restrictions, where QPSK provides the lower end of data transfer speeds, and 16-QAM is used for the highest quality and speeds.
The third generation of GSM is also known as UMTS, or Universal Mobile Telecommunications System. Like IS-95 and its successor IS-2000, the underlying physical layer of UMTS is based on the concept of CDMA. More specifically, the most common form of CDMA used is W-CDMA or Wideband CDMA. W-CDMA makes use of channels with 5 MHz bandwidth and a chip rate of 3.84 Mcps. Data transfer rates of 2Mbps can be achieved, with further increases to 14.4 Mbps after the implementation of HSDPA.
The core frequencies for UMTS include 1920-1980 and 2110-2170 MHz using Frequency Division Duplex and W-CDMA as well as 1900-1920 and 2010-2025 MHz using Time Division Duplex and TDMA or CDMA. Additional spectrum regions include 1980-2010 and 2170-2200 MHz which are used primarily for satellite uplink and downlink. Spreading codes used in W-CDMA are Orthogonal Variable Spreading Factor (OVSF) codes, which remain orthogonal in order to allow multiple users without mutual interference. A second stage of spreading is accomplished using a pseudo-random number (PN) code to help differentiate between the signals.
7. National Instruments Hardware for IS-95 (CDMA) and GSM (TDMA)
The current generation of NI RF hardware (566x, 567x) reaches up to 2.7 GHz with a 20 MHz of bandwidth. In combination with the LabVIEW Modulation Toolkit you can generate and analyze the physical modulation schemes required to implement CDMA and TDMA.
8. CDMA and GSM (TDMA) Software Solutions
- AmFax (www.amfax.co.uk)