The Next Plateau: SR1

Mar 15, 2001 12:00 PM, Paul Dohrman

IS-2000, also referred to as IS-95C, is a radio format well positioned to provide the first access to services that are driving the telecommunications industry to adopt new systems. It is one of three radio-access modes being pursued worldwide, which also include WCDMA and TDD.

One important benefit of IS-2000 is the fact that it is compatible with 2G IS-95. Besides protecting network operators’ capital-equipment investment, there is no need to clear the spectrum to allow installation of the new system. Existing customers will continue to get the same service they had in the past, while 3G customers with 3G-capable equipment will have access to the advanced features made available by IS-2000.

IS-2000 is the radio-access mode based on a standard (IS-2000 RTT) submitted to the ITU (www.itu.int) by the TIA standards body in North America. This standard is supported by an international group of standards development organizations called 3G Partnership Project 2 (3GPP2) (www.3GPP2.org). This group is spearheaded by ANSI and includes the TIA committee, among others. Under 3GPP2 supervision, the plan is to gradually evolve the existing core network (based on the ANSI/TIA/EIA-41standard) to support a virtually seamless transition to 3G performance.

IS-2000 will be introduced in two phases: Spreading Rate 1 (SR1) and Spreading Rate 3 (SR3). Unless otherwise noted, the following will refer to the first phase, SR1.

Advancements

IS-2000 SR1 requires 1.25MHz bandwidth channels, the same bandwidth used in 2G, to transmit data two to 10 times faster than 2G. This will allow operators to overlay existing 2G systems with higher-data-rate 3G systems.

Data can be sent in packets, which improves network efficiency. Packet data uses standard IP. With higher data rates, services such as Internet access, FTP, e-mail with attachments and general network connectivity will be obtainable.

SR1 will approximately double capacity over TIA/EIA-95B by introducing improvements in: channel structure, power control, spatial (antenna) diversity and coding techniques.

Channel Structure

At any given time, the signal from a 2G wireless terminal, which is referred to as the reverse-link signal, contains at most one code channel. (Code channels are baseband data or voice signals that have been spread in frequency over the bandwidth of the system through the use of specialized code sequences.) Many code channels can be present at one time on the same frequency and still be distinguished from one another.

The receiver is designed to make a correlation between the received signal and the appropriate code. A single 2G code channel can carry voice and short text messages simultaneously, but this requires forcing the voice to a lower data rate while the messaging takes place or waiting for a quiet time. With IS-2000, the reverse link can be configured to include a fundamental voice-code channel and up to two supplemental code channels dedicated to carrying high-speed data. This would allow downloading e-mail with no disruption to voice service.

The addition of these channels comes at a price, however. The amount of spectral spreading, or spread factor, is significantly lower on the higher-rate data channels. Because of this characteristic, higher-rate data channels are transmitted at higher power levels, which uses up a larger portion of the available system capacity. When supplemental channels are not needed, the power level decreases, freeing up system capacity for voice traffic. IS-2000 systems will manage the demands of data and voice users by balancing the loading effects of data transmission with the need for voice traffic. Five forward-channel structures are specified in IS-2000, and they are labeled radio configuration (RC) one through five. RC1 and RC2 are identical with cdmaOne at channel rates of 9,600b/s and 14,400b/s; RC3 through RC5 provide the newer channel structures to support high-end services.

In IS-2000 systems, the signaling necessary to maintain a call may be off-loaded to a dedicated control channel. Using a dedicated control channel eliminates the need to use a 2G technique called dim-and-burst signaling, which takes bandwidth away from the transmission of information and gives it to control functions.

A pilot channel, transmitted only on the forward link in IS-95 systems, has been added to the IS-2000 coding. This allows coherent demodulation of the signal from the wireless terminal to the base station. This improves signal reception at the base station and simplifies the design of base-station receivers.

Power Control

In 2G systems, base stations measure the received signal-to-noise ratio from wireless terminals and communicate instructions back to the wireless terminal to increase or decrease power. Referred to as reverse-link closed-loop power control, this feature minimizes the power levels of each transmitting mobile. Lower power levels allow more users on the system. IS-2000 builds on this feature by including closed-loop power control on base-station signals.

Antenna Diversity

Using multiple antennas to take advantage of spatial diversity is a common method of improving signal reception. Wireless terminals, however, will not support multiple antennas because of their small size. IS-2000 systems overcome this problem by setting up multiple antennas at the base-station end. The forward (base station to wireless terminal) channel is split into two signals, and each half is transmitted on antennas spaced at least quarterwave-length apart. This is referred to as forward-transmit diversity. This provides two separate signal paths to the wireless terminal’s receiver without the need for two antennas at the receiving end of the signal path.

Coding Techniques

Because of the shorter code lengths used at higher data rates, less processing gain is applied to the supplemental channels than for voice or other code channels. (Processing gain is the term used to quantify the amount of signal spreading due to the multiplication of the information data with a digital code word.) On signals with lower processing gain, higher transmit power is required to maintain a minimum acceptable level of data errors. In fact, data channels require proportionally higher signal energy per bit as data rates increase. Higher transmit power, however, increases the interference level to all users. The penalty for higher transmitted power is decreased system capacity. To offset this undesired characteristic of high-data-rate signals, a new technique for coding information data has been developed. It is called the turbo coder. Turbo coding mitigates the high transmit power requirement by improving the receiver’s ability to correct errors and recover bad data frames. The net effect with turbo coding is a reduction of approximately 2dB in signal power needed to achieve the same error rate as the IS-95 convolutional encoder would yield. This procedure increases the number of users allowed on the system during periods of heavy data traffic.

Other Benefits/Features

When a wireless terminal is not on a call, it must monitor its assigned paging channel during pre-defined time slots to be alerted of an incoming call. To minimize battery-current drain, IS-2000 implements a quick-paging channel that gives the wireless terminal advance knowledge about when to monitor the paging channel. If a page indicator is not present on the quick-paging channel, the wireless terminal does not monitor the paging channel. This extends the wireless terminal’s standby times and overall battery life.

Multiple code channels, now present on the wireless terminal’s transmitted signal, greatly increase the peak-to-average ratio of the transmitter’s amplitude waveform. A signal with an increased peak-to-average ratio consumes more power and complicates the design of the wireless terminal’s output amplifier. The introduction of an IS-2000 feature called hybrid-phase-shift keying imposes limits on the code that drives the waveform modulation, lowering the reverse-channel signal’s peak-to-average ratio.

Finally, an enhancement to the IS-2000 system referred to as 1XEV will increase data rates to as high as 2.4Mb/s and is expected to be introduced with some of the first system roll-outs.

SK Telecom, a leading Korean carrier, began providing IS-95C service in 2000. Customers now can access mobile data services at 144kb/s. A wider roll-out is scheduled to occur in Korea this month, and hints of IMT-2000 (3G) delays are increasing the importance of this system to operators.

IS-2000 trials are now under way in North America, and systems could be up and running by year-end.

Dohrman (paul_dohrman@agilent.com) is Agilent Technologies customer education engineer in the RF Communications Product Generation Unit.