The purpose of IS-2000 R-SCH is to serve as a dedicated transport pipe for high-rate packet data. In performing this function, the R-SCH has two unique characteristics:
• Because it is acting as a high data rate transport pipe, the R-SCH only carries
user traffic data and does not carry any signaling traffic.
• Because it is used to transmit packet data which is bursty in nature, the exis-
tence of the R-SCH is itself bursty. This means that the R-SCH is set up and torn down rather quickly.
Because the R-SCH does not carry any signaling, it has to coexist with another physical channel that is able to carry signaling traffic for the R-SCH (i.e., reverse fundamental channel or reverse dedicated control channel). In other words, while the R-SCH is active, one of these other physical channels has to be active as well. Since both the reverse fundamental channel and reverse dedicated control channel support 5-ms frames, signaling traffic can be transmitted on these channels quickly to set up and tear down the R-SCH.
The R-SCH can use one of four different radio configurations (Radio Configu- rations 3 through 6) to support rates from 0.6875 Kbps to 1.0368 Mbps. It can also support three different frame duration: 20 ms, 40 ms, and 80 ms. Readers are referred to Section 2.1.3.8.2 of [2] for an exhaustive list of radio configurations, cor- responding data rates, and frame formats supported by the R-SCH.
3.5
Channel Structure
After a physical channel generates a frame, then the physical layer performs the usual functions such as:
• Adding the CRC bits for detecting frame errors, • Coding the bits for correcting bit errors,
• Interleaving for combating fades,
These functions are similar to those of IS-95. After block interleaving, the sym- bols of a physical channel undergo a channel gain. The gain is determined by reverse power control. Figure 3.11 depicts a general block diagram up to right before modulation for enhanced access channel, reverse common control channel, reverse dedicated control channel, reverse fundamental channel, and reverse supplemental channel. The figure shows the function that is unique to IS-2000 and corresponds to Spreading Rate 1.
3.6
Modulation
After being applied a channel gain, the symbol stream of a physical channel (except the reverse pilot channel) is multiplied and spread by its assigned Walsh code for channelization. The Walsh code runs at the chip rate of 1.2288 Mcps for Spreading Rate 1.
After channelization, the chip streams of the reverse pilot channel, reverse dedi- cated control channel, and the second reverse supplemental channel are added together, and the chip streams of the reverse fundamental channel and enhanced access channel, reverse common control channel, or the first reverse supplemental channel are added together. See Figure 3.12. These two summations undergo another layer of spreading by a pair of spreading codes: sIand sQ. These two spread-
ing codes are derived from the long PN code, which in turn is derived from the mobile’s unique identity5. To reiterate, the Walsh codes provide channelization of
the different physical channels transmitted by the mobile, while the long PN code provides identification of the mobile to the base station. Figure 3.12 depicts how the two summations of the physical channels are spread and fed into two separate BPSK modulators. 3.6 Modulation 51 Block interleaver Channel gain Modulation symbols Z
Figure 3.11 Conceptual block diagram of enhanced access channel, reverse common control channel, reverse dedicated control channel, reverse fundamental channel, and reverse supplemen- tal channel.
5. These spreading codes are derived from the long PN code, in contrast to the spreading codes used on forward link that are short PN codes.
Figure 3.12 shows that the chip stream of any particular physical channel is always fed into both BPSK modulators. For example, the reverse dedicated control channel is transmitted by both BPSK signals. The two BPSK signals are two inde- pendent signals separated by phase. This method of using two independent BPSK signals to transmit a single physical channel is similar to that used by IS-95 forward link.
3.7
Capacity Gain: Reverse Link
One reason IS-2000 has a higher physical layer capacity than IS-95 is because IS-2000 uses coherent BPSK whereas IS-95 uses 64-ary orthogonal modulation. See Chapter 5 of [3] for a review of 64-ary orthogonal modulation. Because IS-2000 now has a pilot on the reverse link, it is able to coherently demodulate BPSK signals. For a given probability of bit error, coherent BPSK requires less Eb/N0than 64-ary
orthogonal modulation, and a lower required Eb/N0means higher RF capacity in a
direct sequence spread spectrum system.
In addition, the existence of a pilot on the reverse link allows Walsh codes to be used for channelization. This is because the reverse pilot channel enables the base station to synchronize the Walsh codes so that they are orthogonal to each other
Z wifor R-DCCH (R-DCCH) Z wifor R-SCH 2 (R-SCH 2) (R-PICH) Z wifor R-FCH (R-FCH) Z wifor R-EACH, R-CCCH, or R-SCH 1 (R-EACH), (R-CCCH), or (R-SCH 1) sI sQ sQ sI BF BF BF: Baseband filter cos(2πf tc) sin(2πf tc) Y t( ) + − + +
Figure 3.12 Modulation: Reverse link. (After: [2]. See Chapter 9 for the assigned Walsh codes of physical channels.)
with respect to a common start time; the maintenance of orthogonality is essential for channelization. The use of Walsh codes for channelization also means that IS-2000 can now have multiple channels active at the same time (whereas IS-95 can only have one channel active at a time) on the reverse link.
References
[1] TIA/EIA-95-B, Mobile Station-Base Station Compatibility Standard for Wideband Spread Spectrum Cellular Systems, Telecommunications Industry Association, March 1999. [2] TIA/EIA/IS-2000.2-A, Physical Layer Standard for cdma2000 Spread Spectrum Systems,
Telecommunications Industry Association, March 2000.
[3] Yang, S. C., CDMA RF System Engineering, Norwood, MA: Artech, 1998.
Selected Bibliography
TIA-97, Recommended Minimum Performance Standards for cdma2000 Spread Spectrum Base Stations, Telecommunications Industry Association, February 2003.
TIA-98, Recommended Minimum Performance Standards for cdma2000 Spread Spectrum Mobile Station, Telecommunications Industry Association, February 2003.
C H A P T E R 4