Space Time Spreading

STS is a technique by which the base station duplicates multiple copies of the symbol stream and transmits them through multiple antennas. A common configuration is to duplicate two copies (1 and 2) and to transmit them through two antennas. STS is a transmit diversity scheme that was proposed in [15] and later adopted by the IS-2000 standard [13]. Figure 9.10 shows the concept of STS.

As shown in Figure 9.10, the interleaved symbol stream is split into two streams: odd and even. The odd and even streams are combined to form two identical paths 1

Demux odd even w_ior QOF p w_ior QOF Y t₁( ) Y t₂( ) Σ Σ 1 2

and 2. Path 1 is channelized by one Walsh code, and path 2 is channelized by another (complementary) Walsh code to maintain orthogonality between the two paths. Quasi-orthogonal functions can also be used. Both channelized paths are spread by a short PN code6_{and then transmitted by two different antennas.}

Figure 9.11 shows the implementation of OTD in IS-2000. Here the channel structure is for the forward dedicated control channel, forward fundamental chan- nel, and forward supplemental channel7 _{(same as that shown in Figure 2.14 of}

Chapter 2) except that the symbol stream is split into four different streams: Iodd,

Ieven, Qodd, and Qeven. For path 1 (top complex modulator), the combination of Iodd

and Ieven streams forms the in-phase input, and the combination of Qoddand Qeven

streams forms the quadrature input. For path 2 (bottom complex modulator), the combination of Ioddand Ievenstreams forms the in-phase input, and the combination

of Qoddand Qevenstreams forms the quadrature input. The top complex modulator

generates the path 1 transmit signal Y1(t) for one antenna. The bottom complex

modulator generates the path 2 transmit signal Y2(t) for another antenna. This way,

each path effectively transmits all symbol streams.

Each complex modulator uses a complementary Walsh code but the same short PN codes. The internal structure of the complex modulator is identical to that used by IS-2000 forward link (see Figure 2.15 in Chapter 2).

9.5 Transmit Diversity 155 I_odd Block interleaving Long code scrambling Channel gain Modulation symbols Power control puncturing

Power control bits

Demux

Long code mask

Q_even I_even Qodd I_odd Iodd Qodd Q_odd Qeven Qeven Ieven I_even wior QOF wior QOF pI Y t1( ) Y t₂( ) Complex modulator Σ Σ Σ Σ Complex modulator pQ

Figure 9.11 Implementation of STS in IS-2000. For brevity, symbol repetition is not shown. 6. In actuality there are two short PN codes: pIand pQ. See Chapter 2.

To support transmit diversity, the base station sector also transmits two pilot channels (F-PICH and F-TDPICH), one for each transmit antenna, to perform chan- nel estimation for and coherent detection of the two signals. At the receiver, the two signals are despread using the same two Walsh codes (used at the transmitter) and combined to recover the original symbol stream [14].

Studies have shown that STS outperforms OTD by about 0.3 to 2.4 dB (e.g., [13]) depending on the speed of the mobile (transmit diversity performs best when the mobile is stationary).

9.5.3 Concluding Remarks

Based on transmitter structures shown in Figures 9.9 and 9.11, it is clear that both OTD and STS require modifications of the receiver to support these transmit diver- sity schemes. Therefore, the mobile needs to inform the base station whether or not the mobile’s receiver is capable of supporting transmit diversity. The mobile does so by using OTD_SUPPORTED and STS_SUPPORTED fields in messages such as registration message, origination message, and page response message.

References

[1] TIA/EIA/IS-2000.2-A, Physical Layer Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000.

[2] TIA/EIA/IS-2000.5-A, Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems, Telecommunications Industry Association, March 2000.

[3] Yang, S. C., CDMA RF System Engineering, Norwood, MA: Artech House, 1998. [4] Jalloul, L. M. A., and A. Shanbhag, “Enhancing Data Throughput Using Quasi-Orthogonal

Functions Aggregation for 3G CDMA Systems,” Proceedings of Vehicular Technology Conference, 2002, pp. 2008–2012.

[5] Berrou, C., A. Glavieux, and P. Thitimajshima, “Near Shannon Limit Error-Correcting Coding and Decoding: Turbo Codes,” Proceedings of IEEE International Conference on Communication, 1993, pp. 1064–1070.

[6] Berrou, C., and A. Glavieux, “Near Optimum Error Correcting Coding and Decoding: Turbo-Codes,” IEEE Trans. Comm., October 1996, pp. 1261–1271.

[7] Rothweiler, J., “Turbo Codes,” IEEE Potentials, February/March 1999, pp. 23–25. [8] Sklar, B., “A Primer on Turbo Code Concepts,” IEEE Communications, December 1997,

pp. 94–102.

[9] Ryan, W. E., “Concatenated Convolutional Codes and Iterative Decoding,” In J. G. Proakis (Ed.), Wiley Encyclopedia of Telecommunications, Hoboken, NJ: Wiley, 2002.

[10] Zhang, P., and P. Luukkanen, “Performance of Turbo Codes in 3rd Generation cdma2000 Mobile System,” Proceedings of Vehicular Technology Conference, 1999, pp. 1674–1677. [11] Walton, R., M. Wallace, and S. Howard, “CDMA Downlink Performance Issues,” Pro-

ceedings of the Ninth IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, 1998, pp. 308–312.

[12] Weerackody, V., “Diversity for Direct-Sequence Spread Spectrum System Using Multiple Transmit Antennas,” Proceedings of IEEE International Conference on Communication, 1993, pp. 1775–1779.

[13] Wu, G., et al., “Performance Evaluation of Space-Time Spreading and Orthogonal Transmit Diversity in CDMA2000,” Proceeding of IEEE Seventh International Symposium on Spread Spectrum Techniques and Applications, 2002, pp. 323–327.

[14] Derryberry, R. T., et al., “Transmit Diversity in 3G CDMA Systems,” IEEE Communica- tions, Vol. 40, No. 4, 2002, pp. 68–75.

[15] Papadias, C., et al., “Space-Time Spreading for CDMA Systems,” Proceeding of Sixth Workshop Smart Antennas Wireless Mobile Communications, July 22–23, 1999, Stanford, CA.

Selected Bibliography

Hanzo, L., T. H. Liew, and B. L. Yeap, Turbo Coding, Turbo Equalisation and Space-Time Cod- ing for Transmission over Fading Channels, New York: John Wiley & Sons, 2002.

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