University of Minnesota

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Chip-Interleaved Block-Spread Code Division Multiple Access

With the demand for higher data rates and wireless services rising as fast as the Internet permeated our lives, the need to cope with the idiosyncrasies of the shared wireless medium increases along with the challenges: multiuser interference (MUI), frequency-selective multipath-induced fading, time-selective mobility-induced impairments, and high-performance transceivers that are also bandwidth- and power-efficient. Today's Code-Division Multiple Access (CDMA) systems rely on long aperiodic pseudo-noise codes and strict power control to suppress MUI. On the other hand, CDMA proposals with symbol-periodic short spreading codes relax the need for strict power control at the expense of high-complexity MUI cancellation which is less affordable at the mobile unit in the downlink. But also in the uplink, the frequency-selective fading that asynchronous high-rate transmissions experience as they propagate through multipath-rich channels, degrades performance unless bandwidth is sacrificed.

Figure 1: Serial4

Fig 1. (Upper part) the serial form of the transmitted block of length M(K+L) in CIBS-CDMA versus the conventional CDMA transmission of K symbols, each spread with short code of length M (lower part)

The CDMA system of this project relies on the novel idea of judiciously spreading a block of symbols with long, yet structured and deterministic, user codes. The transmitted block is depicted in Fig. 1. On the other hand, our block-spread transmissions can be viewed as (and are implemented by) chip-interleaving of symbols spread by short codes, as demonstrated in Fig. 2. This not only explains the acronym Chip-Interleaved Block-Spread CDMA (CIBS-CDMA), but also highlights how readily implementable (and backward-compatible) the proposed system is by simply cascading to an existing DS-CDMA system a chip interleaver with guards.

Figure 2: transceiver image

Fig 2. CIBS-CDMA transceiver for a single user

Preliminary results favor the proposed CIBS-CDMA system because:

  1. it is simple with transceivers involving (de-) interleavers and low-complexity matched filters;
  2. it has built-in MUI-resilience regardless of the underlying frequency-selective (uplink or downlink) channels and without power control;
  3. surprisingly, maximum-likelihood (ML) optimal multiuser detection in the presence of multipath becomes possible with chip de-interleaving followed by single-user matched filtering followed by single-user ML decoding;
  4. it offers flexibility as it can revert to DS-CDMA, TDMA, FDMA, and Multi-Carrier (MC) CDMA hybrids;
  5. even with MC codes, the CIBS-CDMA transmissions maintain constant-modulus which translates to power efficiency ;
  6. by adjusting the amount of block spreading, CIBS-CDMA can have high bandwidth efficiency which is instrumental for enhancing system capacity.

To realize the application potential CIBS-CDMA holds for tomorrow's wireless multiple access, basic research directions are proposed. Those include: thorough performance analysis and comparisons with competing alternatives in terms of capacity and bit error rate under imperfect system parameters; mobility studies and development of timing-offset, carrier-frequency offset and channel estimation algorithms for acquisition and tracking; incorporation of multi-antenna transmissions to enhance multipath diversity with space-time diversity gains; investigation of optimal precoding, power-and-bit loading and adaptive modulation; and study of cellular system-level issues.

In addition to basic research directions, the simple transmitter-receiver structure of the proposed low-complexity CIBS-CDMA system makes experimental studies attractive and offers a wealth of educational projects. It is proposed to test the novel CIBS-CDMA system over the testbed currently under development in the Wireless Lab of the ECE Department at the University of Minnesota. Experiments will be carried out by both graduate students in DSP (MSc level), Digital and Wireless Communications (MSc/PhD level) courses, and undergraduate students in their Communications Lab Course (senior level) or their senior design project.

Our work in this project and basic references can be found in the following papers:

  1. S. Zhou, G. B. Giannakis and C. Le Martret, "Chip-Interleaved Block-Spread Code Division Multiple Access," IEEE Transactions on Communications, Feb. 2002.
  2. S. Zhou and G. B. Giannakis, "Chip-Interleaved Block-Spread Code Division Multiple Access," Proc. of 35th Conf. on Info. Sciences and Systems, The Johns Hopkins Univ., Baltimore, March 21-23, 2001.
  3. S. Zhou, G. B. Giannakis and C. Le Martret, "Low-Complexity Bandwidth-Efficient MUI/ISI-resilient CDMA based on Block-Spreading," Proc. of 38th Allerton Conf., Univ. of Illinois at U-C, Monticello, IL, Oct. 4-6, 2000.

 

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