It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity. >

On the capacity of a cellular CDMA system

Spread spectrum-based code division multiple access (CDMA), has taken on a significant role in cellular and personal communications. We concentrate on direct sequence CDMA (DS/CDMA). We show that there is a natural modification of the present systems that is potentially capable of significant capacity increases. By "natural modification" we mean a modification that can be made conceptually clear, not that it is easy to implement. Indeed, the optimal multiuser detector is much too complex and most of the present research addresses the problem of simplifying multiuser detection for implementation. The objective of the article is to make the basic idea intuitive and then show how investigators are trying to reduce the idea to practice. We also indicate multiuser receiver structures with potentially acceptable levels of complexity and address potential obstacles for achieving theoretically predicted performance in practice. As a result of these investigations, an answer to the following question is expected. Is there a suboptimal multiuser detector that is cost effective to build with significant enough performance advantage over present day systems? A definitive answer is not yet available. >

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Multiuser detection for CDMA systems

A process for reducing the amount of overhead data in ATM cell headers prior to transmission both upstream and downstream on an HFC cable plant or other transmission media using SCDMA on at least the upstream path, without loss of either IP or Ethernet addressing information. A virtual link between each remote unit and a central unit is established using multiplexing to establish a virtual link to every remote unit. Downstream ATM cells are optimized by stripping off all but 2 bytes to leave a 50 byte cell. Incoming IP packets have their IP addressing information used to look up Ethernet domain address information for the RU and destination process to which this RU is coupled. An Ethernet header is appended to each IP packet as is RFC 1483 bytes to signal the start of the packet. The packet is parsed into 48 byte ATM cell payloads. VPI/VCI information for the virtual link of the remote unit to which the packet is bound is looked up using the Ethernet address information. The last cell in the packet is indicated by encoding 9th bits. The CU cable data modem formatter then generates optimized downstream 2-byte headers for each downstream ATM cell by stripping off all but 2 bytes of VPI/VCI data. 9th bits are added to each byte, and these 9th bits are encoded with last cell, normal cell and idle cell information and start codes so that cell boundaries can be located.

Lower overhead method for data transmission using ATM and SCDMA over hybrid fiber coax cable plant

A method and apparatus for carrying out synchronous co-division multiple access (SCDMA) communication of multiple channels of digital data over a shared transmission media such as a cable television system coaxial cable, a fiber optic or copper conductor telephone link, terrestial microwave, satellite link, local or wide area network, wireless including cellur network or some combination of these media using suitable interface circuitry. The system includes modems at remote units and a central unit to receive time division multiplexed digital data arranged into timeslots or channels and uses orthogonal codes to encode each channel of multiple data and spread the energy of each channel data over a frame of data transmitted in the code domain. Spreading the data this way makes the system less susceptible to impulse noise. Frames are synchronized as between remote and central units using a ranging scheme which is also useful in any other system transmitting data by frames in a distributed system where synchronizing the frames as between all units regardless of differences in propagation delays is necessary. Each frame in the SCDMA modulation scheme includes a gap or guardband containing no other data. The ranging process involves training each remote unit to impose enough delay prior to re-transmission of a barker code received from the CU such that a barker code sent by the RU arrives at the CU during the gap. The process of setting the delay in each RU is a trial and error process, and each RU starts the ranging process asynchronously. Contention resolution protocols such that only one RU is aligning to the gap at any particular time are taught.

Apparatus and method for digital data transmission using orthogonal codes

In asynchronous code division multiple access (A-CDMA) systems transmitting over multipath channels, both intersymbol interference (ISI) as a result of interchip interference (ICI) and multiple access interference (MAI) cannot be easily eliminated. Although it is possible to design multiuser detectors that suppress MAI and ISI, these detectors often require explicit knowledge of at least the desired users' signature waveform. Recently, Liu and Xu (see Proc. 29th Asilomar Conf. Signals, Systems and Computers, 1996) introduced a blind estimation algorithm for synchronous CDMA (S-CDMA) systems to estimate the multiuser channels. However, this algorithm cannot be directly applied to an asynchronous CDMA (A-CDMA) system. In this paper, we study a similar blind estimation scheme that provides estimates of the multiuser channels by exploiting the structure information of the data output and the users' delays. In particular, we show that the subspace of the data matrix contains sufficient information for unique determination of channels and, hence, the signature waveforms. By utilizing antenna arrays, we extended our approach to overloaded systems, where the number of users may exceed the spreading factor.

Blind multiuser channel estimation in asynchronous CDMA systems

Recent trends in digital communications are opening commercial applications to code division multiple access (CDMA). A novel access technique based on bandlimited quasi-synchronous CDMA (BLQS-CDMA) is described, showing all the advantages of synchronizing conventional direct sequence CDMA to drastically reduce the effect of self-noise. Bandlimitation is achieved with no detection loss by means of Nyquist chip shaping, leading to a simple all-digital demodulator structure. Detection losses due to imperfect carrier frequency and chip timing synchronization are analytically derived and numerical results are checked by computer simulations. Impairments due to satellite transponder distortions are evaluated. The full digital modem structure is presented, together with possible applications to mobile and very small aperture terminal (VSAT) satellite communications. >

Bandlimited quasi-synchronous CDMA: a novel satellite access technique for mobile and personal communication systems

Migration towards a full-digital implementation of modems is currently one of the main trends in transmission systems design. The authors describe a noncoherent all-digital delay lock loop (DDLL) suited for chip timing synchronization in band-limited direct sequence spread spectrum (DS/SS) systems, and they thoroughly analyze its performance. The key features of this novel scheme are represented by its low-complexity processing section together with its good tracking capability. Analytical expressions for the DDLL S-curve and steady-state timing jitter are derived and confirmed by a time-domain computer simulation. Furthermore, the Mean Time to Lose Lock (MTLL) of the loop is evaluated and some numerical results are reported. The proposed chip timing synchronization scheme reveals also an improved tracking performance when compared to the traditional analog DLL for rectangular chip DS/SS signals. >

A digital chip timing recovery loop for band-limited direct-sequence spread-spectrum signals

The authors discuss and analyze the performance of a quasi-synchronous code-division multiple-access (QS-CDMA) mobile satellite communication system. Practical network synchronization schemes cannot guarantee perfect carrier frequency and code timing alignment. Therefore, bit error rate (BER) impairment due to frequency and chip timing errors is evaluated. Theoretical results on QS-CDMA performance are confirmed by a time-domain system computer simulation. In particular, QS-CDMA sensitivity to combined frequency/timing errors is addressed. The suitability of Gold codes for the proposed QS-CDMA mobile satellite network has been demonstrated. >

Performance evaluation of quasi-synchronous code division multiple access (QS-CDMA) for satellite mobile systems

The authors propose an innovative code division multiple access (CDMA)-based mobile system for voice (and data) services. The system makes use of voice activation in order to increase the CDMA efficiency (useful transmitted information vs. occupied bandwidth). The synchronization problems are solved by distributing a master code in the forward link and using forced link activation in the return when no speech activity is detected. The blocking probability is minimized by means of multirate speech coding associated with a centralized network control. Bit error rate calculations show all the advantages of operating in a synchronous mode. >

R. Viola

Papers

Bandlimited quasi-synchronous CDMA: a novel satellite access technique for mobile and personal communication systems

A digital chip timing recovery loop for band-limited direct-sequence spread-spectrum signals

Performance evaluation of quasi-synchronous code division multiple access (QS-CDMA) for satellite mobile systems

High efficiency voice activated CDMA mobile communication system based on master code synchronization

A novel code division multiple access system for high capacity mobile communication satellites