Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2001-05-31
2003-10-21
Corrielus, Jean B. (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06636556
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to spread-spectrum communications, and more particularly to a coding technique for a large area spread-spectrum CDMA system.
DESCRIPTION OF THE RELEVANT ART
The growing popularity of personal communication services coupled with the scarcity of radio bandwidth resources has resulted in the ever-increasing demand for higher spectral efficiency in wireless communications. Spectral efficiency refers to the maximum number of subscribers that can be supported in a cell or sector under a given bandwidth allocation and transmission rate requirement. The unit of spectral efficiency is the total transmission rate per unit bandwidth within a given cell or sector. Obviously, the better the spectral efficiency is, the higher the system capacity will be.
Traditional wireless Multiple Access Control (MAC) systems, such as a Frequency Division Multiple Access (FDMA) system, a Time Division Multiple Access (TDMA) system, result in system capacity that is limited by the time-bandwidth (TB) product. It is impossible to increase the number of supportable subscribers under these MAC schemes. For example, assume that the basic transmission rate of a subscriber is 1/T samples per second and the allocated bandwidth is B Hz. Then, the time-bandwidth product is BT, which is the maximum number of supportable subscribers. It is impossible to support more than BT subscribers in FDMA and TDMA systems.
The situation is completely different under a Code Division Multiple Access (CDMA) system where the system capacity only depends on the Signal-to-Interference Ratio (SIR). Increasing the number of subscriber reduces the SIR, thus lowering the transmission rate. However, a subscriber will not be denied radio resource allocation. In other words, unlike FDMA and TDMA systems, a CDMA system does not have a hard upper bound (i.e. BT) on the number of supportable subscribers.
The capacity of a CDMA system depends on the interference level. As such, the ability to accurately control the interference level is critical to the performance and the successful operation of a CDMA system. There are four sources of interference in a CDMA system: The first type of interference, or noise, comes from various sources in the local environment, which cannot be control by the wireless communication system. The only way to alleviate noise interference is to use a low noise amplifier. The second type of interference is Inter-Symbol-Interference (ISI). The third type of interference is Multiple Access Interference (MAI) that is originated from other subscribers in the same cell. The forth type of interference is Adjacent Channel or Cell Interference (ACI) that is originated from other subscribers in the neighboring channel or cell. It is possible to reduce or eliminate ISI, MAI, and ACI by using high performance codes.
In a CDMA system, each subscriber has his/her own unique identification code. In addition, the subscribers' spread-spectrum codes are orthogonal to each other. The orthogonality requirement is common to all multiple access schemes. If the communication channel is an ideal linear time and frequency non-dispersion system, and the system has high degree of synchronization, then the subscribers will stay orthogonal to each other. In reality, the communication channel is not ideal, and it is very difficult to achieve tight synchronization for communication channels with time and frequency dispersion. As a result, the ability to achieve orthogonality in a non-ideal communication channel with time and frequency dispersion is critical to the successful operation of CDMA systems.
It is commonly known that a mobile communication channel is a typical random time varying channel, with random frequency dispersion, due to Doppler shift effect, and random time dispersion, due to multi-path transmission effect. Random frequency dispersion results in the degradation in time selectivity of the received signal with unexpected fluctuation of the reception power level. Random time dispersion results in the degradation in frequency selectivity, which results in the unexpected variation in the reception level within each frequency component. This degradation results in reduced system performance and significantly lowers the system capacity. In particular, because of the time dispersion of the transmission channel, as a result of multi-path transmission, different signal paths do not arrive at the receiver at the same time. This results in the overlapping of neighboring symbols of the same subscriber and causes Inter Symbol Interference (ISI). On the other hand, the time dispersion of the channel worsens the multiple access interference. When the relative delay of signals of different subscribers are zero, any orthogonal code can achieve orthogonality. However, it is very hard to maintain orthogonality if the relative delay of signals of subscribers is not zero.
In order to reduce ISI, the auto-correlation of each subscriber's access codes must be an ideal impulse function that has all energy at the origin, nowhere else. To reduce the MAI, the cross-correlations between multiple access codes of different subscribers must be zero for any relative delay. In the terms of orthogonality, each access code must be orthogonal to itself with non-zero time delay. The access codes must be orthogonal to each other for any relative delay, including zero delay.
For simplicity, the value of an auto-correlation function at the origin is called the main lobe and the values of auto-correlations and cross-correlations at other points are called side lobes. The correlation functions of ideal multiple access codes should have zero side lobes everywhere. Unfortunately, it is proved in Welch theory that there does not exist any ideal multiple access codes in the field of finite elements and even in field of complex numbers. The claim that ideal multiple access codes do not exist, is called the Welch bound. Especially, the side lobes of auto-correlation function and the side lobes of cross-correlation function are contradicted to each other; as side lobes of one correlation function become small, the side lobes of the other correlation function become big. Furthermore, NASA had done brute force searching, by using a computer, to search for all ideal codes. However, there has not been a breakthrough. Since then, not much research work has been done on the search of the ideal multiple codes.
NASA searched for the good access codes in the Group codes and the Welch bound in the sub-fields of complex numbers. Beyond the field of complex numbers, the ideal codes could exist. For example, B. P. Schweitzer found an approach to form ideal codes in his Ph.D thesis on “Generalized complementary code sets” in 1971. Later, Leppanen and Pentti (Nokia Telecommunication) extended Dr. Schweitser's results in the mixed TDMA and CDMA system. They broke the Welch bound in the high dimensional space. However, the utilization of frequency is very low and thus there is no practical value. There has not been any application of their invention in nearly 30 years. According to their invention, in a system of N multiple access codes, there requires at least N
2
basic codes. Each basic code has length at least N chips. That means it needs N
3
chips to support N addresses. For example, when N=128, with 16 QAM modulation, the coded spectral efficiency is only log
2
16×128/128
3
=2.441×10
−4
bits/Hz. The more access codes, the lower the utilization of the spectral efficiency. This coding methodology reminds us that ideal multiple access codes can be achieved via complementary code sets. We should, however, avoid that the code length grows too fast with the required number of multiple access codes.
In addition, with technique of two-way synchronization, the relative time delay within each access code or between each other in a random time varying channel will not be greater than the maximum time dispersion of the channel plus the maximum timing error. Assuming that value is &Dgr; second, so long
Corrielus Jean B.
David Newman Chrtd.
LinkAir Communications, Inc.
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