Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-01-12
2004-10-26
Tran, Khai (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06810070
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the use of Code Division Multiple Access (CDMA) communication techniques in radio communication systems, and more particularly to systems and methods for selecting the number of carriers for a Direct Sequence Spread Spectrum Multiple Carrier (DS-SS MC) CDMA communication signal using a characteristic of a selected communication channel.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs, to maintain high quality service, and to avoid rising prices.
Throughout the world, one important step in the advancement of radio communication systems is the change from analog to digital transmission. Equally significant is the choice of an effective digital transmission scheme for implementing the next-generation technology. Furthermore, it is widely believed that Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to access networks to transmit voice, data, and/or video from the home, office, street, car, etc. will be provided by cellular service providers using a digital cellular system infrastructure. An important feature desired in these new systems is increased traffic capacity.
Wireless communication systems transmit communication signals on one or more carrier waves. As used herein, the term “signal” refers to an electrical wave, either analog or digital, that is used to convey information, and the term “communication signal” refers to a signal that conveys user information such as, for example, voice, video, or data information. As used herein, the term “carrier” is used to refer to a radio frequency (RF) wave generated at a transmitting station for the purpose of carrying a signal, which may be a communication signal.
In wireless communication systems, the term “channel” refers to an electromagnetic communication path between a transmitter and one or more receivers. In many existing radio communication systems, channel access is achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In FDMA, a channel is a single radio frequency band within a given frequency spectrum into which a communication signal's transmission power is concentrated. Signals that can interfere with such a communication channel include those transmitted on adjacent channels (adjacent channel interference) and those transmitted on the same channel (co-channel interference). Interference from adjacent channels is limited by the use of band-pass filters that filter out energy outside the specified frequency band.
In TDMA systems, a channel comprises, for example, a time slot in a periodic train of time slots of a carrier having a given frequency. These time slots may be organized into groups called frames. A given user's signal energy is confined to one or more of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, with each channel being assigned a different time slot, system capacity is limited by the available time slots as well as by limitations imposed by channel reuse as described above with respect to FDMA.
With FDMA and TDMA systems (as well as hybrid FDMA/TDMA systems), one goal of system designers is to ensure that two potentially interfering signals do not occupy the same time and frequency. In contrast, Code Division Multiple Access (CDMA) allows communication signals to overlap in both time and frequency, while communication channels are defined by an encoding scheme, as discussed below. CDMA is a type of spread spectrum communication that has been around since the days of World War II. Early applications were predominantly military oriented. However, today there has been an increasing interest in using spread spectrum systems in commercial applications since spread spectrum communication can be more robust against interference, allowing more signals to occupy the same bandwidth at the same time. Examples of commercial applications include digital cellular radio, land mobile radio, and indoor and outdoor personal communication networks.
In a CDMA system, an electrical signal embodying an informational data stream (e.g., digitized voice, data, video) to be transmitted is combined with an electrical signal embodying a higher bit rate data stream known as a signature sequence, or spreading sequence, to produce a spread spectrum signal. Each bit of the signature sequence is referred to as a “chip”, and the frequency of the electrical signal embodying the signature sequence is referred to as the “chip rate”. The ratio of the chip rate to the frequency of the electrical signal embodying the informational data stream is generally referred to in the art as the “spreading ratio”.
In an exemplary CDMA system, a spread spectrum signal may be generated by multiplying an electrical signal embodying an informational data stream and an electrical signal embodying a unique signature sequence. The information required to decode the spread spectrum signal (e.g., the unique signature sequence) may be transmitted to an intended receiver over a separate communication channel (e.g., a pilot channel or a control channel). Using this information, the intended receiver can extract the informational data stream from the spread spectrum signal, thereby establishing a communication channel with the transmitter.
In a wireless CDMA system, a plurality of spread spectrum signals may be combined at a transmitter to form a composite signal which modulates a radio frequency carrier, for example by binary phase shift keying (BPSK). In the composite signal, each of the spread spectrum signals overlaps all of the other spread spectrum signals in the time domain and the frequency domain. At an intended receiver, the composite signal is correlated with a signature sequence uniquely identifying one of the electrical signals embodying the informational data stream, such that the electrical signal embodying the desired informational data stream can be isolated and despread.
Traditionally, a signature sequence is used to spread one bit of information. Receiving the transmitted sequence or its complement indicates whether the information bit is a+1 or −1, sometimes denoted “0” or “1”. The signature sequence usually comprises G chips per information bit. The signature sequence may consist of complex numbers (having real and imaginary parts), where the real and imaginary parts are used to modulate two carriers at the same frequency, but ninety degrees different in phase. The entire G-chip sequence, or its complement, is referred to as a transmitted symbol. The conventional receiver, e.g., a rake receiver, correlates the received signal with the complex conjugate of the known signature sequence to produce a correlation value. If BPSK modulation is used, only the real part of the correlation value may be computed. When a large positive correlation results, a “0” is detected; when a large negative correlation results, a “1” is detected.
The “information bits” referred to above can also be coded bits, where the code used is one or more of a block or convolutional code or an orthogonal code. Also, the signature sequence can be much longer than a single transmitted symbol, in which case a subsequence of the signature sequence may be used to spread the information bit. In many radio communication systems, the received signal includes two components: an I (in-phase) component and a Q (quadrature phase) component. This occurs because the transmitted signal has two components (e.g., quadrature phase shift keying, QPSK), and/or the inter
Coats & Bennett, P.L.LC.
Ericsson Inc.
Tran Khai
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