Pulse or digital communications – Spread spectrum
Reexamination Certificate
2000-12-22
2002-09-10
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
C375S213000, C375S284000
Reexamination Certificate
active
06449302
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to code division multiple access communications systems and related methods of operation. More particularly, the present invention relates to cellular communications systems and signal processing apparatus and methods employed in cellular communications systems.
2. Background of the Prior Art and Related Information
Wireless communications systems employing transmission between base stations and multiple mobile users are a key component of the modern communications infrastructure. (Such wireless communications systems, are referred to herein as “cellular” communications systems for brevity and without limiting the term cellular to the specific types of communications systems or specific frequency bands to which the term is sometimes associated.) These cellular systems are being placed under increasing performance demands which are taxing the capability of available equipment, especially cellular base station equipment. These increasing performance demands are due to both the increasing number of users within a given cellular region as well as the bandwidth requirements for a given channel. The increasing number of cellular phone users is of course readily apparent and this trend is unlikely to slow due to the convenience of cellular phones. The second consideration is largely due to the increased types of functionality provided by cellular phone systems, such as Internet access and other forms of data transfer over the cellular phone system. These considerations have resulted in a need for more channels within the available spectrum provided to cellular phone carriers as well as more bandwidth for each channel.
The traditional approach to fitting as many channels as possible into an available frequency spectrum is to place each channel in a narrow frequency band. The individual channels must be sufficiently far apart in frequency to avoid significant interference between the individual cellular system users, however. Also, the narrower the frequency band for a given channel the less bandwidth which is available for the particular channel.
An alternative approach to providing the maximum number of channels in a given frequency spectrum, which has been adopted in more and more digital cellular systems, is code division multiple access spread spectrum communication. When digital information is transmitted from one location to another the data bits are converted to data symbols before transmission. The bandwidth of the transmitted signal is a function of the number of symbols transmitted per data bit sent. In code division multiple access spread spectrum communication, more symbols are transmitted than the data bits to be sent. In particular, for each data bit to be sent a multi symbol code is transmitted. The receiver, knowing the code, decodes the transmitted signal recovering the data bits sent. With a suitable choice of unique codes, many users can communicate in the same bandwidth without interference since each channel is orthogonal through coding. In code division multiple access spread spectrum cellular systems the spreading code is typically chosen to spread the data from an individual channel across a relatively wide frequency spectrum, within of course the spectrum range available to the given cellular provider. This minimizes interference between channels and maximizes the number of channels in the available frequency spectrum. Currently, two standards exist which relate to code division multiple access cellular communications systems. These standards are commonly known as CDMA and WCDMA for Code Division Multiple Access and Wide Code Division Multiple Access. Due to the highly effective use of the available frequency spectrum CDMA and WCDMA are increasingly being adopted as the solution of choice to accommodate increased cellular use.
A problem exists, however, with the practical implementation of spread spectrum cellular systems due to the manner in which the multiple user channels are combined to create the spread spectrum signal. This may be appreciated by referring to
FIG. 1
which illustrates spread spectrum signal generation in a typical prior art cellular base station implementation. As shown in
FIG. 1
, in a spread spectrum system, a code-multiplexed signal generator
10
receives a plurality of data channels D, e.g., n in number, corresponding to the number of users which can be accommodated. A train of symbols is created for each communication channel by multiplying the input symbols for each channel by a separate orthogonal code. The amplitude of each channel may differ based on individual channel power needs. Each symbol train is then added to create a single code multiplexed symbol train (having in-phase and quadrature components, V
1
and V
2
in FIG.
1
). The code multiplexed symbol train is then passed through a filter
20
to create the desired output signal. This filter plays a critical role since it imposes a “spectral mask” over the symbol train that ensures the broadcast signals stay within the spectrum allocated to the cellular carrier. Failure to observe such limitations on spectrum allocation can violate federal regulations as well as causing noise in neighboring bands of a given carrier. The output signal is then provided to a digital to analog converter
30
resulting in an analog signal that is mixed with a carrier signal in a modulator
40
. The resulting RF signal is provided to an RF power amplifier
50
and broadcast to the cellular users.
The problem begins in the combining of the multiple symbol train in the code multiplexor
10
in FIG.
1
. Since many individual symbol trains are combined, the peak power of the overall signal output from the filter will depend on the individual amplitudes of the symbols being combined. It is statistically possible that the individual channel symbols will add to create very large combined symbol peaks. Although statistically not common, such very large symbol peaks must be accommodated in the overall system design. Accommodating such large symbol peaks in the overall system creates practical implementation problems. For example, the presence of potentially very large peaks in the signal being output from the filter to the digital-to-analog converter requires a very high resolution digital-to-analog converter to be used. This adds cost and complexity to the overall system.
Another problem associated with potentially very large signal peaks in a code division multiple access spread spectrum system relates to the difficulty of providing linear amplification of the signal by the RF power amplifier. In cellular systems, it is very important to provide linear amplification of the broadcast signal. This is the case since non-linear amplification of the signal can result in distortion in the signal as well as creation of spectral sidebands that can interfere with other cellular frequency bands. Since cellular frequency bands are strictly regulated, cellular systems must be carefully designed so that such creation of noise outside of the allocated frequency band is avoided. Therefore, linear RF amplification is necessary in cellular base stations. To operate an amplifier in its linear range, however, requires that the amplifier be operated in a relatively low power mode. If large random peaks in the signal are to be accommodated by such an amplifier and still keep it operating in the linear regime, a higher power RF amplifier is required. High power, high quality RF amplifiers are very expensive and this thus adds significant cost to the overall base station system.
The problem of large random peaks in the signal is therefore a significant problem in the practical implementation of spread spectrum cellular communications systems.
The significance of the problem of large random signal peaks has been appreciated in the prior art and solutions to this problem have been attempted. For example, an approach to solving this problem is described in U.S. Pat. No. 6,009,090 to Oishi, et al. The approach of the '090 patent is ill
Al-Beshrawi Tony
Myers Dawes & Andras LLP
Pham Chi
Powerwave Technologies Inc.
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