Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1997-12-09
2003-10-28
Olms, Douglas W. (Department: 2732)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S335000, C370S441000, C375S213000, C375S213000
Reexamination Certificate
active
06639906
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to digital wireless communications. More particularly, the present invention relates to a novel and improved demodulator for processing a set of user signals that facilitates implementation on a single integrated circuit.
II. Description of the Related Art
FIG. 1
is a block diagram of a highly simplified cellular telephone configured in accordance with the use of a Code Division Multiple Access (CDMA) over-the-air interface. In particular,
FIG. 1
illustrates a cellular telephone system configured in accordance with the use of the IS-95 standard, which uses CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. IS-95, and its derivatives such as IS-95A and ANSI J-STD-008 (referred to herein collectively as IS-95), are promulgated by the Telecommunication Industry Association (TIA) as well as other well known standards bodies. Additionally, a cellular telephone system configured substantially in accordance with the use of IS-95 is described in U.S. Pat. No. 5,103,459 entitled “System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System” assigned to the assignee of the present invention and incorporated herein by reference.
A primary benefit of using a CDMA over-the-air interface is that communications are conducted over the same RF band. For example, each mobile unit
10
(typically cellular telephones) shown in
FIG. 1
can communicate with a same base station
12
by transmitting a reverse link signal over the same 1.25 MHz of RF spectrum. Similarly, each base station
12
can communicate with mobile units
10
by transmitting a forward link signal over another 1.25 MHz of RF spectrum. Transmitting signals over the same RF spectrum provides various benefits including an increase in the frequency reuse of a cellular telephone system, and the ability to conduct soft handoff between two or more base stations. Increased frequency reuse allows a greater number of calls to be conducted over a given amount of spectrum. Soft handoff is a robust method of transitioning a mobile unit from the coverage area of two or more base stations that involves simultaneously interfacing with two base stations. Soft handoff can be contrasted with hard handoff where the interface with a first base station is terminated before an interface with a second base station is established.
During typical operation of the cellular telephone system of
FIG. 1
, a base station
12
receives a set of reverse link signals from a set of mobile units
10
. The mobile units
10
are conducting telephone calls or other communications. Each reverse link signal is processed within base stations
12
, and the resulting data forwarded to base station controller (BSC)
14
. BSC
14
provides call resource allocation and mobility management functionality including the orchestration of soft handoffs between base stations. BSC
14
also routes the data received to mobile switching center (MSC), which provides additional routing services for interface with the conventional public switch telephone system (PSTN).
A portion of a prior art base station configured to processing a set of reverse link signals from a set of mobile units
10
is shown in FIG.
2
. During operation, antenna system
40
receives a set of reverse link signals transmitted in the same RF band from the set of mobile units
10
in the associated coverage area. RF receiver
42
downconverts and digitizes the set of reverse link signals yielding digital samples that are received by cell site modems (CSMs)
44
. Each CSM
44
is allocated by controller
46
to processes a particular reverse link signal from a particular mobile unit
10
, and each generates digital data that is forwarded to BSC
14
. A system and method for implementing each CSM on a single integrated circuit is described in U.S. Pat. No. 5,654,979 entitled “Cell Site Demodulator Architecture for a Spread Spectrum Multiple Access Communication System” and copending U.S. application Ser. No. 08/316,177 entitled “Multipath Search Processor For A Spread Spectrum Multiple Access Communication System,” both assigned to the assignee of the present invention and incorporated herein.
In general, a base station must be capable of interfacing with between sixteen and sixty-four mobile units simultaneously in order to provide adequate capacity for a typical urban appellation. This in turn, requires each base station
12
to contain between 16 and 64 CSMs. While base stations using between 16 and 64 CSMs have been implemented and deployed on a wide scale, the cost of such base stations is relatively high. One of the main causes of this cost is the complex and somewhat sensitive interconnects from the RF unit to the various CSMs, and the interconnects between the base stations controllers and the CSMs. Typically, a subset of twenty-four (24) to thirty-twotwenty-six (3226) or so CSMs are placed on a circuit board, and a set of circuit boards are coupled via a backplane, which in turn is coupled to an RF unit using sets of coaxial cables. Such interconnecting is expensive, and somewhat unreliable, and contributes substantially to the overall cost, complexity and maintenance of a base station
12
. Therefore, such a configuration is highly undesirable. The present invention is directed to a method and apparatus for processing a set of reverse link signals received from a set of mobile units without the need for a large set of cell site modems.
FIG. 3
is a block diagram illustrating the signal processing used to transmit a single reverse link traffic channel in accordance with the IS-95 standard provided to facilitate understanding of the invention. Data
48
being transmitted is provided to convolutional encoder
50
in 20 ms segments, called frames, at one of four rates referred to as “full rate”, “half rate”, “quarter rate”, and “eighth rate” respectively, as each frame contains half as much data as the previous and therefore transmits data at half the rate. Data
48
is typically variable rate vocoded audio information where lower rate frames are used when less information is present, such as during a pause in a conversation. Convolution encoder
50
convolutionally encodes data
48
producing encoded symbols
51
, and symbol repeater
52
generates repeated symbols
53
by symbol repeating encoded symbols
51
by an amount sufficient to generate a quantity of data equivalent to a full rate frame. For example, three additional copies of a quarter rate frames are generated for a total of four copies while no additional copies of a full rate frame are generated.
Block interleaver
54
then block interleaves the repeated symbols
53
to generate interleaved symbols
55
. Modulator
56
performs 64-ary modulation on interleaved symbols
55
to produce Walsh symbols
57
. That is, one of sixty-four possible orthogonal Walsh codes, each code consisting of sixty-four modulation chips, is transmitted for every six interleaved symbols
55
. Data burst randomizer
58
performs gating, using frame rate information, on Walsh symbols
57
in pseudorandom bursts such that only one complete instance of the data is transmitted. The gating is performed in increments of six Walsh symbols referred to as “power control groups,” because a power control command is generated at the base station every corresponding period. Sixteen power control groups occur for each 20 ms frame, with all sixteen being transmitted for a full rate frame, eight for a half rate frame, four for a quarter rate frame and two for an eighth rate frame. For each lower rate frame the power control groups transmitted are a subset of the groups transmitted for a higher rate frame.
The gated Walsh chips are then direct sequence modulated using a pseudorandom (PN) long channel code
59
at rate of four long channel code chips to each Walsh chip generating modulated data
61
. The long channel code is unique for each mobile unit
10
and is known by each base station
12
. Modulated data
61
is duplicated
Brown Charles D.
Edwards Christopher O.
Hom Shick
Olms Douglas W.
Wadsworth Philip R.
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