Cryptography – Communication system using cryptography – Variable time delay modulation
Reexamination Certificate
1998-11-12
2002-05-14
Decady, Albert (Department: 2767)
Cryptography
Communication system using cryptography
Variable time delay modulation
C380S031000, C380S038000, C455S103000, C455S454000, C370S209000, C375S246000
Reexamination Certificate
active
06389138
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of The Invention
This invention relates to wireless communications and, more particularly, to a system for generating a complex scrambling code sequence.
2. Description of Related Art
FIG. 1
depicts a schematic diagram of a portion of a typical wireless communications system
10
, which provides wireless communications service to a number of wireless units
12
a-c
, such as mobile or fixed units, that are situated within a geographic region. The heart of a typical wireless communications system is a Mobile Switching Center (“MSC”)
14
, which might be known also as a Wireless Switching Center (“WSC”) or a Mobile Telephone Switching Office (“MTSO”). Typically, the Mobile Switching Center
14
is connected to a plurality of base stations, such as base stations
16
a-c
, that are dispersed throughout the geographic area serviced by the system and to the local and long-distance telephone offices, such as local-office
18
, local-office
20
and toll-office
22
). The Mobile Switching Center
14
is responsible for, among other things, establishing and maintaining calls between the wireless units and calls between a wireless unit and a wireline unit (e.g., wireline unit
24
), which wireline unit is connected to the Mobile Switching Center
14
via the local and/or long-distance networks.
The geographic area serviced by a wireless communications system is divided into spatially distinct areas called “cells.” As depicted in
FIG. 1
, each cell is schematically represented by one hexagon in a honeycomb pattern; in practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell and other factors. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless units in that cell and also comprises the transmission equipment that the base station uses to communicate with Mobile Switching Center
14
. For example, when wireless terminal
12
b
desires to communicate with wireless unit
12
c
, wireless unit
12
b
transmits the desired information to base station
16
c
, which relays the information to Mobile Switching Center
14
. Upon receipt of the information, and with the knowledge that it is intended for wireless unit
12
c
, Mobile Switching Center
14
then returns the information back to base station
16
c
, which relays the information, via radio, to wireless unit
12
c.
In a spread spectrum wireless communications system, the information signal or baseband data sent between the base station
16
c
and the mobile unit
12
c
is multiplied by a spread spectrum signal. Certain spread spectrum systems, such as code-division multiple access (CDMA) systems, spread and/or scramble the baseband data information signal by multiplying the information signal with a spreading and/or scrambling code sequence (“scrambling code sequence”), such as a pseudo-noise (PN) code which is a binary sequence that appears random but can be reproduced by the intended receiving station. When the scrambling code sequence has the same pulse rate as the information signal, the product of the scrambling code sequence and the information signal is scrambled, and the spectrum is unchanged. When the scrambling code sequence has a faster pulse rate than the information signal, the product of the scrambling code sequence and the information signal has its spectrum spread in addition to being scrambled. A single pulse of the scrambling code sequence is called a chip.
FIG. 2
shows a general block diagram of a CDMA transmitter
30
and receiver
31
. The CDMA transmitter
30
spreads and/or scrambles the information or data signal to produce a spread spectrum signal for transmission, and the CDMA receiver
31
de-scrambles and/or de-spreads the spread spectrum signal to retrieve the information or data signal. At the transmitter
30
, the original data is manipulated by the coder and/or processing block
32
, which can perform speech coding, channel coding, bit interleaving, digital modulation as well as other functions, to produce the information or data signal D(t) (“information signal”). A scrambling code sequence generator
33
generates the scrambling code sequence, and a multiplier
34
multiplies the scrambling code sequence with the information signal D(t) to produce the wide band or spread spectrum information signal y(t). Modulator
35
modulates the spread spectrum information signal onto a carrier signal, for example using quadrature modulation, after which the spread spectrum signal is transmitted to the receiver
31
. At the receiver
31
, a demodulator
36
demodulates the signal transmitted from the transmitter
30
to produce the spread spectrum information signal y(t). The spread spectrum signal y(t) is multiplied by a multiplier
37
with a locally-generated version of the scrambling code sequence from a scrambling code generator
38
. The multiplication with the correct scrambling code sequence de-spreads and/or de-scrambles the spread spectrum signal y(t) and restores the information signal D(t). Multiplying the spread spectrum signal y(t) from an undesired user with the scrambling code sequence results in a small amount of noise. A decoder and deprocessing block
39
manipulates the information signal D(t) to obtain the original data.
As shown, each process in the transmitter
30
has a peer in the receiver
31
. When data is being transmitted from the base station and received by the wireless unit, the data is being sent over the forward link. When data is being transmitted from the wireless unit and received by the base station, the data is being sent over the reverse link. In current CDMA systems, there are differences between the forward link and reverse link processes as well as differences in how the scrambling code sequence is generated.
FIG. 3
shows how the reverse link scrambling code sequence is generated for the TIA/EIA-95-B standard (“IS-95B”) using a (2
42
−1) bit long code and a 2
15
bit complex short code. A long code generator
40
generates a long code sequence which is the inner product of a 42-bit user mask
41
, which is uniquely assigned to each user, and a 42-bit long code vector which is the state of a long code generator engine
42
. The long code generator engine
42
can be based on a shift register which maintains the long code vector or state of the shift register, and the mask
41
is used select bits from the long code vector which are exclusive-ord (for example, using AND gate arrangement
43
and mod
2
summer
44
) to produce the long code sequence. By performing the masking operation with the user specific mask
41
, the long code is effectively time shifted a different amount for each user to produce the long code sequence. As such, the base station can identify the particular user. In IS-95B, where quadrature spreading and modulation is used, the long code sequence is provided to a quadrature spreader
45
. Quadrature spreading ensures that other user interference appears to have random phase and amplitude.
The quadrature spreader and/or scrambler
45
multiplies long code sequence with the complex short code sequence. The complex short code sequence is generated using two independent generator polynomials of degree 15 as described in IS-95B to produce the in-phase (I) and quadrature (Q) short code sequences of the complex short code sequence. The I and Q short code sequences are chip synchronous, but otherwise independent of each other. An in-phase (I) mixer
46
effectively multiplies the long code sequence with the I short code sequence, and a quadrature mixer
47
effectively multiplies the long code sequence with the Q short code sequence. As such, the quadrature spreader
45
produces an I scrambling code sequence and a Q scrambling code sequence. In IS-95B, both the I spreading code sequence and the Q spreading code sequence are multiplied with the information signal D(t) to produce the I and Q spread spectrum signals y
i
(t) and y
q
(t) which are quadrature modulated a
Li Quinn
Ramesh Nallepilli S.
Callahan Paul E.
De'cady Albert
Garceran Julio A.
Lucent Technologies - Inc.
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