Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-10-27
2003-11-04
Appiah, Charles (Department: 2682)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S335000, C375S149000
Reexamination Certificate
active
06643280
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to wireless communication systems, and more particularly, to methods and apparatus for generating CDMA long codes in a wireless communication system.
BACKGROUND OF THE INVENTION
Code division multiple access (CDMA) techniques have been employed in many digital wireless communication systems to permit a large number of system users to communicate with one another. Many communication systems utilizing code division multiple access (CDMA) techniques comply with the IS-95 standard, adopted by the Telecommunication Industry Association (TIA). Under the IS-95 standard, a communication system modulates the information signals with Walsh orthogonal function sequences. To produce corresponding orthogonal information signals, these CDMA systems require that the forward link information signals be transmitted in a synchronized manner. A more detailed discussion of the IS-95 standard is provided in “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” Telecommunication Industry Association Doc. No. TIA/EIA/IS-95 (1993), incorporated by reference herein.
Wireless communications systems, including CDMA systems, are increasingly utilized to transmit voice and data communications. A mobile station desiring a connection to a called party employs an access channel to supply the base station with the dialing number of the called party and an identification of the mobile station (calling party). Thereafter, traffic channels are employed for communications between the calling and the called parties by means of the base station. Generally, users do not want the content of the communication to be publicly available. Without proper cryptographic precautions, however, the communications in a wireless system can be intercepted. Thus, communications in CDMA systems and other wireless systems are typically encrypted to prevent an eavesdropper from accessing the transmission.
FIG. 1
illustrates the manner in which speech signals are encrypted in a conventional CDMA system. As shown in
FIG. 1
, at the transmitter
100
, user data is encrypted by a scrambler/spreader
130
using a short code and a long code produced by a short and a long code generator
110
,
120
, respectively. At the receiver
150
, the encrypted signal is operated on by the inverse process, using the same short and long code to reproduce the original data stream. For a more detailed discussion of speech encryption techniques under the IS-95 standard, see, for example, “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” Telecommunication Industry Association Doc. No. TIA/EIA/IS-95 (1993), incorporated by reference above.
Generally, the short code shown in
FIG. 1
is a pseudo-random sequence that differentiates each base station in the CDMA system. Long codes are utilized to differentiate mobile stations. Under the IS-95 standard, the long code is a pseudo-random sequence with a period of 2
42
−1 chips that is used for scrambling on the forward (base station to mobile) CDMA channel and for spreading on the reverse (mobile to base station) CDMA channel. Thus, the long code provides limited privacy and uniquely identifies a mobile station on both the forward and reverse traffic channels.
The Telecommunication Industry Association (TIA) has recently adopted a new standard for CDMA networks, referred to as the IS-2000 standard (CDMA-2000). For a detailed discussion of the IS-2000 standard, see, for example, IS-2000.2, “Physical Layer Standard for CDMA-2000 Spread Spectrum System,” Telecommunication Industry Association Doc. No. TIA/EIA/IS-2000, incorporated by reference herein. Among other changes to the IS-95 standard, the IS-2000 standard contemplates base stations and mobile stations that communicate using a plurality of spreading rates. Thus, while IS-95 originally contemplated communications having a spreading rate equal to the chip rate of 1.2288 mega-chips-per-second (Mcps), referred to herein as a 1× system, IS-2000 contemplates a number of spreading rates, such as a spreading rate equal to three-times the 1× system chip rate, referred to herein as a 3× system. To implement a spreading rate equal to three-times the 1× system chip rate (3×), the short and long code sequences may be generated by clocking the code generators
110
,
120
at three-times the 1× system chip rate (3×1.2288 MHz).
In communication systems that employ more than one spreading rate, however, such as 3× and 1× spreading rates, there are difficulties in handoffs between the 3× and 1× sub-systems. Specifically, the long code state for the two systems are different due to the fact that the long code generators are clocked at different chip rates between the two systems. A need therefore exists for a method and apparatus for generating CDMA long codes that can operate in communication systems employing more than one spreading rate. A further need exists for a method and apparatus for generating CDMA long codes that maintains long code state synchronization between two systems employing different spreading rates.
SUMMARY OF THE INVENTION
Generally, a long code generator is disclosed that maintains a common long code state between a system with multiple spreading rates, such as a spreading rate equal to the 1× chip rate and a spreading rate at a multiple of the 1× chip rate (n×), such as three-times the 1× chip rate (3×). If not otherwise specified herein, the term “chip rate” or “system chip rate” refers to the 1× chip rate. An n× long code generator of the present invention generates n bits for every clock pulse, where the clock operates at the system chip rate. The n× long code generator generates n bits for every clock period by having the long code mask value assume n values for each clock period. Thus, the long code mask value is changed at n times the chip rate, while the state of the long code is advanced at the chip rate. Each of the long codes corresponding to the n long code mask values are multiplexed, for example, using an interlacing technique. Thus, the n long code outputs are multiplexed to a single bit stream.
The long code generator includes a conventional (feedback) shift register, AND gate array, modulo-2 adder and clock. In addition, the disclosed long code generator includes a long code mask generator that generates n long code masks for each clock period. Thus, the long code mask generator operates with a chip rate of n-times the system chip rate (n×1.2288 MHz). A long code multiplexer interlaces each of the long codes corresponding to the n long code mask values, for example, by alternating individual bits from each of the n generated long codes.
In one implementation, a clock source operating at three-times the system chip rate drives a divide-by-three counter that cycles through states 00, 01, 10, respectively, at bit positions
38
and
39
of the 42 bit mask. The state of the divide-by-three counter is logically added to the conventional 42-bit long code mask using an adder, to produce three long code masks for each clock period. Thus, for a counter state of 00, the conventional long code mask will be unchanged, for a counter state of 01, the 38
th
bit of the conventional long code mask will be changed and for a counter state of 10, the 39
th
bit of the conventional long code mask will be changed. The three long code masks generated for each user must be different from one another, and different from the three long code masks generated for all other users to ensure that the long codes generated will be unique among different users.
The present invention effectively cycles through n distinct long code masks for each clock tick, thereby generating an n× rate long code. All the randomness properties necessary for proper CDMA operation are preserved. Furthermore, the present invention is equally applicable on both the forward and reverse links for l
Li Quinn
Ramesh Nallepilli S.
Appiah Charles
Lucent Technologies - Inc.
LandOfFree
Method and apparatus for generation of CDMA long codes does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for generation of CDMA long codes, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for generation of CDMA long codes will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3166022