Telecommunications – Receiver or analog modulated signal frequency converter – Local control of receiver operation
Reexamination Certificate
1999-03-15
2003-03-18
Urban, Edward F. (Department: 2685)
Telecommunications
Receiver or analog modulated signal frequency converter
Local control of receiver operation
C455S024000, C455S067700, C455S069000
Reexamination Certificate
active
06535723
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to wireless communications systems and, in particular, to power control in wireless communications systems having multiple information rates.
BACKGROUND OF THE INVENTION
Wireless communications systems use power control to improve system performance and increase system capacity. Power control involves tracking possible fading of communication channels and using that tracked fading to manage the power at which signals are being transmitted from base stations (in order to compensate for the fading). Conventional Code Division Multiple Access (CDMA) wireless communications systems based on the well-known IS-95 standard use error indicator bits to assist in controlling transmission power at the base station. Power control is implemented in the following manner.
When a call is set up in a CDMA wireless communications system, a base station and a mobile station communicate over a forward and a reverse link. The forward link includes communication channels for transmitting signals from the base station to the mobile station and the reverse link includes communication channels for transmitting signals from the mobile station to the base station. The base station transmits control information to the mobile station over a communication channel referred to herein as a forward control channel, and the mobile station transmits control information to the base station over a communication channel referred to herein as a reverse control channel. The base station transmits voice or data to the mobile station over a communication channel referred to herein as a forward traffic channel, and the mobile station transmits voice or data to the base station over a communication channel referred to herein as a reverse traffic channel. In either traffic, channel, voice or data is transmitted over 20 milli-seconds (ms) time intervals referred to herein as frames.
A set number of voice or data bits are transmitted within each frame, typically reported as the number of bits transmitted per second, referred to herein as a channel rate. The channel rate does not change and is typically dependent on the rate of the coder in the system, i.e. the rate of the speech or data coder. However, the amount of information within each frame of the signal does change, particularly for voice signals. Therefore the number of bits of information transmitted per second, referred to herein as the information rate, can change.
Four different information rates are possible in the traffic channel: full rate, ½ rate, ¼ rate and ⅛ rate. At the full rate the information rate and the channel rate are a equal. The information rate of the forward traffic channel is at the full rate when a large amount of information is being transmitted from the base station to the mobile. The information rate of the forward traffic channel is at the ⅛ rate when a small amount of information is being transmitted from the base station to the mobile. The ½ and the ¼ rate are transitional rates. For example, in a telephone conversation between a mobile station user and a second user communicating with the mobile station user, a voice signal from the second user is transmitted to the base station, which transmits it to the mobile station over the forward traffic channel. During a part of the conversation, the second user is talking. Therefore, the information rate of the forward traffic channel would be high because a large amount of information is being transmitted on the forward traffic channel. In this case, the information rate would be the full rate. During another part of the conversation, the second user is listening. Therefore, the information rate of the forward traffic channel would be low because a small amount of information is being transmitted on the forward traffic channel. In this case, the information rate would be equal to the ⅛ rate.
When the information rate is ½, ¼, or ⅛, the channel rate is higher than the information rate, and the information is repeated several times per frame. For example, with the ½ rate information is repeated twice each frame; with the ¼ rate the information is repeated four times per frame; and with the ⅛ rate the information is repeated eight times per frame. Repeating the information several times per frame permits the information to be transmitted at a correspondingly lower power. The power is scaled by an information rate scaling factor, which is equal to the information rate. For a frame whose information rate is equal to the ⅛ rate, the information rate scaling factor is ⅛, and the power can be reduced to ⅛ of the power of the frame at the full rate.
The bits in the frame are spread in time, referred to herein as interleaved. Interleaving typically spreads out important bits in time so that if there is a deep fade or noise burst the important bits are not corrupted by one deep fade or noise burst. This reduces the number of frames containing errors, referred herein as a frame error rate.
When system conditions are equal, frames that have an information rate lower than the channel rate have a lower frame error rate than frames whose information rate is equal to the full rate. This is due to the synergistic effects of combining interleaving with the repeating of the bits in the frame. The lower frame error rate of the frames having the lower information rate allows these frames to be transmitted at an even lower power. For example, for a frame whose information rate is ⅛, the power can be reduced to below ⅛ the power of a frame whose information rate is the full rate. The base station can adjust the power of a frame having an information rate lower than the full rate.
Referring to
FIG. 1
, in conventional CDMA systems, each forward traffic frame
10
(i.e., frames transmitted over the forward traffic channel) includes voice or data and error control information, typically in the form of a cyclical redundancy code (CRC). By contrast, each reverse traffic frame
20
(i.e., frames transmitted over the reverse traffic channel) includes voice or data and error indicator bits (EIB) for indicating whether the last forward traffic frame is a good frame or in erasure, i.e., a bad frame.
When base station
30
transmits forward traffic frame
10
, mobile station
40
receiving forward traffic frame
10
will check the CRC to determine whether forward traffic frame
10
is good or not. Mobile station
40
will indicate such determination to base station
30
using the EIB in the next reverse traffic frame the mobile station will transmit. For example, a zero error indicator bit indicates no error in the forward traffic frame, and a positive error indicator bit indicates the forward traffic frame is a bad frame. Upon receiving reverse traffic frames from the mobile station, the base station examines the EIB and determines whether its forward link to the mobile station is in fading, and adjusts the power of its forward link accordingly. For example, if the base station receives one or more successive EIB, denoting erred forward traffic frames, the base station may determine that its forward link is in fading and increase the power of its forward link. This is to ensure that the frame error rate is kept to an acceptable percentage, typically between 1% and 3%, depending on the desired system performance.
Therefore, in a conventional CDMA wireless, communications system, a power control decision to either adjust the power or keep the power at its current level occurs once every frame, when the EIB is received. In newly proposed CDMA wireless communications system (hereinafter referred to as CDMA 2000), the forward link power control is much faster. The forward link power control is at 800 Hz rate, which means that power control information, referred to herein as a power control bit, is sent every 1.25 ms, or once for every power control group. Therefore, the base station cannot wait until the end of the forward traffic frame to dete
Jiang Frances
Kamel Raafat Edward
Li Quinn
Salvarani Alexandro Federico
Weaver Carl Francis
Craver Charles
Law Offices of John Ligon
Lucent Technologies - Inc.
LandOfFree
Method of power control for a wireless communication system... 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 of power control for a wireless communication system..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of power control for a wireless communication system... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3059437