Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1998-12-22
2002-06-11
Chin, Stephen (Department: 2734)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S224000, C375S227000, C375S316000, C375S147000, C370S342000, C455S226200, C455S522000
Reexamination Certificate
active
06404826
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to wideband code division multiple access (WCDMA) for a communication system and more particularly to signal-to-interference ratio estimation of WCDMA signals.
BACKGROUND OF THE INVENTION
Present code division multiple access (CDMA) systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected receiver to determine the proper recipient of a data signal. These different data signals arrive at the receiver via multiple paths due to ground clutter and unpredictable signal reflection. Additive effects of these multiple data signals at the receiver may result in significant fading or variation in received signal strength. In general, this fading due to multiple data paths may be diminished by spreading the transmitted energy over a wide bandwidth. This wide bandwidth results in greatly reduced fading compared to narrow band transmission modes such as frequency division multiple access (FDMA) or time division multiple access (TDMA).
New standards are continually emerging for next generation wideband code division multiple access (WCDMA) communication systems as described in Provisional U.S. Patent Application No. 60/082,671, filed Apr. 22, 1998, and incorporated herein by reference. These WCDMA systems are coherent communications systems with pilot symbol assisted channel estimation schemes. These pilot symbols are transmitted as quadrature phase shift keyed (QPSK) known data in predetermined time frames to any receivers within range. The frames may propagate in a discontinuous transmission (DTX) mode. For voice traffic, transmission of user data occurs when the user speaks, but no data symbol transmission occurs when the user is silent. Similarly for packet data, the user data may be transmitted only when packets are ready to be sent. The frames are subdivided into sixteen equal time slots of 0.625 milliseconds each. Each time slot is further subdivided into equal symbol times. At a data rate of 32 thousand symbols per second (ksps), for example, each time slot includes twenty symbol times. Each frame includes pilot symbols as well as other control symbols such as transmit power control (TPC) symbols and rate information (RI) symbols. These control symbols include multiple bits otherwise known as chips to distinguish them from data bits. The chip transmission time (T
c
), therefore, is equal to the symbol time rate (T) divided by the number of chips in the symbol (N).
Referring to
FIG. 1
, there is a simplified diagram of a mobile communication system. The mobile communication system includes an antenna
100
for transmitting and receiving external signals. The diplexer
102
controls the transmit and receive function of the antenna. Multiple fingers of rake combiner circuit
104
combine received signals from multiple paths. Symbols from the rake combiner circuit
104
are applied to a bit error rate (BER) circuit
110
and to a Viterbi decoder
106
. Decoded symbols from the Viterbi decoder are applied to a frame error rate (FER) circuit
108
. Averaging circuit
112
produces one of a FER and BER. This selected error rate is compared to a corresponding target error rate from reference circuit
114
by comparator circuit
116
. Detector circuit
118
produces an output signal corresponding to the comparison. This output signal and a feedback signal from delay circuit
120
are added by circuit
122
to produce a signal-to-interference ratio (SIR) reference signal on lead
124
.
Pilot symbols from the rake combiner
104
are applied to the SIR measurement circuit
132
. The SIR measurement circuit produces a received signal strength indicator (RSSI) estimate from an average of received pilot symbols. The SIR measurement circuit also produces an interference signal strength indicator (ISSI) estimate from an average of interference signals from base stations and other mobile systems over many time slots. The SIR measurement circuit produces an SIR estimate from a ratio of the RSSI signal to We ISSI signal. This SIR estimate is compared with a target SIR by circuit
126
. Detector circuit
128
produces an output signal corresponding to the comparison that is applied to TPC command circuit
130
. The TPC command circuit
130
sets a TPC symbol that is transmitted to a remote base station. This TPC symbol instructs the base station to either increase or decrease transmit power by preferably 1 dB for subsequent transmission.
The diagram of
FIG. 2
illustrates the closed-loop transmit power control sequence between of the base station and the mobile system. The base station receives a group of pilot symbols
200
in a time slot
204
from the mobile system. The base station determines an SIR ratio from the pilot symbols
200
and TPC symbol
202
and adjusts transmit power accordingly. This adjusted transmit power is applied to time slot
210
of downlink
220
. The time slot
210
is offset from time slot
204
by one-halftime slot or 0.3125 milliseconds, so the mobile system has time to adjust transmit power in response to TPC symbol
208
for the next time slot
218
of uplink
230
. The mobile system determines an RSSI estimate from pilot symbols
206
of time slot
210
. For high data-rate channels such as 256-1024 thousand symbols per second (ksps), there are preferably eight pilot symbols in each time slot. For low data-rate channels such as 32-128 ksps (
FIG. 3
) there are preferably four pilot symbols in each time slot. The ISSI estimate includes an average of interference signals over many time slots. The ISSI estimate, therefore, is relatively stable and changes slowly with time. By way of comparison, an RSSI estimate for time slot
310
may include of an average of pilot symbols
308
alone. This small sample produces large variations in the RSSI estimate. For example, for six fingers of rake combiner circuit
104
, the RSSI estimate Ŝ
m
for the m
tm
time slot is given by equation [1]. Here, r
k,m,g
corresponds to the k
th
pilot symbol of the m
th
time slot and the g
th
finger with the pilot symbol data removed.
S
^
m
=
1
16
∑
g
=
1
6
(
&LeftBracketingBar;
∑
k
=
1
4
r
k
,
m
,
g
&RightBracketingBar;
2
)
[
1
]
This RSSI estimate may fluctuate abruptly due to the limited number of pilot symbols available for averaging. The SIR estimate is given by equation [2], where {circumflex over (1)}
m
is the ISSI estimate for the m
th
time slot, which is obtained by averaging the interference from many previous time slots. Since the SIR estimate is a ratio of the RSSI to the ISSI estimate, most of the variation of the SIR estimate is due to the RSSI variation. The variation in the SIR estimate produces erratic TPC control and correspondingly large variations in transmit power.
S
I
^
R
m
=
S
^
m
I
^
m
[
2
]
These large variations in transmit power degrade communications between the base station and the mobile system.
SUMMARY OF THE INVENTION
These problems are resolved by a circuit comprising an estimate circuit coupled to receive a plurality of predetermined signals from an external source. Each of the predetermined signals is spaced apart in time. The estimate circuit produces a first estimate signal in response to at least one of the plurality of predetermined signals. An averaging circuit is coupled to receive a data signal and the at least one of the plurality of predetermined signals. The averaging circuit produces an average signal from the data signal and the at least one of the plurality of predetermined signals.
The present invention improves signal-to-interference estimation by averaging pilot symbols and corrected data symbols. Closed-loop power control is improved. A standard deviation of transmit power is greatly reduced, and the link margin of the system is improved.
REFERENCES:
patent: 5204878 (1993-04-01), Larsson
patent: 5228054 (1993-07-01), Rueth et al.
patent: 5692015 (1997-11-01), Higashi et al.
patent: 60
Dabak Anand G.
Hosur Srinath
Schmidl Timothy M.
Brady III Wade James
Chin Stephen
Deppe Betsy L.
Rountree Robert N.
Telecky , Jr. Frederick J.
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