Pulse or digital communications – Spread spectrum
Reexamination Certificate
2003-06-11
2004-04-27
Ghebretinsae, Temesghen (Department: 2631)
Pulse or digital communications
Spread spectrum
C375S295000, C455S522000, C370S342000
Reexamination Certificate
active
06728292
ABSTRACT:
BACKGROUND
This invention generally relates to spread spectrum time division duplex (TDD) communication systems. More particularly, the present invention relates to a system and method for controlling transmission power within TDD communication systems.
FIG. 1
depicts a wireless spread spectrum time division duplex (TDD) communication system. The system has a plurality of base stations
301
-
307
. Each base station
301
communicates with user equipments (UEs)
321
-
323
in its operating area. Communications transmitted from a base station
301
to a UE
321
are referred to as downlink communications and communications transmitted from a UE
321
to a base station
301
are referred to as uplink communications.
In addition to communicating over different frequency spectrums, spread spectrum TDD systems carry multiple communications over the same spectrum. The multiple signals are distinguished by their respective chip code sequences (codes). Also, to more efficiently use the spread spectrum, TDD systems as illustrated in
FIG. 2
use repeating frames
34
divided into a number of time slots
361
-
36
n
, such as fifteen time slots. In such systems, a communication is sent in selected time slots
361
-
36
n
using selected codes. Accordingly, one frame
34
is capable of carrying multiple communications distinguished by both time slot
361
-
36
n
and code. The combination of a single code in a single time slot is referred to as a resource unit. Based on the bandwidth required to support a communication, one or multiple resource units are assigned to that communication.
Most TDD systems adaptively control transmission power levels. In a TDD system, many communications may share the same time slot and spectrum. When a UE
321
or base station
301
is receiving a specific communication, all the other communications using the same time slot and spectrum cause interference to the specific communication. Increasing the transmission power level of one communication degrades the signal quality of all other communications within that time slot and spectrum. However, reducing the transmission power level too far results in undesirable signal to noise ratios (SNRs) and bit error rates (BERs) at the receivers. To maintain both the signal quality of communications and low transmission power levels, transmission power control is used.
One approach to control transmission power levels is open loop power control. In open loop power control, typically a base station
301
transmits to a UE
321
a reference downlink communication and the transmission power level of that communication. The UE
321
receives the reference communication and measures its received power level. By subtracting the received power level from the transmission power level, a pathloss for the reference communication is determined. To determine a transmission power level for the uplink, the downlink pathloss is added to a desired received power level at the base station
301
. The UE's transmission power level is set to the determined uplink transmission power level.
Another approach to control transmission power level is closed loop power control. In closed loop power control, typically the base station
301
determines the signal to interference ratio (SIR) of a communication received from the UE
321
. The determined SIR is compared to a target SIR (SIRTARGET). Based on the comparison, the base station
301
transmits a power command, bTPC. After receiving the power command, the UE
321
increases or decreases its transmission power level based on the received power command.
Both closed loop and open loop power control have disadvantages. Under certain conditions, the performance of closed loop systems degrades. For instance, if communications sent between a UE and a base station are in a highly dynamic environment, such as due to the UE moving, such systems may not be able to adapt fast enough to compensate for the changes. The update rate of closed loop power control in TDD is 100 cycles per second which is not sufficient for fast fading channels. Open loop power control is sensitive to uncertainties in the uplink and downlink gain chains and interference levels.
One approach to combining closed loop and open loop power control was proposed by the Association of Radio Industries and Business (ARIB) and uses Equations 1, 2, and 3.
T
UE
=
P
BS
(
n
)
+
L
Equation
1
P
BS
(
n
)
=
P
BS
(
n
-
1
)
+
b
TPC
Δ
TPC
Equation
2
b
TPC
=
{
1
:
if
SIR
BS
⟨
SIR
TARGET
-
1
:
if
SIR
BS
⟩
SIR
TARGET
Equation
3
T
UE
is the determined transmission power level of the UE
32
1
. L is the estimated downlink pathloss. P
BS
(n) is the desired received power level of the base station
30
1
as adjusted by Equation 2. For each received power command, b
TPC
, the desired received power level is increased or decreased by &Dgr;
TPC
. &Dgr;
TPC
is typically one decibel (dB). The power command, b
TPC
, is one, when the SIR of the UE's uplink communication as measured at the base station
30
, SIR
BS
, is less than a target SIR, SIR
TARGET
. Conversely, the power command is minus one, when SIR
BS
is larger than SIR
TARGET
.
Under certain conditions, the performance of these systems degrades. For instance, if communications sent between a UE
32
and a base station
30
are in a highly dynamic environment, such as due to the UE
32
moving, the path loss estimate for open loop severely degrades the overall system's performance. Accordingly, there is a need for alternate approaches to maintain signal quality and low transmission power levels for all environments and scenarios.
SUMMARY
Combined closed loop/open loop power control controls transmission power levels in a spread spectrum time division duplex communication station. A first communication station receives communications from a second communication station. The first station transmits power commands based on in part a reception quality of the received communications. The first station transmits a second communication having a transmission power level in a first time slot. The second station receives the second communication and the power commands. A power level of the second communication as received is measured. A path loss estimate is determined based on in part the measured received second communication power level and the first communication transmission power level. The second station transmits a second communication to the first station in a second time slot. The second communication transmission power level is set based on in part the path loss estimate weighted by a factor and the power commands. The factor is a function of a time separation of the first and second time slots.
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Ozluturk Faith M.
Shin Sung-Hyuk
Zeira Ariela
Ghebretinsae Temesghen
InterDigital Technology Corporation
Volpe and Koenig P.C.
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