Transmit power control for an OFDM-based wireless communication system

• US 7,151,740 B2
• Filed: 02/28/2001
• Issued: 12/19/2006
• Est. Priority Date: 02/28/2001
• Status: Expired due to Term

• Current Assignee: AT&T Mobility II LLC (AT&T, Inc.)
• Original Assignee: Cingular Wireless II LLC (AT&T, Inc.)
• Inventors: Heinonen, Jari, Hirano, Mike, Zhang, Hongliang, Leung, Louis
• Primary Examiner(s): Lee, Andrew C.
• Application Number:
  US09/795,630
• Publication Number:
  US 20020145968A1
• Time in Patent Office: 2,120 Days
• Field of Search: None
• US Class Current: 370/208
• CPC Class Codes: H04L 27/2602 Signal structure
• CPC Class Codes: H04W 52/08 Closed loop power control
• CPC Class Codes: H04W 52/10 Open loop power control
• CPC Class Codes: H04W 52/146 Uplink power control
• CPC Class Codes: H04W 52/16 Deriving transmission power values from another channel
• CPC Class Codes: H04W 52/24 using SIR [Signal to Interference Ratio] or other wireless path parameters
• CPC Class Codes: H04W 52/42 in systems with time, space, frequency or polarisation diversity

Abstract

An open loop power control system for an orthogonal frequency division modulation (OFDM)-based fixed/mobile wireless system. A preferred system and method comprise measuring signal strength of received interference-free pilot tones transmitted by a base station; determining pathloss according to received power level of pilot tones; and controlling transmit power level of the remote unit by adjusting transmitting channel attenuation according to the pathloss determined in the preceding step. The method may further comprise monitoring at the base station the received signal level of uplink pilot tones; checking if received signal level of uplink pilot tones is outside pre-set limits around the target level at the start of each call; and sending a command to the mobile station over the broadcast channel to increase or decrease the transmitting power level.

Claims

1. A method of controlling transmit power in an OFDM transmission system, said method comprising:

(a) transmitting from a transmitting end of said transmission system a series of pilot tones on multiple sub-channels of an OFDM transmission;

(b) detecting on each of said sub-channels a power level of received pilot tones at a remote end OFDM receiver;

(c) determining path loss in a physical path between the transmitting end and the remote end based on the level of the received pilot tones; and

(d) controlling transmit power level of a transmitter associated with the remote end OFDM receiver by adjusting transmit power level according to the path loss determined in the determing step.

2. The method as recited in claim 1, wherein said determining step includes determining path loss based on the relationship:

P_t^(m)=K−

P_r^(m).

3. The method as recited in claim 1, wherein said controlling step includes controlling the gain or attenuation Gtatt of the transmit power level according to the relationship Gtatt=(L_E+Gtmax−

Gmax+G_TTA−

L_TD−

Lref+L_B)+Gatt where L_E is the signal power at a base station, Gtmax is the maximum gain in the transmitting path of the remote unit, G_TTA is the gain at a tower top amplifier of a base station, and L_TD is a downlink transmit power.

4. The method as recited in claim 3, wherein said controlling step includes controlling the gain or attenuation Gtatt of the transmit power level according to the relationship:

Gtatt=α

·

Gtatt+(1−

α

)·

Gtatt where α

is a forget factor for a moving average of Gtatt.

5. The method as recited in claim 1, wherein the determining step includes using AGC set points at the remote unit to determine path loss.

6. A system for controlling transmit power in an OFDM wireless transmission system comprising:

(a) a base station that transmits pilot tones on at least one sub-channel of an OFDM transmission;

(b) a remote unit that detects a power level of received pilot tones at a remote end OFDM receiver on each of said sub-channels;

(c) signal processing circuitry in the remote unit that determines path loss in a physical path between the base station and the remote end based on the levels of the received pilot tones; and

(d) an attenuation controller at the remote unit that controls transmit power level of a transmitter by adjusting transmit power level according to the path loss.

7. The system as recited in claim 6, wherein the attenuation controller determines path loss based on the relationship:

P_t^(m)=k−

P_r^(m).

8. The system as recited in claim 6, wherein the controlling step controls a gain or attenuation Gtatt of the transmit power according to the relationship Gtatt=(L_E+Gtmax−

Gmax+G_TTA−

L_TD−

Lref+L_B)+Gatt where L_E is the signal power at a base station, Gtmax is the maximum gain in the transmitting path of the remote unit, G_TTA is the gain at a tower top amplifier of a base station, and L_TD is a downlink transmit power.

9. The system as recited in claim 6, wherein the attenuation controller controls the gain or attenuation Gtatt of the transmit power level according to the relationship:

Gtatt=α

G·

Gatt+(1−

α

)·

Gtatt where α

is a forget factor for a moving average of Gtatt.

10. The system as recited in claim 6, further including:

(a) a detector at the base station that monitors the received signal level of uplink pilot tones transmitted by the remote units and checks if received signal level of uplink pilot tones is outside pre-set limits around the target level at the start of each call; and

(b) a controller that sends a command to the remote end over a broadcast channel to increase or decrease the transmit power level if it falls outside of prescribed limits.

11. The system as recited in claim 6, wherein said processing circuit includes an AGC circuit that provides an indication of path loss.

12. An apparatus that controls the power of a mobile station transmission signal in an OFDM-based wireless system, said apparatus comprising:

(a) a base station having a receiving channel and a transmittal channel;

(b) a subscriber unit physically remote from said base station having a receiving channel and a transmit channel;

(c) an automatic gain control circuit for conveying a path loss measurement between said base station and said subscriber unit wherein said path loss measurement is based on a strength of OFDM signals of known power received by said subscriber unit over multiple OFDM sub-channels; and

(d) a transceiver located in said subscriber unit having the transmitting power level thereof set by adjusting an attenuation reading of said transmitting channel according to the readings of the automatic gain control circuit.

13. A method of controlling transmit power in an OFDM wireless transmission system, said method comprising:

(a) transmitting from a transmit end of said system a series of pilot tones over multiple sub-channels of an OFDM transmission;

(b) detecting respective power levels of received pilot tones at a remote end OFDM receiver;

(c) determining path loss along a physical path between the transmit end and the remote end based on the level of the received pilot tones using AGC set points of a receiver; and

(d) controlling transmit power level of a transmitter by adjusting transmit power level according to the path loss determined in the determining step.

14. A method of controlling transmit power in an OFDM wireless transmission system, said method comprising:

(a) transmitting from a transmit end of said system a series of pilot tones on multiple sub-channels of an OFDM transmission;

(b) detecting a power level of received pilot tones at a receiver;

(c) determining path loss along a physical path between the transmit end and the receiver based on the level of the received pilot tones using AGC set points of a receiver based on the relationship P_t^(m)=K−

P_r^(m); and

(d) controlling transmit power level of a transmitter by adjusting transmit power level according to the path loss determined in the determining step.

15. A system for controlling transmit power in an OFDM wireless transmission system comprising:

(a) a transmitter that transmits from a transmit end of said system a series of pilot tones on multiple sub-channels of an OFDM transmission;

(b) a receiver that detects power levels of received pilot tones at a receiver;

(c) circuitry that determines path loss along a physical path between the transmit end based on the levels of the received pilot tones using AGC set points of a receiver and that controls transmit power levels of a transmitter by adjusting transmit power level according to the path loss.

16. A system for controlling transmit power in an OFDM transmission system comprising:

(a) a transmitter that transmits from a transmit end of said system a series of pilot tones on multiple sub-channels of an OFDM transmission;

(b) a detector that detects power levels of received pilot tones at a receiver;

(c) a circuit that determines path loss along physical path between the transmit end and the receiver based on the levels of the received pilot tones using AGC set points of a receiver based on the relationship P_t^(m)=K−

P_r^(m) and that controls a transmit power level of a transmitter by adjusting transmit power level according to the path loss.