The present invention relates to wireless communication systems in general and, more particularly, to an improved method and apparatus for controlling the power of signals transmitted by base stations and mobile units operating in a CDMA communications system.
Many wireless communications systems in use today employ a form of spread-spectrum communications technology known as code-division multiple-access, or simply xe2x80x9cCDMAxe2x80x9d. In systems utilizing spread spectrum communications technology such as CDMA, signals transmitted by a base station or mobile unit are spread out over a very wide frequency range using pseudo-random noise sequences, making the signals relatively immune to frequency-dependent interference.
The majority of wireless communications systems using CDMA are based on TIA/EIA standard IS-95, which is incorporated by reference herein. IS-95 is known in the industry as a second generation standard. It is anticipated that there will soon be a new industry-wide CDMA standard emerging from various proposals currently under consideration by the International Telecommunications Union (ITU). Such proposals are collectively known as third generation (3G) CDMA proposals and include xe2x80x9ccdma2000 RTT Candidate Submission to ITU-Rxe2x80x9d, which is incorporated by reference herein.
In any CDMA system, the mobile units in a given cell act as geographically disparate signal sources which activate at random times. Consequently, it is not possible to synchronize reverse-link transmissions, i.e., transmission from the various mobile units to the cell""s base station. It is therefore impossible for a base station to perform accurate detection (even using pilot-assisted coherent reception as in 3G CDMA) without relying on a feedback mechanism to adjust the transmit power of each mobile unit. As a result, a dynamic method of power control known as closed-loop power control is commonly employed for controlling reverse-link power.
On the other hand, forward-link CDMA signals transmitted by a base station and destined for the various mobile units in the cell are designed to be mutually orthogonal and, furthermore, transmission of these signals can be synchronized by the base station. Thus, not only is a transmitted CDMA signal destined for a given mobile unit immune to interference from signals destined for other mobile units, but the mobile unit is able to perform coherent detection with a large processing gain. Accordingly, second generation standards such as IS-95 do not provide for closed-loop power control in the forward-link direction. Nevertheless, it has been found from experience with IS-95 CDMA that significant additional performance improvements can be achieved by using a feedback power control mechanism as in the reverse-link. Accordingly, most 3G CDMA proposals call for the use of closed-loop power control.
Closed-loop power control consists of a destination (which could be a base station or a mobile unit) measuring the signal-to-interference ratio of a signal received from a source and comparing the measured signal-to-interference ratio with a predetermined target value. If the measured value is greater than the target value, then the power transmitted by the source may be lowered, while if it is less than the target value, then the power transmitted by the source must be increased in order to meet the target. The desired power adjustment is forwarded by the destination back to the source along an appropriate power control subchannel.
A separate outer-loop power control mechanism is responsible for setting the target signal-to-interference ratio. The guidelines for implementing open-loop power control in the forward-link (3G CDMA) and the reverse-link (IS-95 and 3G CDMA) directions are fairly flexible. Still, thus far, designers have chosen to constrain the target signal-to-interference ratio to a constant value or one which depends on the type of service being offered. While this is satisfactory for situations where the data rate of a service is generally constant and quite low, many emerging applications require high data rates (certain 3G CDMA proposals anticipate that data connections at rates of up to 2 megabits per second per connection should be supported) and are characterized as having a substantially bursty traffic pattern.
As a result, if a conventional closed-loop power control algorithm relying on a fixed or service-specific target signal-to-interference ratio is applied in, say, the reverse-link direction, then whenever there is an increase in the instantaneous traffic from a given mobile unit, all the other mobile units will be required to raise their transmit power significantly because of interference from the given mobile unit. The fact that the other mobile units have raised their transmit power causes interference to the given mobile unit, which is again forced to increase its transmit power, and so on. This phenomenon will be repeated at a time scale of the burst duration which makes it a frequently occurring event.
Frequent increases in transmit power may cause undue stress on the transmitter power amplifier, which may consequently limit the cell capacity. Furthermore, the power control algorithm is required to converge quickly in order to avoid outages during the bit rate transition from a low bit rate to a high bit rate.
Similar effects occur in the forward-link direction, although other mobile units within the same cell as the given mobile unit are relatively immune to interference caused by forward-link transmissions involving the given mobile unit. Still, the above described effects can occur as a result of bursty forward-link transmissions occurring in neighbouring cells.
Clearly, the signal-to-interference ratio alone does not appear to be an adequate characterization of the grade of service required of a bursty link. For instance, when the mobile unit is transmitting at a high bit rate, its required performance and required quality of service are likely different from when the bit rate is considerably lower. Thus, there is a need in the industry to provide a method and apparatus for controlling transmitted power by a base station or mobile unit which takes into account the bursty nature of data traffic.
The invention can be summarized according to a first broad aspect as a method for use in a communications system wherein the transmit power of a wireless link is adjusted so that link performance meets a target level, the method including dynamically adjusting the target level as a function of the traffic characteristics of the link.
The invention can be summarized according to a second broad aspect as a method for use in a closed-loop power control system wherein the transmit power of a source unit communicating with a destination unit across a wireless link is varied in accordance with measured performance and a target performance parameter. The method is one of setting the target performance parameter and is performed at the destination unit. The method includes (a) detecting the start and end of data bursts received from the source unit across the link; and (b) if the performance of the link is adequate, then gradually increasing the target performance parameter when the start of a burst is detected and gradually decreasing the target performance parameter when the end of a burst is detected.
According to another broad aspect, the invention can be summarized as a method of generating power control commands for transmission to a source unit communicating with a destination unit. The method includes (a) measuring an instantaneous performance parameter of signals received from the source unit; (b) determining an instantaneous bit rate, denoted RMINST, of the signals received from the source unit; (c) computing a threshold instantaneous performance parameter as a first function of the measured instantaneous performance parameter, of RMINST, and of at least one target error performance-parameter; and (d) generating a power control command based upon a second function of the measured instantaneous performance parameter and the threshold instantaneous performance parameter.
Preferably, the measured instantaneous performance parameter is a measured instantaneous signal-to-interference ratio, denoted (S/I)MINST, and the threshold instantaneous performance parameter is a threshold instantaneous signal-to-interference ratio, denoted (S/I)LINST. Preferably, two of the target error performance parameters are a target average signal-to-interference ratio, denoted (S/I)*AVG, and a target maximum outage time, denoted T*MAX.
Preferably, step (c) includes (c1) integrating (S/I)MINST over a first time window to compute a measured average signal-to-interference ratio, denoted (S/I)MAVG; (c2) integrating RMINST over a second time window to compute an average bit rate, denoted RMAVG; and (c3) among a plurality of most recent values of (S/I)MINST, determining the longest amount of time during which (S/I)MINST was below (S/I)*AVG, said longest amount of time being denoted TMMAX.
Preferably, the first function is:                               (                      S            I                    )                INST        L            =                                    (                          S              I                        )                    AVG          *                ·        α              ,    where        α    =          {                                                  1              ,                                                                          if                ⁢                                  xe2x80x83                                ⁢                                  T                  MAX                  M                                             greater than                                                 T                  MAX                  *                                ⁢                                  xe2x80x83                                ⁢                or                ⁢                                  xe2x80x83                                ⁢                                                      (                                          S                      I                                        )                                    AVG                  M                                             less than                                                 (                                      S                    I                                    )                                AVG                *                                                                                                                          R                  AVG                  M                                                  R                  INST                  M                                            ,                                                          otherwise              .                                          
The invention can be summarized according to another broad aspect as a computer-readable storage medium which, when processed by a computer at a destination unit, executes a sequence of steps to generate a power control command for transmission to a source unit communicating with the destination unit. The steps include (a) computing a threshold instantaneous performance parameter as a first function of a measured instantaneous performance parameter, a measured instantaneous bit rate and at least one target error performance parameter; and (b) generating a power control command based upon a second function of the measured instantaneous performance parameter and the threshold instantaneous performance parameter.
According to still another broad aspect, the invention may be summarized as a statistical power control block for use in a destination unit communicating with at least one source unit. The statistical power control block includes a unit for computing a threshold instantaneous performance parameter as a first function of a measured instantaneous performance parameter, a measured instantaneous bit rate and at least one target error performance parameter; and a unit connected to the computing means, for generating a power control command based upon a second function of the measured instantaneous performance parameter and the threshold instantaneous performance parameter.
The invention can also be summarized as a functional unit for generating power control commands. The functional unit includes a maximum selector for receiving a plurality of composite correlation levels and selecting the largest thereamong, as well as an integrator connected to the maximum selector, for receiving the largest composite correlation level and integrating it over a predetermined length of time, thereby to produce a measured instantaneous performance parameter at regular intervals. In addition, the functional unit includes the above-described statistical power control block.
When the invention is implemented, a sudden burst of data results in a lowering of the threshold instantaneous signal-to-interference ratio, until such time that the average signal-to-interference ratio or the maximum outage time are no longer acceptable. This provides a smoothing effect of the interference induced to other users and there may result an increase in cell capacity. Similarly, if the data rate is reduced while the limitations on the average error performance and outage times are being respected, then the average bit rate will be higher than the instantaneous bit rate, which increases the threshold instantaneous signal-to-interference ratio, which further maintains the mobile unit""s transmitted power at a relatively high power level for a longer time interval, followed by a gradual reduction as the average bit rate drops with time.