Patent Application
20060223565
The present invention relates to telecommunications and, more particularly, to wireless communications systems.
A typical cellular telecommunications network (e.g., mobile phone network) includes one or more fixed base stations having various transceivers and antennae for radio communications with a number of distributed mobile stations (e.g., mobile phones). The base stations are in turn connected to a mobile switching center (“MSC”), which acts as the interface between the wireless/radio end of the cellular network and a public switched telephone network or other network(s), including performing the signaling functions necessary to establish calls or other data transfer to and from the mobile stations.
Various methods exist for conducting wireless communications between the base stations and mobile stations, including the CDMA (code division multiple access) and TDMA (time division multiple access) multiplexing schemes, both implemented in the United States and elsewhere under various standards. Generally, in a cellular network, transmissions from the mobile stations to the base stations are across a bandwidth known as the reverse link, while transmissions from the base stations to the mobile stations are across another frequency bandwidth known as the forward link. The forward and reverse links may each comprise a number of traffic channels and signaling or control channels, the former primarily for carrying data, and the latter primarily for carrying the control, synchronization, and other signals required for implementing wireless communications.
Currently, both single-diversity mobile stations and dual-diversity mobile stations are available for voice/data use. Single-diversity, “traditional” mobile stations are those that utilize a single antenna/receiver combination for receiving signals. The more advanced, dual-diversity mobile stations utilize, e.g., dual antennas and receivers or the like, for the simultaneous combining of (or selection from) two independently fading signals, which is meant to increase channel capacity across the forward link. In most cellular networks, the dual- and single-diversity mobile stations are on the same carrier. However, at the base stations, there is no direct way to know which mobile stations have single-diversity capacity and which have dual-diversity capacity. This makes increasing the forward link capacity difficult, since one of the actions necessary to achieve this increase is to be able to reduce the minimum transmission gains for the dual-diversity mobile stations.
According to an embodiment of the present invention, a method for optimizing the capacity of a forward link in a cellular telecommunications network involves dynamically adjusting a control setting on the forward link for each mobile station, e.g., the minimum gain level, based on each mobile station's individual performance. The cellular network has one or more base stations and a mixed population of single-diversity mobile stations and dual-diversity mobile stations. In communicating with a particular mobile station, the quality of the transmission over a forward link channel (i.e., physical or logical channel) is monitored at the base station, according to one or more factors. For example, the base station may monitor the reverse link for a power measurement report message (“PMRM”) sent by the mobile station. Depending on the particular configuration of the cellular network, a PMRM may be sent if there are a particular number of frame errors within a particular number of frames, i.e., in such a case the PMRM would indicate poor quality channel conditions. Thus, if no PMRM's are received during a predetermined time period, indicating high quality channel/transmission conditions, then the base station reduces the minimum gain level (or other minimum control setting) by a set amount.
The base station may also monitor the gain level of the transmission over the forward link channel. If the gain level stays at or close to the minimum gain level for a certain time period, and if no PMRM's are received during that time period, indicating high quality channel/transmission conditions, then the base station reduces the minimum gain level by a set amount. The reduction in the minimum gain level can be triggered based on other channel quality factors, such as the forward link frame error rate.
Additional capacity gain on the forward link is achieved by reducing the power allocation to individual mobile stations. Given that the older, single-diversity mobile stations cannot make use of enhancements such as mobile receive diversity, the base station cannot reduce the power allocation for all users and still meet the required quality of service. Hence, the base station is configured to distinguish which mobile stations are capable of supporting lowered power allocation, according to the algorithms described herein.
The present invention will be better understood from reading the following description of non-limiting embodiments, with reference to the attached drawings, wherein below:
With reference to
As indicated in
As one of the forward link control settings in the base station 14, there will typically be a minimum gain level 26, e.g., a minimum DGU. (More generally, the minimum gain level is a measure of the forward link traffic channel power.) Since the base station 14 will vary the transmission gain level 20 based on power control messages received from the mobile station, the minimum gain level 26 sets a lower limit on this variance, to ensure a minimum signal quality or performance level. Returning to
In communicating with a mobile station 16, 18 over the forward link, the voice/data information intended for the mobile station is typically broken into frames at the base station 14. Once the frames are received, the mobile station 16, 18 checks the frames for errors. Periodically, or depending on the number or rate of detected errors, the mobile station 16, 18 generates a power control message 22 as a means of forward link power control or otherwise. The power control message 22 contains an indication of the forward link signal quality, e.g., the error rate. An example of a power control message is the power measurement report message (“PMRM”) generated by mobile stations in certain CDMA-based networks.
At Step 106, if no PMRM's 22 have been received from the mobile station during T_1, the base station 14 lowers the minimum gain level 26 by “X1” dB for the transmission, where X1 is in 1 or 2 dB steps. Conceptually, having the gain level 20 at or close to a minimum gain level 26 for the time interval T_1, without any power control messages providing feedback of an unacceptable error rate or the like, is an indication of a high quality channel/signal. Since transmit power and signal quality are related, this means that the minimum gain level 26 can be decreased without likely significantly affecting channel quality. Lowering the transmit power, besides reducing resources expended by the base station (e.g., electrical power), tends to increase channel capacity because of lowered interference with other radio channels. Additionally, since the minimum gain level 26 is set based on individual mobile station performance, the present method allows for the lowering of minimum gains (capacity allocations) for those mobile stations exhibiting superior performance, e.g., the dual-diversity mobile stations 18. This is also the case for mobile stations, dual-diversity or otherwise, positioned in areas with exceptionally good RF conditions.
As noted above, the base stations 14 will typically also use the PMRM's 22 (or other control messages) to adjust the digital gain level 20. For example, if a PMRM 22 indicates a high error rate, the gain level 20 may be adjusted upwards for an increase in transmit power, to reduce the error rate. Similarly, if a PMRM 22 indicates a low error rate, the gain level 20 may be adjusted downwards for a decrease in transmit power. Lowering the minimum gain level 26, according to the method above, allows for the further decrease in transmit power levels below the previous minimum level. This optimizes forward link capacity, and enables the base station 14 to take advantage of the performance capabilities of the dual-diversity mobile stations 18. In other words, the minimum gain level 26 is controlled on a mobile-by-mobile basis, with the base station 14 lowering the minimum gain level 26 in situations where warranted, without having to know in advance if the mobile station is dual-diversity or not, or if the mobile station is in an area with good RF conditions.
The cellular network 12 may be configured for the mobile stations 16, 18 to generate a PMRM or other power control message if there are a predetermined number of frame errors within a predetermined number of frames. It may also be the case that a PMRM or other power control message is sent after a particular number of frames are received. For the former, the reception of a PMRM at the base station 14 during interval T_1 indicates (possibly) poor quality channel conditions, meaning that the minimum gain level 26 should probably not be lowered. For the latter, the reception of a PMRM during interval T_1 may or may not indicate poor quality channel conditions, depending on the content of the PMRM. For example, if the PMRM contains a command, request, or other feedback that the transmission gain be significantly increased, that would be an indication of poor quality channel conditions. If there is no such command or request, the channel conditions should be sufficient for lowering the minimum gain level.
If the ratio is greater than threshold_1, at Step 114, the base station 14 determines whether or not the gain level 20 for the transmission is at the minimum gain level 26 (or within the range 28) for a time interval denoted “T_2”. If so, at Step 116, the minimum gain level (e.g., minimum DGU) 26 is reduced by a value “X2” dB. X2 is selected so as to not introduce a large step reduction in the minimum gain level, which might result in the signal gain 20 of a transmission being detrimentally reduced to a low level where, e.g., the call/transmission is lost.
An additional embodiment of the present invention involves optimizing forward link capacity by lowering the minimum gain level 26 based primarily on received power control messages 22. Specifically, for each transmission/call the base station 14 monitors the reverse link 24 for incoming PMRM's 22. If no PMRM's are received during a time interval denoted “T_3”, the minimum gain level 26 may be reduced by a value “X3” dB, where X3 is similar in nature to X2 or X1.
An additional embodiment of the present invention involves optimizing forward link capacity by lowering the minimum gain level 26 based on the forward link frame error rate (“FFER”). In particular, as noted above, the mobile stations 16, 18 determine whether the frames received from the base station 14 contain errors. This information is transmitted back to the base station 14 as the FFER, as part of the PMRM's, separate messages, or otherwise. If the FFER is less than an upper threshold of “Y”% over a time interval denoted “T_4”, the minimum gain level 26 may be reduced by a value “X4” dB (again, X4 may be similar to X1−X3). Where an acceptable FFER is typically on the order of no more than 1%, the value Y will be chosen to be under 1, for a resulting upper threshold of 1%.
Since lowering the minimum gain level 26 may result in lower quality transmissions in certain circumstances as transmission conditions change, the base station 14 may be configured to take action to prevent an “overshoot” of the lowered minimum gain level 26. Specifically, if for a particular transmission the mobile station 16, 18 generates a PMRM 22 within a time interval denoted “T_5” from when the minimum gain level 26 was reduced, then the base station 14 increases the minimum gain level 26 by X2 dB. The minimum gain level 26 may also be increased if there are two PMRM's within a time interval denoted “T_6”. This control action may also be made in a multi-level manner, e.g., increase the minimum gain level 26 by X2 dB if a PMRM 22 is received within T_5 of when the minimum gain level 26 was lowered, or by X3 dB if within T_6. The values for T_2−T_6 are selected and optimized in a similar manner as described above for T_1. Though static values are one possible option, these time periods need not be static values. T_1−T_6 may be adapted dynamically based on how some metric is tracking the desired threshold. For example, if the FER is <0.1%, the system could update the minimum gain down at a fixed rate, but when the FER is between 0.1% and 0.3%, the rate of decrease would be slowed down or stopped altogether.
With reference to
For lowering the minimum Eb/No setpoint, in communicating with a mobile station 16, 18, the base station 14 queries the mobile station for the FFER at Step 120. At Step 122, the base station 14, having received the FFER from the mobile station, determines whether the FFER is less than Y%. If so, at Step 124, the base station 14 estimates whether the Eb/No of the mobile station 16, 18 (either the actual Eb/No or Eb/No setpoint, depending on how the base station tries to infer this state, not directly available) is at or close to the minimum level, i.e., within a particular window or range of the forward link minimum Eb/No setpoint. If so, then the forward link minimum Eb/No setpoint for that specific mobile is reduced, as at Step 126. Additionally, if the mobile station 14, 16 reports an error within a time interval “T_7” from when the minimum Eb/No setpoint is reduced, or if there is a string of errors within a time interval “T_8” (typically longer than T_7), as determined at Step 128, then the base station 14 increases the forward link minimum Eb/No setpoint back to a default value, as at Step 130.
Capacity optimization according to the methods described herein can be coordinated across all the sectors or cells of a call in soft-handoff, or it can be done autonomously at each individual cell or sector. Also, the present invention may be applied to other power control algorithms, depending on how they are implemented in a cellular network. Also, the various methods described herein may be used individually or combined together based on the specific characteristics and details of the technology and equipment used in the cellular network.
As should be appreciated, the act of reducing the minimum gain level 26 can be triggered based on various factors as described above. These factors include the FFER, power control bit ratio, and transmission of PMRM's, and are collectively referred to herein as “channel indicators,” i.e., each of these factors provides some sort of indication of the signal quality across the forward link and/or the condition of the forward link between the base station and mobile station. In deciding whether or not to reduce the minimum gain level, it is determined whether or not the channel indicator falls within a quality range, i.e., a range of values that correspond to high quality channel conditions, depending on the particular channel indicator utilized. Typically, the quality range is implemented as an upper threshold or a lower threshold, as described above. Thus, in utilizing the FFER as a channel indicator, the minimum gain level 26 would be reduced if the FFER is below an upper threshold of a designated percentage (Y%) during a time interval, i.e., quality range is between 0 and less than Y%. The minimum gain level 26 may be reduced based on other channel indicators, or upon various combinations of channel indicators.
Since certain changes may be made in the above-described method for optimizing the forward link capacity from a mixed population of single- and dual-diversity mobile stations, without departing from the spirit and scope of the invention herein involved, it is intended that all of the subject matter of the above description or shown in the accompanying drawings shall be interpreted merely as examples illustrating the inventive concept herein and shall not be construed as limiting the invention.
