The present invention relates to telecommunications and, more particularly, to wireless communications systems.
In a cellular system or network utilizing a multi-carrier transmission mechanism, the information to be communicated on the forward link can be transmitted on many frequency bands (i.e., carriers) simultaneously, in parallel to several mobile stations, and/or to one mobile station as the traffic and user load warrant. In communicating with a particular mobile station, the base station transmitter retains the flexibility of transmitting on one or more of the frequency bands within the total bandwidth. In other words, the number of frequency bands on which the base station transmits signals to a particular mobile station may be less than the total number of available frequency bands. Such a system is henceforth referred to as a multi-carrier (“MC”) cellular system or network.
The path loss between a base station transmitter and a mobile station is a measure of the attenuation experienced by a radio signal in propagating from the base station transmitter to the mobile receiver. In a mobile environment, this typically will be a time varying quantity.
The path loss in a base station to mobile station link is inversely proportional to the signal-to-interference ratio (“SIR”) for that link; all other quantities remaining the same, a lower path loss implies a higher SIR for that link. The SIR determines the ability of the receiver to extract the intended information signal out of the total received power. A higher SIR implies a better ability to perform this useful signal extraction. More specifically, in a communications system, the total received power impinging on a receiver consists of three parts: (a) the transmit power of the information signal intended for the receiver; (b) partial powers of signals intended for other users (this could be either due to deficiencies in hardware leading to imperfect isolation of the transmitted signals, or due to deliberate design in introducing controlled mixing of the signals meant for different users by the transmitter, e.g., as in CDMA networks); and (c) random noise introduced by inefficiencies in the transmitting/receiving hardware or otherwise. The SIR is defined as the ratio of: (a)/((b)+(c)).
Due to several well-understood physical phenomena that affect the propagation of radio signals, the path loss is dependent on the frequency at which the signal transmission is made. Hence, in an MC system, the SIR at a mobile station is dependent on the frequency band of transmission. Each band over which a signal is sent to the mobile has a different path loss. If the base station had knowledge of which bands have lower path loss to a mobile, it could use this information advantageously.
An MC cellular system or network includes a base station that communicates with a number of distributed mobile stations over a plurality of frequency bands. A method for exploiting the diversity of the cellular system's frequency bands, for purposes of increasing network capacity, involves the base station transmitting a pilot signal on each frequency band to the mobile stations. The mobile stations measure a quality or characteristic, e.g., SIR, of the pilot signals they receive across the various frequency bands. This information, or some function or portion thereof, is transmitted back to the base station on the reverse link. Thus, the base station is provided with an indication, for each mobile station, of the signal quality as perceived by that mobile station on each frequency band of the MC cellular system. Alternatively, the mobile stations may be configured to provide information relating to the signal quality, e.g., measured pilot signal SIR, across only one or several of the frequency bands.
The base station may be configured to utilize the signal quality information in a number of ways, all of which are intended to increase the amount of data that can be transmitted by the base station per unit time, i.e., channel capacity. For example, on each frequency band, the base station may transmit data signals only to the mobile station(s) with the best signal quality on that frequency band. Additionally, the base station may transmit signals to one or more mobile stations across one or more frequency bands simultaneously, with transmissions on each band being adapted to the corresponding channel conditions. For the same total transmitted power by the base station in an MC cellular network, these solutions tailor the transmissions to the signal quality conditions across the transmission bandwidth, thus maximizing the amount of information transmitted. This will result in faster information transfers, leading to better system performance.
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
Transmissions from the base station 22 to the mobile stations 26a-26c are across a forward link 28, while transmissions from the mobile stations 26a-26c to the base station are across a reverse link 30. In the MC cellular network 20, the forward and/or reverse links will typically include a number of frequency bands within an overall link bandwidth. For example,
The overall bandwidth 32 of the forward link will usually be a function of the total bandwidth allotted to the MC cellular network 20. Most cellular networks are configured according to one or more industry standards or protocols, which are in turn based on, in part, government frequency spectrum allocations. These standards or protocols dictate the total reverse and forward link bandwidth. For example, in certain cellular networks, each link may have a 1.25 MHz bandwidth. The total bandwidth 32 may be broken into a number of frequency bands 34, depending on the particular network and its configuration. Additionally, the frequency bands will not necessarily be non-overlapping, as shown in
To optimize capacity (e.g., data throughput in bits/sec or symbols/sec) in the MC cellular network 20, the base station 22 is provided with information about the signal quality (e.g., SIR) across each frequency band 34 in the MC cellular network 20. Then, the base station 22 utilizes this information to modify and enhance the wireless communications between it and the mobile stations 26a-26c.
The signal quality message(s) 38 includes signal quality information about each frequency band 34 received by the mobile station, namely, an identifier 40 that identifies the frequency band, and a quality descriptor 42 that conveys the measured quality or characteristic, or some pre-specified function of it, of the received pilot signal 36 in that frequency band. (Other information may also be provided.) For example, as shown in
To save transmission resources, instead of reporting on the signal quality across every frequency band 34, the mobile stations 26a-26c may simply report information about the frequency band 34 with the best-observed SIR across the entire frequency bandwidth 32, including the SIR measured on that frequency band, or some function thereof. The information about the identifier of the frequency band with the best signal quality may be implicit, as in the case where the mobile station itself utilizes a multi-carrier scheme to transmit information to the base station 22, with each frequency band on the reverse link 30 having a correspondence with a certain forward link frequency band 34. In this case, the mobile station may indicate which forward link frequency band 34 has the best SIR by simply transmitting a quality descriptor 42 (i.e., SIR measurement information) to the base station 22 on the reverse link frequency band corresponding to the best forward link frequency band 34, without explicitly committing any resources to convey information explicitly identifying the best forward link frequency band 34, e.g., an identifier 40.
Alternatively, instead of reporting on the signal quality across every frequency band 34 or only one frequency band, the mobile stations 26a-26c may provide information on the signal quality across some number “n” of the frequency bands 34, where “n” is less than the total number of frequency bands 34. For example, the mobile stations 26a-26c may measure the SIR of the pilot signals on all the frequency bands 34, and then report on those “n” frequency bands whose pilot signals have the best measured SIR. As explained above, this reporting may be implicit with respect to the frequency band identifier.
Referring back to
The base station 22 utilizes the information provided in the signal quality messages 38 to increase the amount of information that may be transmitted per unit time (i.e., channel capacity) by the base station 22. This may be done in a number of ways.
According to one possible method for adjusting base station transmissions to increase channel capacity, based on the knowledge of which mobile station has the best received signal quality on each frequency band, the base station, on each frequency band, transmits information signals (e.g., voice and other data signals) only to the mobile station with the best signal quality on that frequency band. If several frequency bands are available for transmissions to a mobile station, the base station 22 selects the frequency band with the best-reported signal quality.
This procedure, designated 112a, is shown in
To elaborate regarding Step 118, the modulation order of a transmission signal refers to the amount of information that can be conveyed in a single signal transmission. Higher modulation order transmissions imply the conveyance of more information, and are hence desirable. “Code rate” refers to the ratio of number of information bits to the total transmitted bits including coded/parity bits. A higher code rate implies the conveyance of more information. The ability to successfully carry out transmissions at a certain modulation order and/or code rate is related to the SIR of the link (i.e., frequency band) between the transmitter and the receiver. A higher SIR implies the possibility of utilizing higher order modulations and/or a higher code rate.
Upon utilizing either of the methods described above (i.e., as illustrated in
Since certain changes may be made in the above-described method for exploiting the diversity across frequency bands of a multi-carrier cellular system or network, 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.