Various wireless access technologies have been proposed or implemented to enable mobile stations to perform communications with other mobile stations or with wired terminals coupled to wired networks. Examples of wireless access technologies include GSM (Global System for Mobile communications) and UMTS (Universal Mobile Telecommunications System) technologies, defined by the Third Generation Partnership Project (3GPP); and CDMA 2000 (Code Division Multiple Access 2000) technologies, defined by 3GPP2.
Another type of wireless access technology is the WiMax (Worldwide. Interoperability for Microwave Access) technology. WiMax is based on the IEEE (Institute of Electrical and Electronics Engineers) 802.16 Standard. The WiMax wireless access technology is designed to provide wireless broadband access.
In a WiMax wireless network, base stations are provided to offer wireless access in corresponding coverage areas. An issue associated with using many base stations is that each base station requires a dedicated backhaul connection in an access service network (for example, base stations are connected by these backhaul connections to one or more ASN gateways in a WiMax wireless network).
To alleviate the issue of having too many base stations and corresponding dedicated backhaul connections, the concept of relay stations has been proposed for standardization in IEEE 802.16j, which is also referred to currently as the Multihop Relay (MR) Specification. Within a coverage area of a base station (referred to as an MR base station in the Multihop Relay context), one or more relay stations can be provided to provide coverage for certain zones (which can be dead zones for the base station, for example) within the coverage area. A benefit of using relay stations is that backhaul connections do not have to be provided between the relay stations and other components of the access service network. A mobile station within the coverage area can connect wirelessly with either the base station or a relay station in the coverage area, for both uplink and downlink communications.
Information sent wirelessly on the downlink to a mobile station includes a frame start preamble and payload data. A preamble is sent from the base station to the mobile station to allow the mobile station to acquire the downlink data (payload data). The preamble can include information such as modulation scheme, transmission rate, length of time to transmit an entire data frame, and so forth. In a Multihop Relay wireless network, a preamble may be sent directly from the base station to the mobile station, even though the corresponding downlink data may be relayed through the relay station to the mobile station. The preamble is sent by the base station to the mobile station is typically highly coded and thus well protected from interference (to increase the likelihood of reliable receipt by the mobile station in a coverage area). On the other hand, downlink data sent from the base station is not as highly coded and therefore not as protected from interference. As a result, in certain zones within a coverage area, downlink data and uplink data would have to be relayed through a relay station rather than communicated directly from the base station to the mobile station.
When the mobile station is relatively far away from the base station within a coverage area, the preamble received by the mobile station from the base station suffers a relatively large path loss from the base station to the mobile station. However, even though the mobile station may be far away from the base station, the mobile station can be quite close to a relay station that is relaying downlink data to the mobile station. As a result, the downlink data received from the relay station by a mobile station can have a power level that is significantly higher than the power level of the preamble received by the mobile station.
A wireless receiver in a mobile station typically includes an adjustable gain control (AGC) circuit that is used to adjust a gain applied to a received signal based on some indication of the expected power level of received signals. If the automatic gain control is performed based on the power level of the preamble received directly from the base station, then this may cause received downlink data that is at a high power level from the relay station to be out of range, which can cause the AGC circuit to not be able to receive the downlink data properly. More specifically, the AGC circuit in the mobile station may not be able to track the sharp variation of power of the downlink data as compared to the preamble power, which can cause a data recovery failure. Moreover, open loop power control for uplink transmission, which is performed using downlink path loss and/or uplink noise plus interference level information, would not operate as desired, which can cause enhanced uplink interference. With a relay station present, the downlink path loss is from the relay station to the mobile station instead of the base station to the mobile station; similarly the uplink noise and interference should be measured at the relay station instead, of the base station.
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