As wireless devices become increasingly widespread, there is a huge demand for frequency spectrum to meet the explosive increase in data traffic. Unfortunately, frequency spectrum is very scarce and the efficient utilization of frequency resources is thus crucial. It is known that the frequency spectrum is heavily crowded and fragmented, with most frequency bands already assigned to various licensed services. The other frequency bands, especially low frequency bands, are not sufficient to meet future requirements of wireless broadband services.
Accordingly, a technique of spectrum sharing between different systems is a promising solution to overcome the spectral shortage problem in future system designs. Generally, in practical systems, frequency spectrum has been shared among different users in the same network. For example, the frequency spectrum can be shared between different cells in a cellular network with appropriate network planning and interference mitigation techniques. Also, in unlicensed bands, many Medium Access Control (MAC) protocols, such as ALOHA and Carrier Sense Multiple Access (CSMA), have been proposed to enable spectrum sharing among homogeneous users. However, for heterogeneous systems having different configurations (e.g. frame-based scheduled system vs. contention-based system), spectrum sharing becomes especially complicated.
A frame-based scheduled system, as used herein, refers to a communication system based on a predefined frame structure and allocates resources to different Mobile Stations (MSs) for orthogonal transmission under the control of a base station (BS). In other words, the transmission in such system is scheduled frame by frame. Examples of frame-based scheduled systems include the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system and the Institute of Electrical and Electronic Engineers (IEEE) 802.16e system. In this context, a transmission in a frame-based scheduled system is referred to as a frame-based scheduled transmission.
FIG. 1 shows an example frame structure in a frequency channel in a frame-based scheduled system. The frame structure is known to a BS and all MSs served by the BS. For a Frequency Division Duplex (FDD) system, Frames #0 to #3 are all used for downlink (DL) or uplink (UL) transmission. For a Time Division Duplex (TDD) system, each of Frames #0 to #3 is configured by the upper layer as a DL or UL frame. MAC Protocol Data Units (PDUs) are scheduled to be transmitted in these frames.
A contention-based system, as used herein, refers to a communication system in which MSs share a channel in a contention-based manner. In a typical contention-based system, a sensing-based collision avoidance mechanism is used at the MAC layer and a PDU is not transmitted in a predefined time point. An example of contention-based system is the Wi-Fi system, which allows MSs to transmit/receive data wirelessly via an Access Point (AP). The Wi-Fi system is also known as the wireless local area network (WLAN) system or the IEEE 802.11 system. In this context, a transmission in a contention-based system is referred to as a contention-based transmission.
FIG. 2 is a schematic diagram of the MAC layer mechanism of the Wi-Fi system. The basic IEEE 802.11 MAC layer employs a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism. The same mechanism applies to all stations including the MSs and the AP, i.e. in both downlink and uplink. Any station that wishes to transmit a packet first senses the medium. If the medium is sensed to be idle for a minimum time period known as Distributed Inter Frame Space (DIFS, which is 50 μs for 802.11b), the packet is then transmitted, as can be seen from the operation sequence of Station 1 in FIG. 2. If the medium is sensed to be busy, the station first defers the transmission until the medium is sensed to be idle, as can be seen from the operation sequence of Station 2 in FIG. 2. At this time, however, the station does not transmit the packet immediately as it may lead to a collision if more than one station starts transmission immediately after sensing the medium to be idle. Instead, the station sets a backoff timer to a random number, and does not transmit until this timer has expired. The backoff timer is only decreased when the medium is sensed to be idle; whereas, whenever the medium is sensed to be busy, a deferral state is entered where the backoff timer is not decreased. When the backoff timer expires, the packet is transmitted. If the packet is successfully received by the receiver, the receiver responds to the transmitter with an acknowledgement (ACK). The acknowledgement is sent after a Short Inter Frame Space (SIFS, which is 10 μs for 802.11b) from the data frame being received. Since SIFS<DIFS, no other user will access the medium during this time period. If no acknowledgement is received, either because the packet itself or the acknowledgement was lost, the transmitter starts a new backoff timer, and retransmits the packet when the new backoff timer has expired. Even if the packet is successfully acknowledged, the transmitter needs to start another backoff timer and wait for it to expire before transmitting the next packet. To avoid congestion, when a collision occurs, the backoff values are drawn from distributions with larger and larger expectations for every retransmission attempt. The backoff time is measured in units of time slots, which are 20 μs for 802.11b.
FIG. 3 shows a schematic diagram of an example coexistence scenario in which two systems share a frequency channel. As shown in FIG. 3, for example, a base station BS1 and a mobile station MS1 are stations in a frame-based scheduled system, while a base station BS2 and a mobile station MS2 are stations in a contention-based system. It can also be seen that BS2 and MS2 are located in the coverage of BS1. In this case, when all frames are scheduled to be used by BS1 for its communication with MS1, i.e., when the channel is continuously occupied by BS1, BS2 will have no chance to transmit any data since it always senses that the channel is busy. Obviously this does not meet the requirement of fairness between systems and new mechanisms should be designed to solve this coexistence problem.
IEEE 802.16h is an amendment to expand IEEE 802.16 standard, specifying improved mechanisms such as policies and MAC enhancements to enable coexistence among license-exempt systems and to facilitate the coexistence of such systems. Non-patent document “IEEE Standard 802.16h-2010, Air interface for broadband wireless access systems Amendment 2: Improved coexistence mechanisms for license-exempt operation, Jul. 30, 2010”, which is incorporated herein by reference, provides a coexistence mechanism known as coordinated contention-based protocol (CX-CBP) which applies to license-exempt frequency bands for coexistence with contention-based systems.
FIG. 4 shows a frame structure for CX-CBP. As shown in FIG. 4, MAC Frames #0 and #1 are configured as schedule-based frames and the time interval composed of Frames #0 and #1 is referred to as CX-SBI (Coordinated Coexistence Schedule-Based Interval). MAC Frames #2 and #3 are configured as contention-based frames and the time interval composed of Frames #2 and #3 is referred to as Coordinated Coexistence Contention-Based Interval (CX-CBI).
From the perspective of the 802.16h system which is a frame-based scheduled system and shares a channel with a contention-based system (e.g., 802.11 system), during the CX-SBI, the 802.16h system uses the channel for data transmission regardless of whether the contention-based system is using the channel; while during the CX-CBI, the 802.16h system uses the channel in accordance with a Scheduled Listen-Before-Talk (SLBT) mechanism.
Before any transmission during the CX-CBI, an IEEE 802.16h station shall first check if the channel is idle. If the channel is idle for at least a particular time period before a predefined transmission time point of the IEEE 802.16h station, the IEEE 802.16h station shall start its transmission at the predefined transmission time point. If the channel is busy, the transmission shall be deferred until the next predefined transmission opportunity.
In order to provide transmission opportunity for the contention-based system, the simplest way is that the 802.16h system configures an entire frame in a CX-CBI as a Quiet Period (QP) during which the 802.16h system is quiet without transmitting any data. However, this may result in a waste of transmission resources especially when the contention-based system has a low traffic.
Thus, a Contention Window (CW) mechanism, which enables multiple systems to access the channel while reducing potential collisions, is introduced in the IEEE 802.16h standard to alleviate the resource waste. FIG. 5 shows a frame structure in which the CW mechanism is applied. As shown in FIG. 5, two contention windows (CWs) are predefined in the CX-CBI. The IEEE 802.16h station will randomly select a time point in each CW as a transmission opportunity (referred to as LBT TXOP since the transmission can only be carried out after confirming that the channel is idle. In addition, the transmission is only limited from the LBT TXOP to the end of this CW. For more details about the CW mechanism, reference can be made to “IEEE Standard 802.16h-2010, Air interface for broadband wireless access systems Amendment 2: Improved coexistence mechanisms for license-exempt operation, Jul. 30, 2010”.
However, according to the above technique, the scheduled system alternates deterministically between CX-SBI and CX-CBI to satisfy a particular fixed duty-cycle defined for the system. For example, in FIG. 3, the scheduled system is scheduled to transmit every two frames. That is, the frame configuration, i.e. schedule-based or contention-based, is fixed. However, in most practical scenarios, the coexisting systems typically have asymmetric traffic, e.g., the contention-based system may have much higher or lower traffic load than the scheduled system in different time periods, which will result in the resource waste and degraded performance for both systems.
FIG. 6 shows an example channel use scenario in which the scheduled system has higher traffic than the contention-based system. As can be seen from FIG. 6, at the first TXOP, the scheduled system senses that the channel is busy (occupied by the contention-based system) and cannot transmit any data. At the second TXOP, the scheduled system senses that the channel is idle and can start transmission. However, as described above, the transmission is only limited from the TXOP to the end of the CW. Therefore, the overall channel use rate is very low and a large amount of packets will be queued in the scheduled system. On the other hand, when the contention-based system has higher traffic than the scheduled system, similar problem will occur due to such fixed frame configuration.
There is thus a need for a solution for coordination of channel resources between systems, capable of achieving a tradeoff between minimization of inter-system collision and maximization the performance of each system, especially in the scenario in which the systems have asymmetric traffics.