The present disclosure relates generally to wireless communications and more specifically to medium access control frame structures in wireless communication systems with improved latency support.
An important consideration for advanced wireless communication systems is one-way air-interface latency. Air-interface latency is primarily dependent on the Medium Access Control (MAC) frame duration. In the developing IEEE 802.16m protocol, for example, the proposed target latency is less than approximately 10 msec and some observers have suggested that a much lower latency may be required to compete with other developing protocols, for example, with 3GPP Long Term Evolution (LTE). The IEEE 802.16m protocol is an evolution of the WiMAX-OFDMA specification for the IEEE 802.16e protocol. However, the legacy IEEE 802.16e TDD frame structure has a relatively long duration and is incapable of achieving the latency targets set for IEEE 802.16m.
Evolutionary wireless communication systems should also support for legacy system equipment. For example, some IEEE 802.16e and IEEE 802.16m base stations and mobile stations are likely to coexist within the same network while upgrading to the newer system. Thus IEEE 802.16e mobile stations should be compatible with IEEE 802.16m base stations, and IEEE 802.16e base stations should support IEEE 802.16m mobile stations. Thus frame structures for air-interfaces are proposed with a view to achieving lower latency and in some embodiments to maintaining backward compatibility.
A legacy system is defined as a system compliant with a subset of the WirelessMAN-OFDMA capabilities specified by IEEE 802.16-2004 (specification IEEE Std 802.16-2004: Part 16: IEEE Standard for Local and metropolitan area networks: Air Interface for Fixed Broadband Wireless Access Systems, June 2004) and amended by IEEE 802.16e-2005 (IEEE Std. 802.16e-2005, IEEE Standard for Local and metropolitan area networks, Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems, Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands, and IEEE Std. 802.16-2004/Cor1-2005, Corrigendum 1, December 2005) and IEEE 802.16Cor2/D3, where the subset is defined by WiMAX Forum Mobile System Profile, Release 1.0 (Revision 1.4.0: 2007-05-02), excluding specific frequency ranges specified in the section 4.1.1.2 (Band Class Index).
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.
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Generally, base units 101 and 102 transmit downlink communication signals 104 and 105 to serving remote units on at least a portion of the same resources (time and/or frequency). Remote units 103 and 110 communicate with the one or more base units 101 and 102 via uplink communication signals 106 and 113. The one or more base units may comprise one or more transmitters and one or more receivers that serve the remote units. The remote units may also comprise one or more transmitters and one or more receivers.
In one embodiment, the communication system utilizes OFDMA or a next generation single-carrier (SC) based FDMA architecture for uplink transmissions, such as interleaved FDMA (IFDMA), Localized FDMA (LFDMA), DFT-spread OFDM (DFT-SOFDM) with IFDMA or LFDMA. In OFDM based systems, the radio resources include OFDM symbols, which may be divided into slots, which are groupings of sub-carriers. An exemplary OFDM based protocol is IEEE 802.16(e).
Generally, the wireless communication system may implement more than one communication technology as is typical of systems upgraded with newer technology, for example, the evolution of GSM to UMTS and future UMTS releases thereof. In
Regarding frame structure, it is generally necessary to design frames having a relatively short duration in order to reduce latency. Thus to deliver low latency in 802.16m systems with backward compatibility, it is necessary to develop a sub-frame structure based on the legacy 802.16(e) frame. In order to address the latency requirements, it is necessary to design frames with shorter than 5 msec duration. However, to efficiently serve legacy traffic, it is also necessary that 802.16(m) systems have 5 msec legacy frames. Thus two broad classes of frames would be required for an 802.16(m) system having reduced latency and support for legacy 802.16(e) devices. The first class includes a full-frame (having a 5 msec duration) with one DL interval and one UL interval similar to the 802.16(e) TDD legacy frames. The second class of frames includes a sub-frame. For example, a 5 msec frame having N DL intervals and N UL intervals. This frame may also contain N transmit/receive transition gap (TTG) and receive/transmit transition gap (RTG) intervals. N could be kept small, typically N=2, in order to limit TTG and RTG related overhead. According to this exemplary scheme, the legacy 802.16(e) TDD frames can only be a full-frame and the 802.16(m) frames are preferably sub-frame 1:2, although they could also be full-frames. The h-frames can be either full-frame or sub-frame 1:2.
The 802.16(m) 5 msec frame can be perceived to be composed of following types of basic regions: e-DL region used for transmission of downlink traffic to 802.16(e) terminals; e-UL: region allocated for transmission of data and control messages by 802.16(e) terminals; m-DL: region allocated for transmission to 802.16(m) terminals; and m-UL: region allocated for transmission by 802.16(m) terminals. The e-DL and e-UL regions can also be used for transmissions to/from 802.16(m) terminals. In general, the structures of the 802.16(m) region (sub-channel and pilot structures) can be different from those of the 802.16(e) regions. Depending on the population of legacy and newer generation terminals, it may be necessary to allocate the entire 5 msec frame for 802.16(e) services or 802.16(m) services.
Using these different types of regions, various types of 5 msec frame structures can be created to suit the traffic service requirements. These are: e-frames composed of only e-DL and e-UL regions used to serve legacy 802.16(e) TDD terminals (802.16(m) terminals can also be served in these frames in legacy mode); m-frames composed of m-DL and m-UL regions only for serving only 802.16(m) terminals; h-frames containing both e-DL/e-UL and m-DL/m-UL regions for serving 802.16(e) and 802.16(m) terminals. The 802.16(m) portion and the 802.16(e) portion should be time division multiplexed so that the 802.16(m) control channel, pilot, and sub-channelization can provide flexibility.
Depending on the device type population and traffic pattern, it may be necessary to treat an m-frame or an h-frame as a legacy virtual frame in a cell/sector. The m-DL and m-UL regions in these frames may have different sub-channel/pilot structures than the legacy systems; those regions need to be treated as “dead zones”, which the legacy terminals should not use. The full-frame, being similar in structure to the legacy 802.16(e) frame, can be easily mapped to a legacy virtual frame with full utilization of the frame resources. However, the sub-frame 1:N, which can also be mapped to legacy 802.16(e) virtual frame, will contain “dead zone(s)” where no 802.16(e) (TDD) transmission can be allowed to ensure DL/UL synchronization.
An 802.16(m) base unit can provide service to legacy 802.16(e) terminals in full-frames. To provide service in the sub-frame 1:N, the 802.16(m) base unit can map a legacy virtual 5 msec frame to N adjacent sub-frames and the train of sub-frames can be organized as a train of legacy 5 msec virtual frames. There are N choices for the time division duplex frame (TDD) split position in a legacy virtual frame. The system wide synchronization requirement for the TDD system imposes additional constraints on the downlink and uplink transmission intervals, creating dead zones during which no transmission should be done to and from legacy 802.16(e) TDD terminals. However, transmissions to and from 802.16(m) terminals are possible in these dead zones.
A generic message structure and its parameters to indicate a dead zone is shown in Table 1.
In the above message, the parameter “location” indicates a position within the frame in time (which may be denoted by the symbol number within the frame or absolute time or time offset from the start of the frame or offset from some other specified time); the interpretation of the parameter “location” depends on the value of the parameter “dedicated pilot tag”. If “dedicated pilot tag” is 1, the pilot symbols after “location” are dedicated; if it is 0, it indicates that the pilot symbols after the “location” are not dedicated pilots. Thus a zone with dedicated pilots can be described by two occurrences of this message: the first message with dedicated pilot tag=1 and location=“T1”, followed by a 2nd message with dedicated pilot tag=0 and location=“T2”, where T2>=T1; a legacy terminal which has been allocated resources within this zone should use only pilots within its burst for channel estimation. A legacy terminal which has not been allocated resources within this zone will ignore the pilots in this zone and also it will not need to decode any of the data transmissions within the dedicated pilot zone. This combined with the BS not making an allocation to any 16e mobile in the zone indirectly disables or isolates the 16e mobiles from this zone. Thus, 16e mobile effectively ignores whatever is in the zone.
An example message which can be used for indicate dead zones is the STC_DL_ZONE_IE( ) of IEEE 802.16e specification; the parameters “OFDMA symbol offset” and “Dedicated pilots” in this message corresponds to the parameters “location” and “dedicated pilot tag” in the above generic message in Table 1.
Another message structure and its parameters which can be used to implement dead zones are shown in Table 2.
The four parameters describe a rectangular dead zone of time-frequency resources. In this message, the parameter “starting symbol” indicates a position within the frame in time (which may be denoted by the symbol number within the frame or absolute time or time offset from the start of the frame or offset from some other specified time) where the dead zone begins; “symbol count” indicates the duration of the dead zone, starting from the “starting symbol”. The parameter “starting sub-channel” indicates the location in the sub-carrier frequency where the dead zone begins; this is in units of sub-carrier or sub-channel, which is a group of sub-carriers; “sub-channel count” indicates the length of the dead zone in the frequency dimension. An example of this generic message type is the PAPR_Reduction_and_Safety_Zone_Allocation_IE( ) of the IEEE 802.16e specification. In this message, the parameters “OFDMA_symbol_offset”, “Subchannel offset”, “No. OFDMA symbols” and “No. sub-channels” corresponds to the parameters “starting symbol”, “starting sub-channel”, “symbol count” and “sub-channel count” of the generic dead zone message type 2, respectively; the PAPR_Reduction_Safety_Zone parameter in the PAPR_Reduction_and_Safety_Zone_Allocation_IE( ) should be set to “1” to indicate a reduced interference zone to the legacy terminal; this will effectively direct the terminal not to perform any uplink transmission in that zone.
Striking a balance between efficient legacy support and low-latency 802.16(m) service is challenging with a homogeneous frame size. The full-frames discussed above provide efficient legacy support while sacrificing latency performance for 802.16(m) terminals. The sub-frames provide low-latency support for 802.16(m) terminals while sacrificing capacity for legacy terminals in the form of dead zones.
In one embodiment, a heterogeneous configuration contains both full-frames and sub-frames, wherein the full-frames and sub-frames are interleaved over time. Within a cell, the full-frames are primarily used for serving legacy terminals present in the cell, whereas the sub-frames are primarily used to serve the 802.16(m) terminals. However, for servicing packets with urgent delay constraints, either frame type can be used to service either type of terminal. The full-frames and the sub-frames are organized in a repeating pattern, called a super-frame.
In the super-frame of
Thus a next generation wireless communication infrastructure entity, for example, an 802.16(m) base unit in
In one embodiment, the configuration characteristic of the regions is selected from a group comprising: a number regions; region size; region type (e.g., uplink or downlink for a TDD system); and the ordering of the regions. Multiple characteristics may also be specified. In one embodiment, for a TDD system, the control message specifies whether the regions of the frame are uplink regions or downlink regions. Thus the regions are selected from a group of regions comprising: an uplink region and a downlink region. The control message may also specify the number of uplink regions or downlink regions within each frame of a super-frame. In some embodiments, the control message specifies a size of uplink regions or downlink regions within each frame of a super-frame. In
There are various ways to configure frames that provide legacy compatibility and reduce latency based on the proposed framework. Another factor to consider in the design of a new protocol frame structure is support for both TDD and FDD. Preferably, similar frame and sub-frame structures can be applied for both TDD and FDD.
In one embodiment, a frame is divided into multiple blocks of equal size, wherein the blocks may support one or more protocols, for example, IEEE 802.16(e) and/or 802.16(m). Such a frame would enable an 802.16(m) wireless communication infrastructure entity to allocate radio resources to both 802.16(e) and 802.16(m) wireless terminals. Generally, the radio frame includes a plurality of blocks, including a first block and last block, wherein each block comprises a plurality of symbols. In one embodiment, each block comprises substantially the same number of symbols. The first block includes a first protocol preamble, for example, a legacy protocol preamble like 802.16(e). The remaining blocks in the frame are devoid of the first protocol preamble.
Generally, the radio frame includes at least one first protocol block and/or at least one second protocol block, for example, 802.16(e) and/or 802.16(m) blocks. In some embodiment, the frame includes both first and second protocol blocks. In another embodiment, the frame includes only second protocol blocks, for example, 802.16(m) blocks. The radio frame includes an allocation control message for allocating resources within a protocol block. In frames that include first and second protocol blocks, the radio frame includes a first protocol allocation control message for allocating resources in the first protocol block, and a second protocol allocation control message for allocating resources in the second protocol block. In one embodiment, the allocation control message is a first protocol allocation control message for allocating resources within a first protocol block of a radio frame, for example, a subsequent frame, that is different than the radio frame within which the first protocol allocation control message is located. In one embodiment, the first allocation control message is located in the first block. The first block may be a first or second protocol block, for example, an 802.16(e) or 802.16(m) block.
The sub-blocks may be described based on their position in the frame and the characteristics of the sub-block. For example, a 5 msec frame supporting both 802.16(e) and 802.16(m) protocols may be characterized as one of the region types discussed above. There are five types of 802.16(m) sub-blocks. Each sub block has a unique characteristic designed to achieve the backward compatibility goals and efficient 802.16(m) performance. An 802.16(m) DL Lead Sub-Block contains a legacy 802.16(e) pre-amble in the first symbol. The remaining symbols of the frame may be allocated to 802.16(m). This sub-block may only be transmitted in the first sub-frame. An 802.16(m) DL Lead Compatible sub-block also contain a 802.16(e) FCH and 802.16e DL-MAP in addition to the 16e pre-amble for backward compatibility with legacy terminals. The remaining symbols are allocated to 802.016(m). The Lead Compatible sub-block may be transmitted only in the first sub-frame. An 802.16(m) Synchronization Sub-Block contains a broadcast control that may be used to synchronize an 802.16(m) terminal and describe broader aspects of the 802.16(m) frame. This sub-block occupies a unique position in the 5 ms frame as a reference for synchronization. The second sub-frame is an appropriate, but not necessary, position for this synchronization sub-block. An 802.16(m) DL Sub-Block is a generic 16m sub-block that contains 802.16(m) Downlink data and 802.16(m) control. This may be occupying the 2nd, 3rd or 4th sub-frames. An 802.16(m) UL Sub-Block is a generic 802.16(m) sub-block contains 802.16(m) Downlink data and 802.16(m) control. This block may occupy the 2nd, 3rd or 4th sub-frame.
There are five types of 802.16(e) sub-blocks that may be allocated in the 802.16(m) frame structure. These sub-blocks conform to the legacy specification of 802.16(e) frames and cannot be distinguished from legacy 802.16(e) frames by a legacy mobile. A Legacy DL Lead Sub-Block is identical to legacy frames containing a 802.16(e) pre-amble, 802.16(e) FCH, 802.16(e) DL-MAP. This sub-block will contain 802.16(e) downlink data and would typically contain an UL MAP. A legacy DL Secondary Sub-Block is identical to legacy 802.16(e) numerology and contains 802.16(e) DL data. The Legacy DL Secondary Sub-Block may only follow a Legacy DL Lead Sub-Block. A Legacy DL Tertiary Sub-Block block is identical to a legacy 802.16(e) numerology and contains 802.16(e) DL data. The Legacy DL Tertiary Sub-Block may only follow a Legacy DL Secondary Sub-Block. A legacy UL Tertiary Sub-Block contains legacy uplink data and may also contain legacy uplink control. A legacy UL Tail Sub-Block contains legacy uplink data and may also contain legacy uplink control.
In one implementation, the sub-block type allocated depends on the frame position. The following sub-blocks may be allocated to the first sub-frame position: 802.16(m) Lead Sub-Block; 802.16(m) DL Lead Compatible Sub-Block; and Legacy DL Lead Sub-Block. The following sub-blocks may be allocated to the second sub-frame position: 802.16(m) Synchronization Sub-Block; 802.16(m) DL Sub-Block; 802.16(m) UL Sub-Block; and Legacy DL Secondary Sub-Block. The following sub-blocks may be allocated to the third sub-frame position: 802.16(m) DL Sub-Block; 802.16(m) UL Sub-Block; Legacy DL Tertiary Sub-Block; and Legacy UL Tertiary Sub-Block. The following sub-blocks may be allocated to the fourth sub-frame position: 802.16(m) DL Sub-Block; 802.16(m) UL Sub-Block; and Legacy UL Tail Sub-Block.
Using these different types of regions, various types of frame structures can be created to suit the traffic service requirements also discussed above. Generally, the first block in the frame is a DL region with the first symbol allocated for the preamble. The last symbol or the last 2 or 3 symbols for cells with relatively large radiuses of the DL block, if the next block is an UL block, will be allocated for TTG. If the last block is an UL block, then the last portion of the 5 msec frame is allocated for RTG. For additional DL/UL split, the first symbol of the DL block (following an UL block) is allocated for RTG.
The size of the 802.16(e) basic MAP is between approximately 2 and approximately 4 OFDM symbols. The rest of the first block contains an 802.16(m)-DL region 808. The last block contains an 802.16(m) UL region and the other 2 blocks contain 802.16(m) DL or 802.16(m) UL regions. Both full-frame and Sub-frame 1:2 can be constructed using this configuration. The control overhead for frame 800 is small since it does not support 802.16(e) data traffic. As many as 2 bits may be required to signal the construction of frame 800. Even though the frame 700 of
Table 2 illustrates an m-frame sub-frame structure control signal.
Table 5 illustrates an HEM-II sub-frame structure control signal.
Table 7 illustrates an exemplary HEM-I sub-frame structure control signal.
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In the example above, the allocation of frame resources for legacy and 802.16(m) traffic and the allocation for DL and UL intervals are in terms 12-symbol blocks. This scheme requires small control overhead, however, allows only a limited set of legacy and 802.16(m) partitions and a limited set of TDD splits. In this section an alternative scheme is described which allows flexible allocation of legacy and 16m partition sizes as well as allows wider range of TDD splits enabling more flexibility in adapting to the DL/UL traffic ratios. In this scheme, there is a super-frame structure comprising one or more of: legacy 802.16(e) frame, 802.16(m) frame, and/or hybrid frame. In some embodiments, the length of the super-frame can be any multiple of 5 msec, thus a hybrid frame of 5 ms is an included special case of the super-frame structure. In other embodiments, the super-frame length could be different from 5 ms. The 802.16(e) frames are same as the legacy frames. The 802.16(m) frames are not required to support 802.16(e) services and they need not have any legacy component. They can have either Full-frame structure or a Sub-frame 1:N structure consisting of N m sub-frames. The m sub-frame can be configured to have a possibly wide range of TDD split. In the hybrid frames that support both 802.16(e) and 802.16(m) terminals within the same 5 msec period, the 5 msec interval is partitioned into 802.16(e) and 802.16(m) regions. Two different types of partitioning are described.
A wireless communication infrastructure entity, for example, an 802.16(m) base station generally transmits a sequence of radio frames, for example, for allocating radio resources to wireless terminals compliant with a first protocol and wireless terminals compliant with a second protocol. In one embodiment, at least fifty percent (50%) of the radio frames in the sequence include a first protocol preamble, for example, an 802.16(e) preamble, in order to facilitate any 802.16(e) mobile units ability to maintain synchronization to the system. In this embodiment, a radio frame that includes a first protocol preamble may or may not also contain a first protocol allocation control message.
The second protocol, for example, 802.16(m), allocation control message may be located in a predetermined location within the radio frame. By locating the second protocol allocation message in a known or predetermined location, the complexity of an 802.16(m) mobile station can be reduced, since it may be able to avoid attempting to blindly detect the location of the message. Blind detection typically involves attempting to decode a message over multiple resource sets until a proper message cyclic redundancy check (CRC) is obtained. The first protocol resource region generally includes pilot sub-carriers. In one embodiment, the radio frame includes a message indicating that first protocol terminals should not use pilot sub-carriers in the second protocol resource region (e.g., by a messaging indicating a dedicated pilot zone with an absence of allocations to the first protocol terminals within the dedicated pilot zone, or by a message indicating a safety zone, or other means). Sub-carriers in second region may not exist or may be in a different location than pilots in the first region. In another embodiment, the message identifies a dedicated pilot interval that includes the second protocol resource region. The radio frame may also include a message identifying a boundary of the first protocol resource region (e.g., by a messaging indicating a dedicated pilot zone with an absence of allocations to the first protocol terminals within the dedicated pilot zone, or by a message indicating a safety zone, or other means).
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In some embodiments of the invention, the first protocol allocation control message (e.g., 802.16(e) MAP) can allocate resources within the first protocol resource region (e.g., 802.16(e) region or zone) to a wireless terminal compliant with both the first protocol and the second protocol (e.g., an 802.16(m) terminal). In this case, the 802.16(m) terminal assigned/allocated resources within the 802.16(e) region may be required to receive and/or transmit using the 802.16(e) protocol. Assigning/allocating resources to an 802.16(m) mobile within an 802.16(e) region in this manner can be advantageous for load balancing purposes—for example, there may be times when the 802.16(m) region may become fully allocated/utilized while the 802.16(e) region is not fully utilized. This can occur dynamically based on traffic patterns and scheduling policies. In such a case, some of the 802.16(m) terminals can be assigned resources in the 802.16(e) region in order to accommodate a higher total amount of traffic for 802.16(m) terminals.
While the present disclosure and the best modes thereof have been described in a manner establishing possession and enabling those of ordinary skill to make and use the same, it will be understood and appreciated that there are equivalents to the exemplary embodiments disclosed herein and that modifications and variations may be made thereto without departing from the scope and spirit of the inventions, which are to be limited not by the exemplary embodiments but by the appended claims.
Number | Date | Country | |
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60956031 | Aug 2007 | US |