Patent Application
20160142241
Particular embodiments relate generally to synchronization signals in wireless communications, and more particularly to frame formats for cell search procedure synchronization signals.
When a wireless device powers on or moves between cells in a wireless network, the wireless device receives and synchronizes to downlink signals in a cell search procedure. The cell search procedure identifies a preferable cell and performs time and frequency synchronization to the network in downlink (e.g., from a base station to a user equipment).
A user equipment (UE) may use primary and secondary synchronization signals (PSS and SSS), such as those described in Section 6.11 of Third Generation Partnership Project (3GPP) TS 36.211, version 11.2.0, for performing a cell search procedure, such as the cell search procedure described in Section 4.1 of 3GPP TS 36.213, version 12.1.0. 3GPP specifies that for frequency division duplex (FDD) PSS is transmitted in the last orthogonal frequency division multiplexing (OFDM) symbol of slots 0 and 10 within a radio frame and that SSS is transmitted in the OFDM symbol preceding PSS, such as illustrated in
The UE then detects the SSS. From the detected SSS, the UE acquires the physical cell id and achieves radio frame synchronization. The UE also detects whether the cyclic prefix length is normal or extended. A UE that is not preconfigured for a particular duplex mode (e.g., TDD or FDD) may detect the duplex mode by the frame position of the detected SSS in relation to the detected PSS. The UE may estimate fine frequency offset by correlating PSS and SSS. Alternatively, the UE may use cell-specific reference signals (CRS) to estimate fine frequency offset.
After synchronizing with the PSS and the SSS, the UE may receive and decode cell system information, which contains cell configuration parameters such as the Physical Broadcast Channel (PBCH). The number of OFDM symbols used for PDCCH (Physical Downlink Control Channel) is signaled by PCFICH (Physical Control Format Indicator Channel) according to Section 6.7 of 3GPP TS 36.211, version 11.2.0. The PCFICH is decoded before the UE receives PDCCH. The number of OFDM symbols signaled by PCFICH may be 1, 2 or 3 for large bandwidth allocations (e.g., more than 10 resource blocks) and 2, 3 or 4 OFDM symbols for small bandwidths (e.g., less than or equal to 10 resource blocks). The first OFDM symbols of a sub-frame are used for PDCCH.
Section 6.10.1 of 3GPP TS 36.211, version 11.2.0, illustrates CRS mappings for one, two, and four antenna ports. As illustrated in the 3GPP specification, CRS are not mapped on the same OFDM symbols as used for PSS and SSS.
According to some embodiments, a method of synchronizing a wireless device with a network node comprises receiving a radio subframe transmitted from the network node. The radio subframe comprises a first Primary Synchronization Signal (PSS) associated with a first Orthogonal Frequency Division Multiplexing (OFDM) symbol and paired with a first Secondary Synchronization Signal (SSS) associated with a second OFDM symbol. The radio subframe also comprises a second PSS associated with a third OFDM symbol and paired with a second SSS associated with a fourth OFDM symbol. The method further comprises detecting at least one of the first PSS and the second PSS within the radio subframe and detecting at least one of the first SSS and the second SSS within the radio subframe. The method determines system information associated with the network node based on the detected at least one PSS and the detected at least one SSS.
In particular embodiments, the first, second, third, and fourth OFDM symbols do not include OFDM symbols reserved for Physical Downlink Control Channel (PDCCH). In particular embodiments, the first, second, third, and fourth OFDM symbols do not include the last and second-to-last OFDM symbol of slot zero and the last and second-to-last OFDM symbol of slot ten within a frame configured for frequency division duplex and the third-position OFDM symbol of slot three and slot thirteen and the last OFDM symbol of slot two and slot twelve within a frame configured for time division duplex. In particular embodiments, the first, second, third, and fourth OFDM symbols do not include OFDM symbols that include Cell Reference Signals (CRS).
In particular embodiments, the method comprises detecting both the first PSS and the second PSS within the radio subframe and accumulating the first PSS and the second PSS. The method further comprises determining system information associated with the network node based on the accumulated first PSS and second PSS.
In particular embodiments, the method comprises accumulating the detected at least one PSS and a PSS detected in a previously received subframe and determining system information associated with the network node based on the detected at least one PSS and the PSS detected in a previously received subframe.
Particular embodiments may exhibit some of the following technical advantages. Particular embodiments may include a PSS and SSS cell search frame format that is backward compatible such that legacy UEs will not detect these cell search signals or need to be aware of their existence. Particular embodiments use synchronization sequences other than those specified in LTE release 12. In particular embodiments, cell search signals are placed in resource blocks that are not scheduled to legacy UEs. In particular embodiments, PSS and SSS frame formats may use a large fraction of the reserved resource blocks which results in low overhead. Particular embodiments may allocate PSS/SSS pairs in subsequent OFDM symbols such that a high resolution frequency offset estimate can be calculated with low computational complexity.
In particular embodiments, a PSS/SSS pair is not transmitted such that PSS is transmitted in one slot and SSS in the next, or vice versa. The first symbol of each slot has a longer cyclic prefix than the other OFDM symbols of the slot. In embodiments that transmit a PSS/SSS pair in the same slot, the timing between PSS and SSS within each pair is constant, such that the phase rotation between PSS and SSS may be used for a fine granularity frequency offset estimator. In particular embodiments, a UE may use coherent accumulation to improve the received SINR. In particular embodiments, a base station may use beamforming or repetition of PSS and SSS to increase successful cell detection rate and reduce cell detection latency. Other technical advantages will be readily apparent to one skilled in the art from the following figures, description and claims.
For a more complete understanding of the present invention and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:
In particular networks, a UE might receive cell search signals at a low signal to interference plus noise ratio (SINR), which results in degraded or impossible cell attachment. 3GPP specifies that the same synchronization signals are transmitted each 5 ms. A UE might attempt to accumulate several occasions of these signals; however, fading radio channel and frequency errors negatively impact this possibility. A fading radio channel exhibits time variations both in amplitude and phase. The speed of these variations depends on both the speed of the UE and how the radio propagation environment is changing. In both cases, these variations may result in received signals that cannot be accumulated coherently in order to increase SINR. The phase variations may lead to a destructive superposition at this accumulation.
Furthermore, frequency error between a base station transmitter and a UE may also result in a channel with large phase variations over time. A UE typically has an oscillator that determines the frequency reference for its receiver with an accuracy of around 20 ppm. With a carrier frequency of 2 GHz, this results in a frequency error of 400 kHz. In order to estimate this frequency error, several PSS detectors could possibly be used in parallel, each with a different hypothesis of the frequency error. With an interval of 5 ms between the PSS transmission, the resolution of these frequency hypothesis signals would need to be 1/(5·[(10)]̂(−3))/100=2 Hz with a required accuracy of one percent. Thus, estimating frequency errors up to 400 kHz is a computationally complex solution.
An alternative may be for a UE to use a non-coherent accumulation in its receiver. Non-coherent accumulation, however, does not increase the SINR. It only improves the statistics of the receiver (i.e., the sensitivity to variations in individual noise samples).
A particular technique to improve coverage of cell search signals uses several antenna elements and beamforming to improve the SINR. A directional cell search procedure is proposed by C. Nicolas Barati et al. in “Directional Cell Search for Millimeter Wave Cellular Systems”, Cornell University Library. In this procedure a base station periodically transmits synchronization signals in random directions to scan the angular space. The need for synchronization and broadcast signals that can be used in the initial cell search for scanning over a range of angles is discussed by Sundeep Rangan et al. in “Millimeter-Wave Cellular Wireless Networks: Potentials and Challenges”, Proceedings of the IEEE, Volume: 102, Issue 3, 2014, pages 366-385.
An object of the present disclosure is to obviate at least these disadvantages and provide an improved method to transmit synchronization signals with a density and directionality that enables successful cell search in low SINR environments. Particular embodiments are described with reference to
Radio network node 120 transmits and receives wireless signals 130 using antenna 140. In particular embodiments, radio network node 120 may comprise multiple antennas 140. For example, radio network node 120 may comprise a multi-input multi-output (MIMO) system with two, four, or eight antennas 140.
In network 100, each radio network node 120 may use any suitable radio access technology, such as long term evolution (LTE), LTE-Advanced, UMTS, HSPA, GSM, cdma2000, WiMax, WiFi, and/or other suitable radio access technology. Network 100 may include any suitable combination of one or more radio access technologies. For purposes of example, various embodiments may be described within the context of certain radio access technologies. However, the scope of the disclosure is not limited to the examples and other embodiments could use different radio access technologies.
As described above, embodiments of a network may include one or more wireless devices and one or more different types of radio network nodes capable of communicating with the wireless devices. The network may also include any additional elements suitable to support communication between wireless devices or between a wireless device and another communication device (such as a landline telephone). A wireless device may include any suitable combination of hardware and/or software. For example, in particular embodiments, a wireless device, such as wireless device 110, may include the components described with respect to
This disclosure describes several frame formats for transmitting and receiving synchronization signals using LTE as an example. Particular embodiments may be applicable to FDD, TDD, or both. With respect to FDD, particular embodiments may apply to all subframes except subframe 0. With respect to TDD, particular embodiments may apply to all subframes except subframes 0 and 1. Subframe 5 may receive special treatment for both TDD and FDD, as described in more detail in the embodiments below.
In this example, SSS and PSS are inserted in subframe 6. A first PSS/SSS pair is transmitted in OFDM symbols 1 and 2 of the first time slot of subframe 6, a second pair is transmitted in OFDM symbols 3 and 4, and a third pair is transmitted in OFDM symbols 5 and 6. In the second slot, the first PSS/SSS pair is transmitted in OFDM symbols 1 and 2. In this example, six PSS/SSS pairs are included within subframe 6. In particular embodiments, the PSS and SSS of a pair are located in the same slot (i.e., a pair does not straddle a slot boundary). This is because a longer cyclic prefix is used for OFDM symbol 0 and the timing between the PSS and the SSS of pair that straddles the slot boundary would differ from other pairs that do not straddle the slot boundary.
In particular embodiments, the SSS and PSS are placed at sub-carriers centered on the DC carrier. Such a configuration enables a UE to detect them without knowing the total system bandwidth.
In this example, the first OFDM symbol of the subframe is not used for PSS or SSS because it contains PDCCH. PSS is transmitted in OFDM symbols 1, 3 and 5 of the first slot and OFDM symbols 1, 3 and 5 of the second slot. In particular embodiments, each of the six PSS comprise the same sequence.
In particular embodiments, the SSS of each pair is transmitted in the next OFDM symbol after the PSS. For example, in the first slot SSS1 is transmitted in OFDM symbol 2 and is paired with the PSS transmitted in OFDM symbol 1. In the second slot, SSS5 is transmitted in OFDM symbol 4 and is paired with the PSS transmitted in OFDM symbol 3. In particular embodiments, each of the six SSS comprise a different sequence. In this example, the sequences transmitted by SSS2 and SSS5 are punctured by CRS 516.
When a PSS sequence is detected, a UE may continue to detect SSS in the next OFDM symbol after the PSS. The SSS sequences are different on different positions within the subframe, such that the UE may determine the frame timing after SSS detection. Each SSS sequence is associated with a specific OFDM symbol within the subframe and a UE can determine which SSS sequences are punctured by CRS.
In particular embodiments, the first OFDM symbol in a PSS/SSS pair may be used for SSS and the second for PSS. A particular advantage of this ordering is that the order matches the legacy PSS/SSS ordering, which may enable reuse of some existing hardware or software components.
In the example embodiments described above, the first OFDM symbol in a PSS/SSS pair is used for PSS and the second OFDM symbol is used for SSS. A particular advantage of this ordering is that PSS is not punctured. When the PSS is not punctured, the same PSS sequence, and thus also detector, may be used irrespective of the position of the PSS within the subframe.
In the example embodiment illustrated in
Particular embodiments are also applicable to TDD. In the following examples, the corresponding PSS and SSS are separated by two OFDM symbols. This separation may enable a UE to distinguish between duplex modes. As described above, the PSS and SSS may also be arranged with PSS in the first OFDM symbol of the pair and the SSS in the second, or vice versa.
In this example, the corresponding PSS and SSS are separated by two OFDM symbols (e.g., PSS1 in position 1 and SSS0 in position 4, PSS2 in position 2 and SSS1 in position 5, PSS2 in position 3 and SSS2 in position 6, and so on). In this example, PSS1 is transmitted in OFDM symbol 1 of both slots and the PSS1 sequence is punctured by CRS 1116. PSS2 is transmitted in OFDM symbols 2 and 3 of both slots and the PSS2 sequence is not punctured by CRS 1116. In this example, the PSS/SSS pairs do not straddle the slot boundary.
Although the examples above are described with respect to a subframe with a normal length cyclic prefix, particular embodiments may also apply to subframes with an extended length cyclic prefix.
Although particular PSS/SSS patterns are illustrated above, additional patterns will be apparent to those skilled in the art. Furthermore, any of the patterns described above, or combination of patterns, may be repeated in other subframes within the frame.
The method begins at step 1710, where a network node generates synchronization signals. For example, radio network node 120 may generate a plurality of PSS sequences and SSS sequences. Each PSS sequence is paired with an SSS sequence to form a PSS/SSS pair. In particular embodiments, a first PSS sequence and a second PSS sequence may comprise identical sequences. In particular embodiments, a first PSS sequence and a second PSS sequence may comprise different sequences.
At step 1712, the network node maps the synchronization signals to radio subframes. For example, radio network node 120 may map the plurality of PSS/SSS pairs to a subframe according to any one of the frame formats described above, such as those described with respect to
At step 1714, the network node transmits the synchronization signals. For example, radio network node 120 transmits the radio frame comprising the subframes with the mapping of PSS/SSS pairs. In particular embodiments, radio network node 120 may perform directional signal transmission. For example, radio network node 120 may transmit a first PSS/SSS pair in a first direction and a second PSS/SSS pair in a second direction. In particular embodiments, radio network node 120 may transmit a first PSS/SSS pair in different directions over time.
The method begins at step 1810, where a wireless device receives signals transmitted from a radio network node. For example, wireless device 110 may receive wireless signal 130 from radio network node 120. Wireless signal 130 may comprise primary and secondary cell search signals. For example, wireless signal 130 may comprise a plurality of PSS/SSS pairs according to any one of the frame formats described above with respect to
At step 1812, the wireless device tries to detect a legacy PSS signal. For example, wireless device 110 may detect a legacy PSS sequence on the last OFDM symbol of slot 0. If the wireless device detects a legacy PSS at step 1814, then the method continues to step 1816 where the wireless device tries to detect a legacy SSS signal.
After detecting both primary and secondary cell search signals, the method is complete. However, if the wireless device at step 1812 is unable to detect a legacy PSS signal (e.g., because the SINR is too low), then the method continues to step 1818.
At step 1818, the wireless device tries to detect a PSS sequence, such as a PSS sequence according to one of the formats described above. In particular embodiments, wireless device 110 may accumulate multiple PSS received within a subframe or received across multiple subframes. A particular advantage is that wireless device 110 may combine signals to create a stronger signal. In particular embodiments, radio network node 120 may transmit a first PSS in a first direction and a second PSS in a second direction. A particular advantage if this transmission method is that wireless device 110 may receive a stronger PSS when radio network node 120 transmits the PSS in the direction of wireless device 110.
If the wireless device successfully detects PSS, then the method continues to step 1822. If the wireless device does not successfully detect PSS, then the method returns to step 1810 where the wireless device continues to detect signals received from the radio network node.
At step 1822, the wireless device tries to detect an SSS sequence according to one of the formats described above. Similar to detecting the PSS, the wireless device may accumulate multiple SSS and the radio network node may transmit different SSS in different directions. After detecting both primary and secondary cell search signals, the method is complete.
Modifications, additions, or omissions may be made to the method of
Processor 1920 includes any suitable combination of hardware and software implemented in one or more integrated circuits or modules to execute instructions and manipulate data to perform some or all of the described functions of the wireless device. Memory 1930 is generally operable to store computer executable code and data. Examples of memory 1930 include computer memory (e.g., Random Access Memory (RAM) or Read Only Memory (ROM)), mass storage media (e.g., a hard disk), removable storage media (e.g., a Compact Disk (CD) or a Digital Video Disk (DVD)), and/or or any other volatile or non-volatile, non-transitory computer-readable and/or computer-executable memory devices that store information.
In particular embodiments, processor 1920 in communication with transceiver 1910 receives cell search signals from radio network node 120. Other embodiments of the wireless device may include additional components (beyond those shown in
In some embodiments, network interface 2040 is communicatively coupled to processor 2020 and refers to any suitable device operable to receive input for radio network node 120, send output from radio network node 120, perform suitable processing of the input or output or both, communicate to other devices, or any combination of the preceding. Network interface 2040 includes appropriate hardware (e.g., port, modem, network interface card, etc.) and software, including protocol conversion and data processing capabilities, to communicate through a network.
In particular embodiments, processor 2020 in communication with transceiver 2010 transmits, to wireless device 110, cell search signals. In particular embodiments, processor 2020 in communication with transceiver 2010 transmits sell search signals such as the PSS and SSS described above to wireless device 110.
Other embodiments of radio network node 120 include additional components (beyond those shown in
Some embodiments of the disclosure may provide one or more technical advantages. As an example, in some embodiments, the methods and apparatus disclosed herein may facilitate detecting synchronization signals in a low SINR environment. Cell search procedure may be performed more efficiently to improve overall system performance.
Some embodiments may benefit from some, none, or all of these advantages. Other technical advantages may be readily ascertained by one of ordinary skill in the art.
Modifications, additions, or omissions may be made to the systems and apparatuses disclosed herein without departing from the scope of the invention. The components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses may be performed by more, fewer, or other components. Additionally, operations of the systems and apparatuses may be performed using any suitable logic comprising software, hardware, and/or other logic. As used in this document, “each” refers to each member of a set or each member of a subset of a set.
Modifications, additions, or omissions may be made to the methods disclosed herein without departing from the scope of the invention. The methods may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order.
Although this disclosure has been described in terms of certain embodiments, alterations and permutations of the embodiments will be apparent to those skilled in the art. Accordingly, the above description of the embodiments does not constrain this disclosure. Other changes, substitutions, and alterations are possible without departing from the spirit and scope of this disclosure, as defined by the claims below.
Abbreviations used in the preceding description include:
