Patent Application
20160366657
Synchronization in time and carrier is important to maximize the performance of cellular networks. In many cases, such as in the Time Division Duplex (“TDD”) or Network Multiple Input Multiple Output (“MIMO”), tight synchronization is a requirement for the cellular network. In other cases, such as Frequency Division Duplex (“FDD”), synchronization may not be a requirement but improves performance. Time synchronization is where multiple base stations have the same (or very similar) time reference so that, for instance, they can both transmit at nearly the same time. Carrier synchronization is where multiple base stations have the same (or very similar) carrier frequency so that, for instance, they would both transmit a signal at nearly the same frequency.
Carrier differences, also known as carrier offset, between multiple cells is caused from oscillators being slightly different. For instance, LTE requires that the carrier frequency of a base station observed over a 1 ms period must be accurate to within 50 parts of billion (“ppb”). By way of example, a 1 GHz signal must have an error within+−50 Hz. Note that this does not mean that the core oscillator must be specified within 50 ppb because it may be further disciplined from an outside source.
Time synchronization additionally requires an absolute time reference. The oscillator error then results in a time drift, resulting in the need for more frequency update based on the absolute time reference. Using a basic analogy, if two people want to synchronize time, they may set their watches to the same time, which would provide them with an absolute time reference. They may then agree to meet at a certain time in the future. Depending on their watch's error over time and depending on how far in the future they have agreed to meet, the time drift between their watches may become a problem. Both of these are real issues in cellular networks.
Another synchronization issue is carrier drift, which is defined as the amount the carrier changes over time. Oscillator's errors can be broken down into an offset and a drift. The offset is a fairly steady state error from the ideal frequency, whereas a drift is the changing of the frequency over time. The offset is typically much higher than the drift, even if measured over hours.
Many synchronization solutions already exist today to the point where synchronization is typically not an issue with large macrocells. They can employ expensive oscillators, utilize network timing via Network Timing Protocol (“NTP”) with local time servers, backhaul based synchronization, and/or global positioning system (“GPS”) technology. However, with the advent of small cells, these solutions are not always practical due to cost constraints and GPS not being available for indoor deployments.
As synchronization in now a recognized problem, there is recent activity in the 3GPP LTE Release 12 standardization to suggest certain over the air synchronization methods. The over the air synchronization method listens to downlink broadcast signals of adjacent cells to try and lock onto their timing and frequency. It is largely focused on heterogeneous networks where the underlay is synchronizing to the overlay but can theoretically be applied to non-heterogeneous networks. These methods however are limited to TDD networks and other constraints, such as requiring quiet periods, and provide limited synchronization accuracy.
The synchronization requirement depends on the type of waveform, characteristics of the waveform, duplexing scheme, feedback period from the mobiles, network architecture, and type of transmission/reception.
Some specific synchronization requirement examples based on standard deployments of cellular systems include:
An emerging architecture/cellular scheme is multicell joint transmission and joint reception. In joint transmission, multiple cells transmit simultaneously to a single mobile device. Similarly in joint receptions, multiple cells receive simultaneously from a single mobile device.
Tighter synchronization is often required under these schemes, which require synchronization to a small fraction of a symbol for the required MIMO level synchronization. In some cases, time and frequency offset measurements can additionally be used to compensate for the lack of synchronization in signal processing.
Accordingly, there exists a need for methods, systems, and computer program products for over the air synchronization for cellular networks.
Methods and systems are described for over the air synchronization in a cellular network. In one aspect, data that is in the presence of interference either from other proximate base stations or from a local synchronization base station is received from a base station that will be synchronized to. A joint signal processing technique is employed on the data to obtain known downlink reference signals having sufficient signal-to-interference plus noise ratio (“SINR”) for time or frequency synchronization. Time or frequency synchronization is determined based on the known downlink reference signals.
In another aspect, data that is in the presence of interference either from mobile devices connected to other base stations or from mobile devices connected to a synchronizing base station is received from a mobile device that is synchronized to a base station. A joint signal processing technique is employed on the data to obtain uplink signals having sufficient SINR for time or frequency synchronization. Time or frequency synchronization is determined based on the uplink signals.
Advantages of the claimed invention will become apparent to those skilled in the art upon reading this description in conjunction with the accompanying drawings, in which like reference numerals have been used to designate like or analogous elements, and in which:
Prior to describing the subject matter in detail, an exemplary hardware device in which the subject matter may be implemented shall first be described. Those of ordinary skill in the art will appreciate that the elements illustrated in
The bus 114 may comprise any type of bus architecture. Examples include a memory bus, a peripheral bus, a local bus, etc. The processing unit 102 is an instruction execution machine, apparatus, or device and may comprise a microprocessor, a digital signal processor, a graphics processing unit, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), etc. The processing unit 102 may be configured to execute program instructions stored in memory 104 and/or storage 106.
The memory 104 may include read only memory (ROM) 116 and random access memory (RAM) 118. Memory 104 may be configured to store program instructions and data during operation of device 100. In various embodiments, memory 104 may include any of a variety of memory technologies such as static random access memory (SRAM) or dynamic RAM (DRAM), including variants such as dual data rate synchronous DRAM (DDR SDRAM), error correcting code synchronous DRAM (ECC SDRAM), or RAMBUS DRAM (RDRAM), for example. Memory 104 may also include nonvolatile memory technologies such as nonvolatile flash RAM (NVRAM) or ROM. In some embodiments, it is contemplated that memory 104 may include a combination of technologies such as the foregoing, as well as other technologies not specifically mentioned. When the subject matter is implemented in a computer system, a basic input/output system (BIOS) 120, containing the basic routines that help to transfer information between elements within the computer system, such as during start-up, is stored in ROM 116.
The storage 106 may include a flash memory data storage device for reading from and writing to flash memory, a hard disk drive for reading from and writing to a hard disk, a magnetic disk drive for reading from or writing to a removable magnetic disk, and/or an optical disk drive for reading from or writing to a removable optical disk such as a CD ROM, DVD or other optical media. The drives and their associated computer-readable media provide nonvolatile storage of computer readable instructions, data structures, program modules and other data for the hardware device 100. It is noted that the methods described herein can be embodied in executable instructions stored in a computer readable medium for use by or in connection with an instruction execution machine, apparatus, or device, such as a computer-based or processor-containing machine, apparatus, or device. It will be appreciated by those skilled in the art that for some embodiments, other types of computer readable media may be used which can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, RAM, ROM, and the like may also be used in the exemplary operating environment. As used here, a “computer-readable medium” can include one or more of any suitable media for storing the executable instructions of a computer program in one or more of an electronic, magnetic, optical, and electromagnetic format, such that the instruction execution machine, system, apparatus, or device can read (or fetch) the instructions from the computer readable medium and execute the instructions for carrying out the described methods. A non-exhaustive list of conventional exemplary computer readable medium includes: a portable computer diskette; a RAM; a ROM; an erasable programmable read only memory (EPROM or flash memory); optical storage devices, including a portable compact disc (CD), a portable digital video disc (DVD), a high definition DVD (HD-DVD™), a BLU-RAY disc; and the like.
A number of program modules may be stored on the storage 106, ROM 116 or RAM 118, including an operating system 122, one or more applications programs 124, program data 126, and other program modules 128.
The hardware device 100 may be part of a base station (not shown) configured to communicate with mobile devices 140 in a communication network. A base station may also be referred to as an eNodeB, an access point, and the like. A base station typically provides communication coverage for a particular geographic area. A base station and/or base station subsystem may cover a particular geographic coverage area referred to by the term “cell.” A network controller (not shown) may be communicatively connected to base stations and provide coordination and control for the base stations. Multiple base stations may communicate with one another, e.g., directly or indirectly via a wireless backhaul or wireline backhaul.
The hardware device 100 may operate in a networked environment using logical connections to one or more remote nodes via communication interface 112, including communicating with one or more mobile devices 140 via a transceiver 110 connected to an antenna 130. The mobile devices 140 can be dispersed throughout the network 100. A mobile device may be referred to as user equipment (UE), a terminal, a mobile station, a subscriber unit, or the like. A mobile device may be a cellular phone, a personal digital assistant (PDA), a wireless modem, a wireless communication device, a handheld device, a laptop computer, a wireless local loop (WLL) station, a tablet computer, or the like. A mobile device may communicate with a base station directly, or indirectly via other network equipment such as, but not limited to, a pico eNodeB, a femto eNodeB, a relay, or the like.
The remote node may be a computer, a server, a router, a peer device or other common network node, and typically includes many or all of the elements described above relative to the hardware device 100. The communication interface 112, including transceiver 110 may interface with a wireless network and/or a wired network. For example, wireless communications networks can include, but are not limited to, Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), and Single-Carrier Frequency Division Multiple Access (SC-FDMA). A CDMA network may implement a radio technology such as Universal Terrestrial Radio Access (UTRA), Telecommunications Industry Association's (TIA's) CDMA2000®, and the like. The UTRA technology includes Wideband CDMA (WCDMA), and other variants of CDMA. The CDMA2000® technology includes the IS-2000, IS-95, and IS-856 standards from The Electronics Industry Alliance (EIA), and TIA. A TDMA network may implement a radio technology such as Global System for Mobile Communications (GSM). An OFDMA network may implement a radio technology such as Evolved UTRA (E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Flash-OFDMA, and the like. The UTRA and E-UTRA technologies are part of Universal Mobile Telecommunication System (UMTS). 3GPP Long Term Evolution (LTE) and LTE-Advance (LTE-A) are newer releases of the UMTS that use E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A, and GAM are described in documents from an organization called the “3rd Generation Partnership Project” (3GPP). CDMA2000® and UMB are described in documents from an organization called the “3rd Generation Partnership Project 2” (3GPP2). The techniques described herein may be used for the wireless networks and radio access technologies mentioned above, as well as other wireless networks and radio access technologies.
Other examples of wireless networks include, for example, a BLUETOOTH network, a wireless personal area network, and a wireless 802.11 local area network (LAN). Examples of wired networks include, for example, a LAN, a fiber optic network, a wired personal area network, a telephony network, and/or a wide area network (WAN). Such networking environments are commonplace in intranets, the Internet, offices, enterprise-wide computer networks and the like. In some embodiments, communication interface 112 may include logic configured to support direct memory access (DMA) transfers between memory 104 and other devices.
In a networked environment, program modules depicted relative to the hardware device 100, or portions thereof, may be stored in a remote storage device, such as, for example, on a server. It will be appreciated that other hardware and/or software to establish a communications link between the hardware device 100 and other devices may be used.
It should be understood that the arrangement of hardware device 100 illustrated in
In the description that follows, the subject matter will be described with reference to acts and symbolic representations of operations that are performed by one or more devices, unless indicated otherwise. As such, it will be understood that such acts and operations, which are at times referred to as being computer-executed, include the manipulation by the processing unit of data in a structured form. This manipulation transforms the data or maintains it at locations in the memory system of the computer, which reconfigures or otherwise alters the operation of the device in a manner well understood by those skilled in the art. The data structures where data is maintained are physical locations of the memory that have particular properties defined by the format of the data. However, while the subject matter is being described in the foregoing context, it is not meant to be limiting as those of skill in the art will appreciate that various of the acts and operation described hereinafter may also be implemented in hardware.
To facilitate an understanding of the subject matter described below, many aspects are described in terms of sequences of actions. At least one of these aspects defined by the claims is performed by an electronic hardware component. For example, it will be recognized that the various actions can be performed by specialized circuits or circuitry, by program instructions being executed by one or more processors, or by a combination of both. The description herein of any sequence of actions is not intended to imply that the specific order described for performing that sequence must be followed. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.
The proposed solution introduces a new synchronization scheme based on Over the Air (OTA) synchronization for adjacent cells. It differs from the OTA synchronization studied as part of LTE because it operates in the presence of interference.
Turning now to
With reference to
For downlink based synchronization, shown in
With reference again to
The signal processing techniques employed depend on the synchronization requirements and network architecture. These techniques are preferable when the interference from local and/or adjacent base stations is too high to employ traditional signal processing techniques. These techniques attempt to suppress interference through jointly modeling multiple signals, traditionally called multiuser detectors, multiuser parameter estimators or MIMO receivers. For example, a simple case of a successive interference canceller may jointly model signal 208 and 206 by first modeling 208, which observed from base station 202 is much higher power than signal 206, then suppress that signal through subtracting it from the combined received signal to leave a clean signal 206 to obtain synchronization from. This technique could be used on the data or the reference signals but the reference signals are typically a better option because they are likely known at the adjacent cell.
In the case of FDD, separate listening antennas and hardware that is tuned to the downlink frequency can be employed because traditional base station design typically only includes receivers for the uplink frequency. In the case of TDD, where the transmit and receive antennas are typically the same but the hardware is designed to have a separate transmit and receive path, it may be possible to receive on the same set of antennas. It may also be necessary to employ additional antennas based on hardware limitations of isolation between the transmit and receive chains.
Related to the antennas problem for TDD is an interference issue for frequency reuse 1 networks where one intends to receive signal 206 at base station 202 but the signal from 208 is being transmitted from 202, which results in a disparity in received power at the receive antennas at base station 202. In such a case, the analog receiver hardware may be insufficient and create signal distortion before it is digitized or have insufficient dynamic range to digitize the very high level interference with the low level desired signal. The analog hardware components may saturate or operate in undesirable regions, resulting in an undesirable signal after the analog to digital converters. One technique to help alleviate this issue is to use an analog cancelation technique where the known transmitted signal is fed back into the receive chain and suppressed with an analog cancelation technique, similar to the successive interference canceller discussed previously but employed in analog hardware. The combination of the analog and digital interference suppression may be enough to obtain a clean synchronization signal.
An alternative technique is to use quiet periods, such as where base station 202 turns off for a period of time so that the synchronization signals can be processed without interference. These techniques are sometimes acceptable, such as when the base station is obtaining its first synchronization settings. However, there are many downsides to relying on quiet periods for synchronization. The time and frequency drift over time due to slightly varying oscillators, causing a need to update and track the drift, resulting in quiet periods periodically being necessary which lowers the throughput. The quiet periods would also need to be network wide or else there will still be interference on the synchronization signals and interference suppression techniques would need to be used in addition to quiet periods.
With reference again to
Synchronization in frequency involves matching the transmit and/or receive frequency of a base station to the transmit and/or receive frequency of another base station. Ideally, the base station that is being synchronized to can be considered a perfect reference, otherwise you can get cascading errors where a measurement error on top of the original error of the base station being synchronized to causes the synchronization to be further off. If another base station then synchronizes based off of that base station and has measurement errors off in the same direction, it can cause cascading errors. A base station may be considered a perfect reference if it has GPS lock and synchronizes its oscillator and timing based off of it. If a perfect reference is not identified, it may be necessary to average the synchronization parameters calculated from multiple neighboring base stations, or some other method based off of multiple measurements from multiple base stations. It may also be desirable to synchronize to an imperfect base station if the two base stations are performing joint transmission or joint reception.
Synchronization in time involves similar issues that can be solved in similar ways. A new issue also arises due to propagation time, where we wish to synchronize to base station 200 by transmitting at the same time. If we synchronize based off of signal 206, it arrives at base station 202 at a certain time after it was transmitted, which is called the propagation time. A simple solution to this problem is to preprogram or ascertain in some ways the physical distance to base station 200 so that the propagation time can be calculated and timing adjusted based on the propagation time.
Similar issues exist for uplink signal based synchronization, with some differences discussed herein. Turning now to
With reference to
For uplink-based synchronization, illustrated in
With reference again to
In block 606, time or frequency synchronization is determined based on the uplink signals. Accordingly, a system for over the air synchronization in a cellular network includes means for time or frequency synchronization is determined based on the uplink signals. For example, as illustrated in
Returning to
A key difference between a frequency reuse 1 version of uplink versus downlink synchronization is that the interfering signals are arriving at the receiver antennas with much less power, as compared to downlink based synchronization. In some cases, this makes the synchronization easier to perform via uplink signal because of the reduced interference levels on the synchronization signals.
The indirect nature of the uplink synchronization may make it less accurate due to synchronizing to a mobile that itself has a small synchronization error. Again, a cascading error problem may occur.
The indirect nature of the uplink synchronization creates a new timing lock challenge. Each mobile is synchronized to their connected base station so that they are received at approximately the same time. Unless the mobile is equidistance to both base stations, the received timing at the synchronizing base station 302 will not be the same as the base station 300 being synchronized to as a result of propagation time. Depending on the timing requirement, various methods or a combination of methods may be employed to counter this issue, such as identifying mobiles nearest to equidistance, obtaining location based information on mobiles, expected timing statistics, and backoff based on known distance between base stations. Expected timing statistics take timing estimates from multiple mobiles and based on expected distribution statistics of timing information determine the best time correction. Taking a fixed backoff is done based on the known distance between cells based on one or more measurements.
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the subject matter (particularly in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation, as the scope of protection sought is defined by the claims as set forth hereinafter together with any equivalents thereof entitled to. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illustrate the subject matter and does not pose a limitation on the scope of the subject matter unless otherwise claimed. The use of the term “based on” and other like phrases indicating a condition for bringing about a result, both in the claims and in the written description, is not intended to foreclose any other conditions that bring about that result. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention as claimed.
Preferred embodiments are described herein, including the best mode known to the inventor for carrying out the claimed subject matter. One of ordinary skill in the art should appreciate after learning the teachings related to the claimed subject matter contained in the foregoing description that variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventor intends that the claimed subject matter may be practiced otherwise than as specifically described herein. Accordingly, this claimed subject matter includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed unless otherwise indicated herein or otherwise clearly contradicted by context.
This application claims the benefit of U.S. Provisional Application No. 62/174,191, filed Jun. 11, 2015, which is incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62174191 | Jun 2015 | US |
