Patent Publication Number: US-9414336-B2

Title: System and method for defining a burst period

Description:
BACKGROUND 
     The telecommunications industry is a dynamic industry. Today&#39;s telecommunications industry comprises a variety of different radio access technologies including Code Division Multiple Access (CDMA2000), UTRAN (UTMS (Universal Mobile Telecommunications System) Terrestrial Radio Access Network), Global System for Mobile Communications (GSM), GSM EDGE Radio Access Network (GERAN), Generic Access Network (GAN), Wireless Fidelity (WiFi), Wireless Local Area Network (WLAN), General Packet Radio Service (GPRS), Worldwide Interoperability for Microwave Access (WiMAX), 1× Evolution-Data Optimized (1×EV-DO), High-Speed Downlink Packet Access (HSDPA), Digital Enhanced Cordless Technology (DECT), and High Rate Packet Data (HRPD). Other RATs or other network technologies based on these RATs may be familiar to one of skill in the art. 
     Future telecommunications technologies are evolving including next generation networks or next generation equipment that may be referred to as long-term evolution equipment (LTE). As the technologies evolve, the vocabulary also changes. Traditional networks referred to base stations, cells, and user terminals. In next generation systems, the equivalent or similar equipment is referred to as enhanced Node B (eNB), hotspots, and user equipment (UE). For ease of understanding, a device used by the user is referred to herein as user equipment (UE). A device in which the user equipment accesses a network is referred to herein as network access equipment (e.g. base stations, or eNBs). The area in which a UE can access the network access equipment is referred to as a cell. One skilled in the art will appreciate that there may be instances where a first UE can be used by a second UE to access a network. In this example, the first UE is may also be referred to as network access equipment. 
     Other terms that will be used through out this disclosure include uplink and downlink. Uplink (UL) is a communication originating at the UE. Downlink (DL) is a communication terminating at the UE. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
         FIG. 1  illustrates a wireless system in which the present invention may be utilized. 
         FIG. 2  illustrates a periodic SRS transmission scheme. 
         FIG. 3  is a table showing results from a simulation of the periodic SRS transmission scheme of  FIG. 2 . 
         FIG. 4  illustrates a periodic SRS transmission scheme in accordance with the present invention. 
         FIG. 5  is a table showing results from a simulation of the periodic SRS transmission scheme of  FIG. 4 . 
         FIG. 6  is a schematic flowchart of a method in accordance with an embodiment of the present invention. 
         FIG. 7  is a schematic flowchart of a method in accordance with another embodiment of the present invention. 
         FIG. 8  is a schematic flowchart of a method in accordance with an embodiment of the present invention. 
         FIG. 9  is a schematic block diagram of an illustrative user equipment in accordance with an embodiment of the present invention. 
         FIG. 10  illustrates a simplified block diagram of network access equipment in accordance with an embodiment of the present invention. 
         FIG. 11  illustrates synchronization timing module that may be sorted at the user equipment in accordance with the present invention. 
         FIG. 12 a    illustrates an example of multiple SRS bursts with activation and deactivation messages and SRS burst duration of four subframes in accordance with the present invention. 
         FIG. 12 b    illustrates an example of multiple SRS bursts with a single activation message in accordance with the present invention. 
         FIG. 12 c    illustrates an example of single SRS burst scheduling in accordance with the present invention. 
         FIG. 13  illustrates SRS transmissions on either of two combs in accordance with the present invention. 
         FIG. 14  illustrates an example of SRS multiplexing in the code domain from multiple transmission antennas, where each antenna is assigned with a unique cyclic shift, in accordance with the present invention. 
         FIG. 15  illustrates an example of SRS multiplexing in the time domain for multiple antennas in accordance with the present invention. 
         FIG. 16  illustrates an example of SRS multiplexing in the code domain and hopping in frequency within a SRS burst in accordance with the present invention. 
         FIG. 17  illustrates an example of SRS multiplexing in the frequency domain and frequency hopping within a SRS burst in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Uplink (UL) synchronization is a necessary component of a UE operating in a mobile, wireless network. Reliable UL synchronization generally requires regular UL transmissions containing reference signals. The reference signals enable the network access equipment to measure and then adjust the timing differences between the UE and the network access equipment. These timing differences have many causes including clock drift, changes in the distance between the UE and the network access equipment due to the velocity of the UE, and changes in the multipath environment. However, traditional UL synchronization techniques may degrade as the UE&#39;s velocity increases. The following disclosure is provided in light of these technical challenges. 
     It should be understood at the outset that although illustrative implementations of one or more embodiments of the present disclosure are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. Thus, although the following is an example of how the present invention may be used in a 3 rd  Generation Partnership Project (3GPP) long term evolution (LTE) network, one skilled in the art will understand that these concepts can be applied to other wireless networks. 
       FIG. 1  illustrates a wireless system  100  in which the present invention may be utilized. The wireless system  100  comprises network access equipment (NAE)  102 , network access equipment antenna  104 , user equipment (UE)  110  and UE antenna  112 . The UE may be a cell phone, a personal digital assistant, a smart phone, a pager, a laptop computer, or any device which communicates with network access equipment  102 . The network access equipment  102  is coupled to the Internet  130 . The wireless system  100  allows the UE  110  to perform any, or a combination, of a variety of functions such as, making and receiving calls, surfing the web, sending text messages, and receiving emails. 
     The UE  110  and the NAE  102  communicate using a protocol that is specific to the type of wireless network  100 . For example, for LTE, there is a basic format of the uplink (UL) channel. The transmission can be one of a number of different bandwidths (viz. 1.25, 5, 15 or 20 Megahertz (MHz)). In the time domain, the UL is broken into frames and sub-frames. A slot can be made up of seven orthogonal frequency division multiplexed (OFDM) symbols. Two slots make up a sub-frame. A frame is a collection of 10 continuous sub-frames. The first symbol of the sub-frame may be where the sounding reference symbol (SRS) is placed. The demodulation (DM) reference symbol (RS) is placed in the fourth symbol of each slot, and the control channel is taken up by at least one resource block(s) on the very outside edges of the frequency band. Because the SRS symbol is not sent in the control channel, the SRS symbol is sent intermixed with the data. The SRS that is made available at the beginning of each sub-frame is broken down into several resource blocks (e.g., 12 sub-carriers wide and several symbols in time). A UE may use one or all of those resource blocks given the transmission bandwidth. The SRS symbol is used by the network access equipment, e.g., enhanced Node B (eNB), to measure the channel quality, e.g., channel quality indicator (CQI), for the uplink. 
       FIG. 2  illustrates a periodic SRS transmission scheme. With a known symbol sequence modulated in the SRS  202 , the eNB can not only measure the CQI, but the eNB can also measure the UE&#39;s timing drift as well. At the cell edge, the limited signal to noise ratio (SNR) requires using multiple SRS  202  transmissions, with certain combination techniques, to be discussed in further detail, to increase the timing estimate accuracy. 
     As illustrated in  FIG. 2 , the measurement period  204  is the same as the timing advance (TA) adjustment period  208 ; though sometimes the TA adjustment may not be updated if the timing drift is smaller than the TA granularity. In  FIG. 2 , the SRS symbols  202  that are used for one timing estimate are spread over one period  204  with equal intervals  206 . With low UE velocity, the timing drift from the beginning to the end of the measurement period  204  is not significant and its effect on the timing estimation is small. However, when the UE&#39;s velocity is high, the timing drift becomes more substantial. Most TA adjustment measurements incorporate several samples over time (e.g., readings from several SRS). The movement of the UE from sample to sample during this measurement period  204  becomes significant with adverse effects on the overall timing estimate accuracy by the eNB.  FIG. 3  is a result from a simulation that shows that the timing estimate degrades as the UE velocity increases when the SRS symbols  202  are transmitted in the manner illustrated in  FIG. 2 . 
     As illustrated in  FIG. 3 , as the UE velocity increases from 30 kilometers/hour (km/h) to 500 km/h, the mean timing estimate error increases from 0.26 microseconds (μs) to 0.69 μs, and the 95 th  percentile increases from 0.22 μs to 0.48 μs. In order to overcome this timing error increase, the number of SRS symbols  202  sent in a measurement period  204  could be increased, but this solution decreases the capacity available for other messages utilizing the radio link, and also decreases the UE&#39;s battery life. 
     Another alternative, that will decrease the capacity and drain the UE&#39;s battery less, is illustrated in  FIG. 4 . As illustrated in  FIG. 4 , the SRS symbols  202  are sent in a burst  402 , and then the SRS symbols  202  are not sent during a quiet period  404 . The overall rate of the SRS symbol transmission is constant, e.g., the same numbers of SRS symbols  202  are sent during a TA adjustment period  408 , e.g. 20; however, the interval  206  and  206 ′ between the SRS symbols  202  is changed to allow for a burst period  402  and a quiet period  404 . 
     In one embodiment, the interval  206 ′ is varied depending on the velocity of the UE. For example, as the velocity of the UE increase, the interval  206 ′ decreases. The interval  206 ′ is lower-bounded by the channel coherent time. To get time domain diversity, the interval  206 ′ should not be so small that a completely coherent radio propagation channel realization can be avoided in one measurement period. 
     In one embodiment, the burst period  402  and the quiet period  404  comprise the timing advance (TA) adjustment period  408 . The TA adjustment period  408  may also be determined by the UE velocity. The lower the velocity, the longer the TA adjustment period. The TA adjustment period is determined by how much timing error the UL can tolerate before communications from one UE begins to interfere with communications of another. Thus, the TA adjustment period  408  takes into account the radio roundtrip timing for a UE with a given velocity. 
     In one embodiment, the network access equipment notifies the UE of the interval  206 ′. The notification can take place during a timing adjustment update message, or through an independent message. 
     For example, in one embodiment, a UE is in an LTE_Idle state when no active communication with the eNB is imminent. When there is a possibility for active communication (i.e., packets need to be sent on the uplink or downlink), the UE will be in an LTE_Connected state. In the transition from LTE_Idle to LTE_Connected state, the UE will acquire uplink timing alignment that needs to be maintained. Once UL timing alignment is acquired, the eNB will signal the UE default SRS burst pattern parameters as soon as necessary so that there is not enough time to pass before the UE loses timing alignment. An example of the signaling would be that the UE should use an SRS burst period and length suited for the highest mobility UE and the time that it should start the burst. The eNB will monitor these burst and other parameters available to it (e.g. changes in channel conditions) that it can use to determine the UE&#39;s velocity. At any time that the eNB determines that the UE is moving at a velocity that warrants a different set of burst parameters, it will signal them to the UE. 
     While the UE is in LTE_Connected mode, it may also be monitoring other parameters that indicate a change in its velocity (e.g., GPS measurements). It may signal this to the eNB and allow the eNB to incorporate that information to determine velocity. If the result is an indication that the velocity requires a different set of SRS burst parameters, the eNB will signal them to the UE. 
       FIG. 5  is a result from a simulation that shows that the timing estimate varies only slightly as the UE velocity increases when the SRS symbols  202  are transmitted in the manner illustrated in  FIG. 4 . As shown in  FIG. 5 , as the UE velocity increases from 30 km/h to 500 km/h the mean timing estimate error increases only slightly from 0.26 to 0.32 μs, and the 95 th  percentile is almost constant. Thus, the synchronization scheme shown in  FIG. 4  has a mean timing estimate degradation of 0.08 μs compared with the 0.33 μs degradation when the synchronization scheme of  FIG. 2  is used. Additionally, the synchronization scheme shown in  FIG. 4  has almost no 95 th  percentile degradation compared with the 0.26 μs degradation when the synchronization scheme of  FIG. 2  is used. While, the synchronization schemes illustrated in  FIG. 2  and  FIG. 4  use the same capacity, e.g., radio resources, and battery power for uplink timing synchronization, the synchronization scheme in  FIG. 4  results in less degradation of the timing estimate performance with different UE velocities. 
       FIG. 6  is a schematic flowchart of a method in accordance with an embodiment of the present invention. As shown, method  600  begins at block  601  and at block  603  defines a burst period (e.g., burst period  402  illustrated in  FIG. 4 ). Method  600  then proceeds to block  605  where method  600  determines an interval (e.g., interval  206 ′ illustrated in  FIG. 4 ). Method  600  then proceeds to block  607  where method  600  sends timing signal instructions (e.g., the burst period and interval timing). Method  600  may optionally proceed to block  609  which allocates a quiet period (e.g., quiet period  404  illustrated in  FIG. 4 ). Method  600  then proceeds to send the timing signal instructions, which will include the optional quiet period timing instruction. 
       FIG. 7  is a schematic flowchart of a method in accordance with another embodiment of the present invention. As shown, method  700  begins at block  701  and at block  703  defines a burst period (e.g., burst period  402  illustrated in  FIG. 4 ). Method  700  then proceeds to block  705  where method  700  determines an interval (e.g., interval  206 ′ illustrated in  FIG. 4 ). Method  700  then proceeds to block  707  where method  700  sends timing signal instructions (e.g., burst period and interval duration). Method  700  may optionally proceed to block  709  which allocates a quiet period. Method  700  then proceeds to send timing signal instructions which include quiet period duration. Method  700  may proceed from block  707  directly to block  711 , or method  700  may proceed from block  707  to block  709  and then to block  711 . At block  711 , method  700  checks if there has been a change in timing. There are many methods that may be used to determine if there has been a change in timing. In one embodiment, the time drift rate, e.g., the timing drift of a current measurement compared to a previous measurement, can be used. Other methods include, using an accelerometer or GPS locations to determine if there has been a change in timing. A change in location, as determined by GPS positions, could indicate a change in timing. In one embodiment, the network access equipment determines if there has been a change in timing based on measurements made at the network access equipment. In another embodiment, the network access equipment determines if there has been a change in timing based on information received from the UE. In another embodiment, the network access equipment uses information based on measurement and information from the UE to determine there has been a change in timing. 
     If there has been a change in timing, the method  700  then proceed back to block  703  to re-define the burst period. Next, method  700  proceeds to block  705  to re-determine the interval. And, method  700  proceeds to block  707  to send the timing signal instructions including the re-defined burst period at the re-determined interval. If there has been no change in timing, or if method  700  determines that the burst period does not have to be re-defined, and the interval does not need to be re-determined, method  700  then proceeds back to block  707  to send the original timing signal instructions. Alternatively, the network access equipment does not send timing signal instructions until there is a change in the timing signal instructions. 
       FIG. 8  is a schematic flowchart of a method  800  in accordance with the present invention.  FIG. 8  starts at Start  801 . Method  800  then proceeds to block  803 , where a message that includes the timing signal instructions is received. These timing signal instructions comprise burst period and interval timing instructions, along with the optional quiet period instructions. In one embodiment, these timing signal instructions may also include the number of timing signals to be sent. The method  800  then proceeds to block  805 , where the timing signals are transmitted in accordance with the timing signal instructions. In one embodiment, the timing signals are the SRS timing signals  202 , as shown in  FIG. 4 , where the SRS timing signals  202  are sent for the instructed burst period  402  and at the instructed interval  206 ′. 
     As mentioned previously, the invention may be practiced with a UE  110  in a wireless operating environment. Shown in  FIG. 9  is a schematic block diagram of an illustrative UE  110 , While a variety of known components of UEs  110  are depicted, in an embodiment a subset of the listed components and/or additional components not listed may be included in the UE  110 . The UE  110  includes a main processor  902  and a memory  904 . As shown, the UE  110  may further include a communication subsystem  906 , random access memory  930 , a microphone  908 , an earpiece speaker  910 , an input/output interface  912 , a removable memory card  914 , a removable memory card interface  916  a universal serial bus (USB) port  918 , a keypad  920 , a liquid crystal display (LCD)  922 , which may include a touch sensitive surface, a camera module  924 , a camera logic  926 , and a global positioning system (GPS) sensor  928 . In an embodiment, the UE  110  may include another kind of display that does not provide a touch sensitive screen. In an embodiment, the main processor may communicate directly with the memory  904 . 
     The main processor  902  or some other form of controller or central processing unit operates to control the various components of the UE  110  in accordance with embedded software or firmware stored in memory  904  or stored in memory contained within the main processor  902  itself. In addition to the embedded software or firmware, the main processor  902  may execute other applications stored in the memory  904 , or in RAM  930 , or made available via information carrier media such as portable data storage media like the removable memory card  914  or via wired or wireless network communications. The application software may comprise a compiled set of machine-readable instructions that configure the main processor  902  to provide the desired functionality, or the application software may be high-level software instructions to be processed by an interpreter or compiler to indirectly configure the main processor  902 . 
     The communication subsystem  906  may be provided to convert between wireless signals and electrical signals, enabling the UE  110  to send and receive information from a cellular network or some other available wireless communications network or from a peer UE  110 . In an embodiment, the communication subsystem  906  may include multiple antennas to support beam forming and/or multiple input multiple output (MIMO) operations. As is known to those skilled in the art, MIMO operations may provide spatial diversity which can be used to overcome difficult channel conditions and/or increase channel throughput. The communication subsystem  906  may include antenna tuning and/or impedance matching components, RF power amplifiers, and/or low noise amplifiers. 
     The communication subsystem  906  may also provide frequency shifting, converting received RF signals to baseband and converting baseband transmit signals to RF. In some descriptions the communication subsystem  906  may be understood to include other signal processing functionality such as modulation/demodulation, coding/decoding, interleaving/deinterleaving, spreading/despreading, inverse fast Fourier transforming (IFFT)/fast Fourier transforming (FFT), cyclic prefix appending/removal, and other signal processing functions. For the purposes of clarity, the description here separates the description of this signal processing from the RF and/or radio stage and conceptually allocates that signal processing to the main processor  902  or other processing unit. 
     Main processor  902  may provide various analog processing of inputs and outputs, for example analog processing of inputs from the microphone  908  outputs to the speaker  910 . To that end, the main processor  902  may have ports for connecting to the built-in microphone  908  and the speaker  910  that enable the UE  110  to be used as a cell phone. The main processor  902  may further include a port for connecting to a headset or other hands-free microphone and speaker configuration. The main processor  902  may provide digital-to-analog conversion in one signal direction and analog-to-digital conversion in the opposing signal direction. In some embodiments, at least some of the functionality of the main processor  902  may be provided by other digital processing components. 
     The main processor  902  may perform modulation/demodulation, coding/decoding, interleaving/deinterleaving, spreading/despreading, inverse fast Fourier transforming (IFFT)/fast Fourier transforming (FFT), cyclic prefix appending/removal, and other signal processing functions associated with wireless communications. In an embodiment, for example in a code division multiple access (CDMA) technology application, for a transmitter function the main processor  902  may perform modulation, coding, interleaving, and spreading, and for a receiver function the main processor  902  may perform despreading, deinterleaving, decoding, and demodulation. In another embodiment, for example in an orthogonal frequency division multiplex access (OFDMA) technology application, for the transmitter function the main processor  902  may perform modulation, coding, interleaving, inverse fast Fourier transforming, and cyclic prefix appending, and for a receiver function the main processor  902  may perform cyclic prefix removal, fast Fourier transforming, deinterleaving, decoding, and demodulation. In other wireless technology applications, yet other signal processing functions and combinations of signal processing functions may be performed by the main processor  902 . 
     The main processor  902  may communicate with a wireless network  938 . In some embodiments, the communication may provide Internet connectivity, enabling a user to gain access to content on the Internet and to send and receive e-mail or text messages. The input/output interface  912  interconnects the main processor  902  and various memories and interfaces. The memory  904 , RAM  930  and the removable memory card  914  may provide software and data to configure the operation of the main processor  902 . 
     The keypad  920  couples to the main processor  902  to provide one mechanism for the user to make selections, enter information, and otherwise provide input to the UE  110 . The keyboard  902  may be a full or reduced alphanumeric keyboard such as QWERTY, Dvorak, AZERTY and sequential types, or a traditional numeric keypad with alphabet letters associated with a telephone keypad. The input keys may include a trackwheel, an exit or escape key, a trackball  932 , and other navigational or functional keys, which may be inwardly depressed to provide further input function. Another input mechanism may be the LCD, which may include touch screen capability and also display text and/or graphics to the user. 
     The camera  924 , if equipped, enables the UE  110  to take digital pictures. The main processor  902  communicates with the camera  924  via the camera logic  926 . The GPS sensor  928  is coupled to the main processor to decode global positioning system signals, thereby enabling the UE  110  to determine its position. Various other peripherals may also be included to provide additional functions, e.g., radio and television reception. 
     The DE  110  may be a battery-powered device and may include a battery interface  934  for receiving one or more rechargeable batteries  936 . In some embodiments, the battery  936  may be a smart battery with an embedded microprocessor. The battery interface  934  is coupled to a regulator (not shown), which assists the battery  936  in providing power to the UE  110 . The battery  936  may be used to power all components and modules in the UE  110 . 
     Software applications  940  may also be stored in the memory  904  of the UE  110 . The software applications  940  may include a messaging application  942 . The messaging application  942  can be any suitable software program that allows a subscriber or user of the UE  110  to send and receive wireless communications, including text messages. Messages that have been sent or received are typically stored in memory  904 , or in some other suitable storage element in the UE  110 . In an alternative embodiment, some of the sent and received messages may be stored remotely. 
     The UE  110  may further include a device state module  944 , a personal information manager (PIM)  946 , an internet browser  950 , and other various modules  948 . 
     It will be appreciated that different UEs may have different capabilities. Thus, the above description is not intended to be exhaustive of all of the potential capabilities that could be provided in a UE. Further, some UEs may have fewer capabilities than described above. For example, a UE may not have a camera or GPS sensor. 
       FIG. 10  illustrates a simplified block diagram of network access equipment  102 . There are many configurations of network access equipment known to those of skill in the art, and the configuration of network access equipment will vary depending on the network. Network access equipment  102  comprises a transmit module  1001 , a receive module  1003  and a processor  1005 . One skilled in the art will appreciate that the transmit module  1001  and receive module  1003  may be combined into a single transceiver module. The transmit module  1001  and the receive module  1003  may be coupled to the same antenna  112 , shown in  FIG. 1 , or to different antennas depending upon the configuration. In one embodiment, the processor  1005  comprises a memory not shown. The synchronization timing instruction module  1007  may be stored in the processor  1005 , or in memory associated with the processor. The synchronization timing instruction module  1007  comprises a burst period module  1009 , an interval module  1011 , a message generator module  1019 , and an optional quiet period module  1017 . The interval module  1011  further comprises a frequency control module  1013  which further comprise an estimation module  1015 . 
     The burst period module  1009  defines the burst period  402  shown in  FIG. 4 . The interval module  1011  determines the interval  206 ′ shown in  FIG. 4 . The burst period module  1009  sends the defined burst to the message generator module  1019 , and the interval module sends the determined interval  206 ′ to the message generator module  1019 . The message generator module  1019  then sends timing signal instructions (e.g., burst period and interval duration) to the transmit module  1001 . The transmit module  1001  may be used in conjunction with the antenna  112 , shown in  FIG. 1 , to transmit the timing signal instructions from the network access equipment (e.g., eNB) to the UE. 
     In one embodiment, the network access equipment  102  contains a look-up table in memory. The network access equipment  102  estimates the velocity of a user equipment, as is described below, and uses the velocity estimate to determine a burst period and interval from the look-up table. 
     In one embodiment, the interval module  1011  comprises a frequency control module  1013 . The frequency control module  1013  ascertains the repeating frequency of the timing signals in order to maintain synchronization. In one embodiment, the frequency control module  1013  may ascertain UL synchronization with the network access equipment using a small interval between timing sequences, e.g., the interval that would be required if the UE were traveling at 500 km/h, i.e., 200 Hz. Once UL synchronization has been established, the frequency control module  1013  may increase the interval between timing sequences for a given number of TA adjustment periods. For example, the frequency control module may increase the interval between timing sequences to the interval that would be required if the UE were traveling at 360 km/h, i.e., 100 Hz. If the UE continues to maintain UL synchronization, then the frequency control module  1013  may increase the interval again, until UL synchronization is lost. The network access equipment can then instruct the UE to return the last known successful interval between timing sequences, as that interval is the most optimum for the system at that time, or the UE can return to the last known successful interval without an instruction from the network access equipment. This process could be repeated periodically, or upon an event notification. 
     In another embodiment, the frequency control module  1013  could set the interval to a nominal rate, e.g., interval associated with a UE moving at 120 km/h. Then the frequency control module  1013  could decrease the interval until UL synchronization is lost or nearly lost. 
     In another embodiment, the frequency control module  1013  further comprises an estimation module  1015 . The estimation module  1015  is capable of estimating the mobility of the user. In one embodiment, the estimation module  1015  can estimate both the absolute and relative velocity of the UE. The estimation module  1015  may include a low pass filter to improve the accuracy of velocity measurements. In general, the UE&#39;s relative velocity, i.e., the velocity towards or away from the network access equipment, will have a greater affect on the UL synchronization timing. In one embodiment, the estimation module  1015  may use the timing drift to estimate the relative velocity, where relative velocity=(T*C)/(2*burst period). Where T is the time drift in seconds, C is the speed of light, and the burst period is also in seconds. 
     In another embodiment, the estimation module  1015  receives input from the UE to determine the velocity. For example, the UE may provide position information (e.g., obtained from GPS) to the network access equipment. This position information may be used by the estimation module to determine the velocity of the UE. 
     In another embodiment, the interval module  1011  establishes a number of timing signals (e.g., SRS symbols  202 ) required per burst to maintain synchronization. While the simulations shown in  FIGS. 3 and 5  show transmission of 20 SRS symbols, this is specific to an LTE system. If the LTE timing changes, or for other networks, it may be necessary to determine the number of timing signals required per burst to maintain synchronization. One manner of determining the number of timing signals is to send the maximum number of timing signals and then decrease the number of timing signals sent until the optimum number is reached. 
       FIG. 11  illustrates synchronization timing module  1101  that may be stored in memory  904 , RAM  930 , or within memory in the main processor  902  of the UE  110 . The synchronization timing module  1011  comprises a message receive module  1103  and a timing signal generation module  1105 . The message receive module  1103  may be used in conjunction with the communication subsystem  906 , shown in  FIG. 9 , to receive timing signal instruction messages from the network access equipment  102 . The timing signal instructions are sent to the timing signal generation module  1105 . The timing signal generation module  1105  transmits the timing signals in accordance with the timing signal instructions. Thus, the UE will transmit timing signals, e.g., SRS signals  202  at the instructed interval  206 ′ and for the instructed burst period  402 . The timing signal generation module may be used in conjunction with the communication subsystem  906 , shown in  FIG. 9 , to transmit the timing signals to the network access equipment  102 . 
     In one embodiment, the synchronization timing module  1101  also includes an optional estimation module  1107 . The estimation module  1107  is used to estimate the velocity of the UE. In one embodiment, the estimation module  1107  obtains information from the GPS sensor  928 , shown in  FIG. 9 , and provides that information to the communication subsystem  906 . The GPS information is then transmitted to the network access equipment  102 . Alternatively, the estimation module  1107  may obtain information from the GPS sensor, estimate the velocity of the UE, and then provide the estimate to the communication subsystem  906  for transmission to the network access equipment  102 . 
     In another embodiment, the start transmission of the SRS bursts shown in  FIG. 4  may be triggered by a network access equipment with an activation message. Such an activation message may be implemented as a Radio Resource Control (RRC) message or a Media Access Control (MAC) Control Element (CE) or physical layer signaling, for example via the Physical Downlink Control Channel (PDCCH). The UE may continuously transmit the SRS bursts until the UE receives a deactivation message as illustrated in  FIG. 12 a   . Such a deactivation message maybe implemented as a RRC message or a MAC CE or physical layer signaling, for example via the PDCCH. 
     In another embodiment, the SRS bursts could be configured by a network access equipment to be transmitted over a configured duration as illustrated in  FIG. 12 b   . The duration parameter could be either preconfigured, for example, semi-statically or dynamically signaled in the activation message. 
     In yet another embodiment, the SRS burst could be configured to be transmitted just once after receiving each activation message as illustrated in  FIG. 12   c.    
     An SRS, such as that used in 3GPP LTE Release 8, occupies a variable bandwidth comprised of a contiguous set of subcarriers, uses every other subcarrier within the bandwidth, and is constructed from orthogonal reference signal sequences. These characteristics mean that an SRS may be identified by its frequency resources (its bandwidth and its starting location in frequency), the subset of subcarriers it occupies within its frequency resources (an SRS “comb”), the OFDM symbols during which it is transmitted relative to frame timing, its code sequence (such as its base sequence or its sequence group), and its cyclic shift of the code sequence.  FIG. 13  illustrates SRS transmissions on either of two combs, where the SRS occupies a bandwidth B that is a total of 48 subcarriers, starts a location K in frequency, and is on either of two combs (having a frequency offset of either k 0 =0 or k 0 =1 subcarriers). The n th  subcarrier used by the SRS comb contains a reference signal sample, r α (n), and there are 24 total subcarriers used by the comb, so the SRS sequence length is 24. Each r α (n) may have a cyclic shift (set by the cyclic shift factor α) that may be implemented in the frequency domain as a complex multiplication of a code sequence r α (n)=e jαn   r (n)=e j2πS     i     n/S     max     r (n), where  r (n) is the code sequence, S i ={0, 1, . . . , S max −1} is the cyclic shift index, and S max =8. Note that S max  sets the number of cyclic shifts, and while S max =8 cyclic shifts are used in this embodiment, alternative embodiments can use S max  as small as 1 (for no cyclic shifts) or as large as the sequence length (24 in this embodiment). 
     Note that the resources (such as the frequency resources, comb, relative OFDM symbol times, code sequence, or cyclic shift) used for the SRS may be time varying, such as in the case of SRS frequency hopping. Therefore, the identification of the SRS can be by the resource hopping sequence, as well as by a fixed mapping of resources. 
     When a UE has multiple transmit antennas, each antenna may be assigned with a separate SRS. Further, the SRS from different antennas may be transmitted from different frequency resources, different combs, with different code sequences, or from different subframes or a combination of different frequency resources, code sequences, or subframes. 
     In one embodiment, the SRS from all antennas may transmit simultaneously as illustrated in  FIG. 14 , where SRS from each antenna are transmitted from the same frequency resources or subcarriers but with different code sequences or different cyclic shifts of a code sequence. 
     In another embodiment, the SRS from different antennas may be transmitted from different subframes as illustrated in  FIG. 15 . Alternatively, SRS from some antennas could be transmitted in the same subframes while SRS from other antennas could be transmitted from different subframes. 
     In one embodiment, the SRS transmissions from each antenna within a SRS burst may be from the same frequency resources or subcarriers. 
     In another embodiment, the SRS transmissions from each antenna within a burst may be on different frequency resources or subcarriers, for example, hopping from one frequency resource to another as illustrated in  FIG. 16 , where SRS from different antennas are assigned with the same frequency resources but with different code sequences or different cyclic shifts of a code sequence. 
     In still another embodiment, the SRS transmissions from different antennas may be on different frequency resources as shown in  FIG. 17 . 
     In one embodiment, the SRS frequency resource allocation for each antenna may be preconfigured with a fixed pattern for each SRS burst as illustrated in the example in  FIG. 17 . 
     In another embodiment, the frequency resource allocation pattern may be different from one burst to another. 
     In one embodiment, the SRS transmissions from each antenna within a SRS burst may use the same cyclic shift of a code sequence, but from a different frequency resource or different subframe. 
     In another embodiment, the SRS transmissions from each antenna within a SRS burst may use a different cyclic shift of a code sequence. 
     In a related embodiment, the SRS transmission from each antenna within an SRS burst uses a cyclic shift of a code sequence that is different from all other antennas transmitting in the same OFDM symbols, but additionally the SRS for all antennas use the same SRS comb, such that all sounding reference signals transmitted by the UE during an OFDM symbol occupy the same subcarriers and occupy half of the subcarriers in the contiguous set of subcarriers that contains the sounding reference signals. While allowing UEs to transmit on multiple combs is reasonable, constraining SRS transmissions from a UE to be on the same SRS comb can be advantageous because it can reduce the interference between UEs transmitting SRS. Because multipath radio channels degrade the orthogonality of UEs transmitting different cyclic shifts of a code sequence (especially close cyclic shifts), UEs transmitting on different combs often mutually interfere less. Using different cyclic shifts instead of different combs to multiplex the SRS on the antennas of a UE may increase the SRS interference between the antennas of a UE. This is an acceptable trade off, because the power received from the antennas of a UE tends to be reasonably close. The antennas are close (on the order of centimeters apart), whereas UEs can be far apart (on the order of kilometers), and so the received power of UEs can be many 10s of dBs apart. This disparity in received power means that a UE will tend to strongly interfere with other UEs more than antennas of a UE mutually interfere. Therefore, it can be more important to use the better isolation properties of SRS combs to multiplex UEs rather than antennas, and so it can be beneficial to constrain UEs to use cyclic shifts of a single comb. 
     Note that it is possible to construct SRS signals that occupy every N th  subcarrier, instead of every other subcarrier as is used in 3GPP Release 8. Still, in this case, all sounding reference signals transmitted by the user equipment in an OFDM symbol will occupy the same subcarriers and occupy no more than half of the subcarriers in the contiguous set of subcarriers that contains the sounding reference signals. 
     In another related embodiment, the SRS transmissions from all antennas within an SRS burst are on the same SRS comb, and each antenna uses an SRS with a cyclic shift of a code sequence that is different from all other antennas transmitting in the same OFDM symbols, and furthermore the cyclic shifts are cyclically contiguous. The cyclically contiguous constraint requires the cyclic shift S i  used for the i th  antenna to be different from the cyclic shift S j  used on a different j th  antenna, and that min(mod(S i −S j +S max , S max ), mod(S i −S j +S max , S max ))=1 for i={1, 2, . . . M} and one or two j&#39;s for each i, where i and j are indices corresponding to the M transmit antennas carrying the SRS, and S max  is the number of cyclic shifts, and mod (x,y) is the modulo division of the integer x by the integer y. Put another way, the cyclically contiguous cyclic shifts are cyclic shifts that are different for each antenna, and when the cyclic shifts are indexed in increasing order such that S k+1 &gt;S k , min(mod(S k+1 −S k +S max , S max ), mod(S k −S k+1 +S max , S max ))=1 for k={1, 2, . . . M−1}. The benefit of further constraining the cyclic shifts to be contiguous is similar to constraining UEs to use a single SRS comb. For a given SRS comb, adjacent cyclic shifts of an SRS code sequence will often mutually interfere more than more distant cyclic shifts of the code in multipath channels. As discussed above, because different antennas of a UE tend to be received at closer power levels than different UEs, mutual interference between a UE&#39;s antennas can degrade performance less than interference between UEs. Therefore, constraining the cyclic shifts to be contiguous will tend to have more interference between each UE&#39;s antennas, but less interference between UEs, resulting in enhanced control of interference. 
     In yet another embodiment, the SRS from different antennas may be configured with a combination of different frequency resources, code sequences or cyclic shifts and subframes. 
     In one embodiment, the SRS parameters may be semi-statically signaled to a UE through, for example, high layer signaling. 
     In another embodiment, the SRS parameters may be dynamically signaled to a UE through, for example, low layer signaling. 
     In still another embodiment, some of the parameters may be semi-statically signaled while other parameters could be dynamically signaled. 
     Some of the SRS parameters may be cell-specific, meaning that they are applicable to all the UEs, while other SRS parameters may be UE-specific, meaning that they are applicable to only one UE. 
     Cell-specific SRS parameters may be semi-statically broadcasted to all the UEs in a cell. UE-specific SRS parameters are usually signaled only to the intended UE, either semi-statically or dynamically. 
     Cell-specific SRS parameters may include SRS subframes and SRS bandwidth on which SRS may be allocated. These parameters may be signaled explicitly or implicitly. In the latter case, the parameters may be derived from other signaled parameters. 
     UE-specific SRS parameters may include SRS subframes, SRS bandwidth, starting frequency location, frequency hopping parameter, SRS duration, SRS burst length, and cyclic shifts of a code sequence. 
     The UE-specific SRS parameters may include per antenna SRS parameters for UEs with multiple transmit antennas such as frequency resource allocation, code sequence, and subframes. 
     In one embodiment, the SRS parameters for each antenna may be explicitly signaled to a UE through a RRC message, a MAC Control Element, or physical layer signaling, for example via the PDCCH. 
     In another embodiment, the SRS parameters for only one antenna, for example the first antenna, may be explicitly signaled through a RRC message, a MAC Control Element, or physical layer signaling, for example via the PDCCH. The SRS parameters for the rest of the antennas may be derived from the signaled parameters with a predetermined or preconfigured rule. For example, the parameters for the rest of the antennas may be derived with pre-configured offsets to the parameters for the first antenna such as cyclic offsets. 
     SRS burst transmissions may be triggered either through a high layer, for example the RRC layer, or a low layer, for example the MAC layer or physical layer. Low layer triggering has generally lower signaling latency and should be preferred, while the configuration of SRS transmission could be signaled with high layer signaling. 
     In an embodiment, a method in a network access equipment is provided. The method includes defining a burst duration for sending a plurality of reference signals. The method further includes determining an interval between ones of the plurality of reference signals in the burst duration. The method further includes defining a burst period for sending a plurality of reference signal bursts. The method further includes sending a reference signal instruction message containing an activation message and at least one of the burst duration, the interval, and the burst period. 
     In an embodiment, an alternative method in a network access equipment is provided. The method includes defining a first sounding reference signal burst and a second sounding reference signal burst for a user equipment with multiple transmit antennas. The first sounding reference signal burst is defined for a first antenna, and the second sounding reference signal burst is defined for a second antenna. The first sounding reference signal burst and the second sounding reference signal burst are different. 
     In an embodiment, another alternative method in a network access equipment is provided. The method includes defining multiple sounding reference signals for a user equipment with multiple transmit antennas. Each sounding reference signal is defined for one transmit antenna and the multiple sounding reference signals are different. All sounding reference signals in an OFDM symbol occupy a set of subcarriers, and the set of subcarriers occupies no more than half of the subcarriers in a contiguous set of subcarriers that contains the sounding reference signals. The multiple sounding reference signal bursts use different cyclic shifts of code sequences, and the cyclic shifts are cyclically contiguous. 
     It will be appreciated that the modules described above and other applications in the embodiments can be implemented using known programming techniques, languages and algorithms. The titles of the modules are provided as a convenience to provide labels and assign functions to certain modules. It is not required that each module perform only its functions as described above. As such, specific functionalities for each application may be moved between applications or separated into different applications. Modules may be contained within other modules. 
     While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods may be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
     Also, techniques, systems, subsystems and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component, whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.