Configurable random access channel structure for range extension in a wireless communication system

The present invention provides a method involving a configurable random access channel structure. One embodiment of the method includes generating a random access channel burst that includes a cyclic prefix and a selected number of repetitions of a preamble. The number of repetitions is selected based on at least one of a cell radius and a radio transmission frequency. This embodiment of the method also includes transmitting the random access channel burst over an air interface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to processor-based systems, and, more particularly, to arbitrating bus transactions in processor-based systems.

2. Description of the Related Art

Wireless communication systems typically include one or more base stations or access points for providing wireless connectivity to mobile units in a geographic area (or cell) associated with each base station or access point. Mobile units and base stations communicate by transmitting modulated radiofrequency signals over a wireless communication link, or air interface. The air interface includes downlink (or forward link) channels for transmitting information from the base station to the mobile unit and uplink (or reverse link) channels for transmitting information from the mobile unit to the base station. The uplink and downlink channels are typically divided into data channels, random access channels, broadcast channels, paging channels, control channels, and the like. The uplink and downlink channels may be shared or dedicated.

Mobile units can initiate communication with the base station by transmitting a message on one or more of the random access channels (RACHs). Uplink random access messages are non-synchronized and therefore may be transmitted at any time based on the synchronized downlink timing by any mobile unit within the coverage area of the base station. The receiver in the base station must therefore continuously monitor the random access channels and search the signals received on the random access channels for predetermined sequences of symbols (sometimes referred to as the RACH preamble) in random access messages transmitted by mobile units. To make the search process feasible, the format of the random access messages must be standardized. For example, conventional random access messages in the Universal Mobile Telecommunication Sservices (UMTS) Long Term Evolution (LTE) system are transmitted in a subframe during a transmission time interval (TTI) of 1 ms in 1.08 MHz bandwidth. The random access messages subframe is divided into a 0.8 ms preamble and a 102.6 μs cyclic prefix that includes a copy of a portion of the sequence of symbols in the preamble. The remaining 97.4 μs in the transmission time interval is reserved as a guard time to reduce or prevent inter-symbol interference between different random access messages or shared data channels.

The coverage area of a base station is related to the duration of the cyclic prefix and the guard time. For example, the conventional guard time of approximately 0.1 ms corresponds to a round-trip delay for a signal that travels approximately 15 kilometers. Thus, a random access channel message format that includes approximately 0.1 ms for the guard time is appropriate for reducing or preventing inter-symbol interference for coverage areas or cell sizes having a radius of up to approximately 15 kilometers. Similarly, the duration of the cyclic prefix is related to the size of the coverage area and the propagation channel delay spread. For example, a cyclic prefix of approximately 0.1 ms is suitable for coverage areas having radii of up to approximately 15 kilometers. Although a range of 15 km may be considered sufficient for conventional wireless communication systems, the base station range of proposed wireless communications systems, such as the UMTS LTE, is expected to increase to at least 100 km. Proposals to extend the range of the random access channel supported by base stations include increasing the transmission time interval to 2 ms.

FIG. 1shows a proposed modification to a random access message100. In this proposal, the extended transmission time interval includes a 0.8 ms RACH preamble105. The length of the cyclic prefix (CP)110increases in proportion to the desired coverage area. For example, every 0.1 ms of additional cyclic prefix length will account for additional 15 km coverage. The guard time115also increases at the same rate as the cyclic prefix length. Thus, with the 0.8 ms RACH preamble, the time available for guard time and cyclic prefix is 2 ms−0.8 ms=1.2 ms. This RACH range extension proposal attempts to reduce the receiver complexity of the RACH preamble detection. However, the range is then extended at the expense of the increased overhead required to transmit the longer cyclic prefix.

FIG. 2conceptually illustrates one conventional RACH receiver200. The receiver200monitors signals received within the 2 ms transmission time interval of the random access channel. If the mobile unit is very close to the receiver200, then the subframe may begin very near the beginning of the transmission time interval, as indicated by the subframe205. However, if the mobile unit is near the edge of the coverage area of the base station, then the subframe may begin very late in the transmission time interval, as indicated by the subframe210. A conventional preamble detection scheme may be used in this range extension scenario by shifting the starting reference time to the end of extended cyclic prefix, e.g., by shifting the Fast Fourier Transform data collection window by 0.6 ms for a 90 km coverage area, as shown inFIG. 2. The accumulated data can then be processed to search for a peak over a delay of approximately 0.6 ms.

SUMMARY OF THE INVENTION

The present invention is directed to addressing the effects of one or more of the problems set forth above. The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.

In one embodiment of the present invention, a method is provided for configuring a random access channel structure. One embodiment of the method includes generating a random access channel burst including a cyclic prefix and a selected number of repetitions of a preamble. This embodiment of the method also includes transmitting the random access channel burst over an air interface. Another embodiment of the method includes receiving a signal including information indicative of a random access channel burst including a cyclic prefix and a selected number of repetitions of a preamble. This embodiment of the method also includes detecting a mobile unit that transmitted the random access channel burst based on the received signal. The number of repetitions of the preamble is selected based on at least one of a cell radius and a radio transmission frequency.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 3conceptually illustrates one exemplary embodiment of a wireless communication system300. In the illustrated embodiment, the wireless communication system300includes base stations305(1-3) for providing wireless connectivity to one or more mobile units310. The indices (1-3) may be dropped when referring to the base stations305collectively. The base stations305may provide wireless connectivity according to various standards and/or protocols, including Orthogonal Frequency Division Multiplexing (OFDM). However, persons of ordinary skill in the art having benefit of the present disclosure should appreciate that alternative embodiments of the wireless communication system300may include base stations305that operate according to different standards and/or protocols. Furthermore, techniques for providing wireless connectivity to mobile units310are known in the art and in the interest of clarity only those aspects of providing wireless connectivity that are relevant to the present invention will be discussed herein.

The base stations305may be configured to provide wireless connectivity over different ranges and/or using different frequency bands. In the illustrated embodiment, the base station305(1) is configured to provide wireless connectivity over a range of approximately 45 km in one or more selected frequency bands, e.g. at carrier frequencies of 450 MHz or 900 MHz. The base station305(2) is configured to provide wireless connectivity over a range of approximately 75 km using the same frequency bands as the base station305(1). The base station305(3) is configured to provide wireless connectivity over a range of approximately 45 km in a different set of frequency bands, e.g. at carrier frequencies of 2.1 GHz or 2.6 GHz. Persons of ordinary skill in the art having benefit of the present disclosure should appreciate that the configurations of the base stations305described above are intended to be illustrative and not to limit the present invention.

The mobile unit310can initiate communication with the base stations305by transmitting a message on one or more random access channels (RACHs). The format of the random access channel burst transmitted by the mobile unit310may be selected based upon the radius or range associated with one or more of the base stations305. In the illustrated embodiment, the mobile unit310initially selects a default format for the random access channel burst. For example, the mobile unit310may be configured to generate and transmit random access channel bursts in a subframe during a transmission time interval (TTI) of 1 ms in 1.08 MHz bandwidth. The random access messages subframe is divided into a 0.8 ms preamble and a 102.6 μs cyclic prefix that includes a copy of a portion of the sequence of symbols in the preamble. The remaining 97.4 μs in the transmission time interval is reserved as a guard time to reduce or prevent inter-symbol interference between different random access messages. This particular format may be suitable for communication with a base station (not shown) that is configured to provide wireless connectivity over a range of approximately 15 km in one or more selected frequency bands, e.g. at carrier frequencies of 450 MHz or 900 MHz.

When the mobile unit310moves within range of the base station305(1), it may receive a broadcast message from the base station305(1) indicating a modified format for the random access channel bursts. In the illustrated embodiment, the range of the base station305(1) is an extended range of approximately 45 km. Accordingly, the format of the random access channel bursts may be modified so that the preamble is repeated once, i.e. the preamble is transmitted twice in the random access channel burst. In one embodiment, the format of the random access channel bursts may also be modified so that the random access channel burst occupies more than one subframe. When the mobile unit310moves within range of the base station305(2), it may receive a broadcast message from the base station305(2) indicating a modified format for the random access channel bursts. In the illustrated embodiment, the range of the base station305(2) is an extended range of approximately 75 km. Accordingly, the format of the random access channel bursts may be modified so that the preamble is repeated twice. The format of the random access channel bursts may also be modified so that the random access channel bursts occupies three subframes.

The format of the random access channel bursts transmitted by the mobile unit310may also be modified based on the frequency bands used by the base stations305. In the illustrated embodiment, the mobile unit310may move from an area served by the base station305(1) to an area served by the base station305(3). Although the base stations305(1,3) have approximately the same range, the base station305(3) transmits at a higher carrier frequency range than the base station305(1). Propagation conditions for lower carrier frequencies (such as 450 MHz or 900 MHz) may be significantly better than for higher carrier frequencies (such as 2.1 GHz or 2.6 GHz). Accordingly, the base station305(3) may transmit a broadcast message indicating that the format of the random access channel bursts may be modified so that the preamble is repeated twice or more. The broadcast message may also indicate that the format of the random access channel bursts may be modified so that the random access channel bursts occupies three or more subframes.

The duration of the cyclic prefix and/or the guard time interval may also be modified. In one embodiment, the duration of the cyclic prefix may remain the same for the different formats used by the base stations305and the guard time interval may occupy the portion of the subframe (or subframes) that is not used by the cyclic prefix or the preambles. In alternative embodiments, other partitions between cyclic prefix and guard period can be envisioned: In one case, the 1.2 ms portion of the subframe that is not allocated to the preamble could be evenly allocated to the cyclic prefix and the guard time so that the RACH coverage is extended to 90 km as shown inFIG. 2. Alternatively, the 1.2 ms portion of the subframe that is not allocated to the preamble could be unevenly distributed between the cyclic prefix length and the guard time. The uneven distribution of the allocated time to the cyclic prefix and the guard time could extend the coverage to the 100 km if the cyclic prefix length is equal to or greater than 0.667 ms. However, inter-symbol interference may occur when the cyclic prefix and guard time allocations are uneven in cases where the preamble is transmitted by a mobile unit near the cell edge. Furthermore, the signal strength received from mobile units near the edge of an extended cell, e.g., mobile units that are as much as 90 or 100 km from the base station, may be very low, which may reduce the likelihood of detecting the preamble of the random access channel message.

FIG. 4Aconceptually illustrates exemplary random access channel burst formats that may be selected by base stations depending on range and/or transmission frequency band. Format400includes a cyclic prefix (CP), a preamble that is formed using a Zadoff-Chu sequence, and a guard time (GT) interval. The format400is transmitted in a single 1 ms subframe. In one embodiment, the format400may be used to transmit random access channel bursts intended for base stations that have a range of approximately 15 km. Format405includes a cyclic prefix (CP), a preamble that is formed using a Zadoff-Chu sequence, one repetition of the preamble, and a guard time (GT) interval. The format405is transmitted in two subframes. The guard time interval in the format405is longer than the guard time interval used in the format400, but the cyclic prefix has the same length. In one embodiment, the format405may be used to transmit random access channel bursts intended for base stations that have a range of approximately 45 km.

Three subframes are used to transmit the random access channel bursts in the formats410,415. Format410includes a cyclic prefix (CP), a preamble that is formed using a Zadoff-Chu sequence, two repetitions of the preamble, and a guard time (GT) interval. The guard time interval in the format410is longer than the guard time interval used in the format405, but the cyclic prefix has the same length. In one embodiment, the format410may be used to transmit random access channel bursts intended for base stations that have a range of approximately 75 km. Format415includes a cyclic prefix (CP), a preamble that is formed using a Zadoff-Chu sequence, one repetition of the preamble, and a guard time (GT) interval. The guard time interval in the format415is longer than the guard time interval used in the format410, but the cyclic prefix has the same length. At least in part because of the relatively long guard time interval, the format415may be used to transmit random access channel bursts intended for base stations that have a range of approximately 200 km. Table 1 lists exemplary values of the parameters used to define the random access channel burst formats400,405,410,415.

FIG. 4Bconceptually illustrates one exemplary embodiment of a transmitter420for transmitting random access channel bursts. In the illustrated embodiment, a basic Constant Amplitude Zero Auto Correlation (CAZAC) sequence425of length P is generated. A Zadoff-Chu sequence cp(n) of length P is generated as

cp⁡(n)={exp⁡[j⁢⁢2⁢π⁢⁢pP⁢(n+n⁡(n+1)2)]for⁢⁢P⁢⁢oddexp⁡[j⁢⁢2⁢π⁢⁢pP⁢(n+n22)]for⁢⁢P⁢⁢even
By selecting different values for p, different root Zadoff-Chu sequences425can be generated. The number of sequences425is (P−1) for a prime number P. Orthogonal Zadoff-Chu sequences425can be generated by a cyclic shift operation of each of the root Zadoff-Chu sequences. The sequence425may be converted from serial stream to parallel stream by an S/P converter430and transformed to the frequency domain by a discrete Fourier transform (DFT) of length NDFTusing a Fourier transform element435. The frequency domain signal is mapped to the RACH resources within the entire signal frequency band using a mapper440. For example, 1.08 MHz bandwidth may be used for RACH transmission out of 1.25 MHz system bandwidth. The unused sub-carriers are set to zeros. The frequency domain signal is converted back to time-domain by an inverse fast Fourier transform (IFFT) of size NFFTusing an inverse transformer445. The output of the IFFT is parallel to serial converted by a P/S converter450. The signal is repeated block-by-block by the configured repetition factor RPF in a block repetition element455to generate a repeated sequence of length (NFFT×RPF). Then, CP samples and GT are added by a summer460to from an output signal465that may be transmitted in one or more access slots.

FIG. 4Cconceptually illustrates generation of a cyclic prefix (CP) for one exemplary random access channel burst format. In the illustrated embodiment, the exemplary random access channel burst470includes two copies of the preamble475of length NFFT. The cyclic prefix has a length of NCPand the guard time interval (GT) has a length of NGT. The cyclic prefix may then be formed by selecting NCPsymbols from the end of one copy of the preamble475and then using these symbols to form the cyclic prefix.

FIG. 5conceptually illustrates one exemplary embodiment of a receiver500. In the illustrated embodiment, the receiver500is a part of a base station (not shown). However, persons of ordinary skill in the art having benefit of the present disclosure should appreciate that the present invention is not limited to receivers500that are implemented in base stations. In alternative embodiments, the receiver500may be implemented in any device used to provide wireless connectivity to one or more mobile units over an air interface. Exemplary devices include, but are not limited to, access points, access serving networks, access networks, and base station routers. The receiver500may implement multiple processing threads that are used to process and/or analyze accumulated signal energy to detect messages provided by mobile units. The processing threads in the receiver500may function independently and/or concurrently to analyze portions of the received signal energy, i.e., the processing threads may operate in parallel. Persons of ordinary skill in the art having benefit of the present disclosure should appreciate that the processing threads may be implemented in a single physical processor, such as a central processing unit, that supports multiple threads or in multiple physical processors.

In the illustrated embodiment, the receiver500receives signals including the random access channel bursts transmitted by mobile units. The receiver500includes a pre-processor505that may remove a cyclic prefix from the received signal. The pre-processor505may also select and take a portion (or portions) of the signal of a selected length from the RACH burst prior to correlating the processed preambles in the frequency domain correlator. The length of the portion may be selected to correspond to the number of samples NFFTof a fast Fourier transform of the portion of the signal corresponding to a preamble. For example, the FFT size may correspond to the preamble length plus the round-trip delay for the target search window size. The pre-processed signal may then be transmitted to a frequency domain correlator510that is configured to process preambles using a plurality of reference signals513in a frequency domain. The reference signals513may correspond to different RACH sequences that may be found in preambles of RACH bursts. The frequency domain correlator510may produce a set of frequency domain correlated outputs for each of the plurality of reference signals513.

The embodiment of the frequency domain correlator510shown inFIG. 5includes a sequential searcher that operates with a search window size selected dependent upon an estimate of round trip delay. In one embodiment, a sequential search in increments of 14.61 km is used. In the first pass, search is done for a round trip delay (RTD) corresponding to distances from 0 to 14.61 km. In the second pass, search is done for 14.61-29.22 km. The sequential search is done up the RTD corresponding to the GT. The frequency domain correlator510also includes a fast Fourier transformer (FFT)515for transforming the pre-processed preamble from a time domain to a frequency domain and producing an FFT output including parallel streams of frequency domain signals. The FFT output may be passed to a RACH selector520for selecting one or more parallel streams of frequency domain signals from the FFT output corresponding to a RACH and producing a RACH selector output. The signal is then transmitted to a multiplier525for multiplying the RACH selector output with one or more of the reference signals to produce multiplier output(s) that include parallel streams of multiplied signals corresponding to the different reference signals. An inverse discrete Fourier transformer (IDFT)530may then be used to transform the multiplier output from the frequency domain to a code domain and produce a frequency domain correlated output for the reference signal.

An energy detector535may be used to detect the RACH burst sequence by comparing energies associated with the frequency domain correlated outputs to a threshold energy value. For example, the frequency domain correlated outputs may correspond to the energy associated with a portion of a signal that may correspond to one or more preambles in a RACH burst. The energy detector535may therefore add or sum the energy associated with each of the frequency domain correlated outputs. If these outputs correspond to the preamble, then the sum of the energies in these outputs should exceed a selected threshold. In the illustrated embodiment, the energy detector535includes a search window limiter540for limiting the frequency domain correlated output to a selected search window size to produce a limited output. For example, the search window size may be selected to correspond to a cell radius or a range of cell radii. The energy detector535may also include an energy module545for determining an energy of the limited output and an energy combiner550for summing the energy associated with different outputs, which may correspond to repetitions of the preamble in a RACH burst. The energy detector535may also include a threshold module555for comparing the energy value to a threshold energy value.

FIGS. 6A and 6Bconceptually illustrate random access channel burst detection sequences for two random access channel burst formats.FIG. 6Adepicts a random access channel burst detection sequence for a random access channel burst format including a cyclic prefix and a preamble. In the illustrated random access channel burst detection sequence, a receiver makes a first pass in which a received signal is searched over a search window corresponding to a range of cell radii from 0 km to about 15 km. The search window for the first pass is delayed by a time corresponding to the duration of the cyclic prefix and extends for a time corresponding to the duration of the preamble, which in this case is a Zadoff-Chu sequence. The receiver may then make a second pass in which the received signal is searched over a search window corresponding to a range of cell radii from about 15 km to about 30 km. The search window for the second pass is delayed by a time corresponding to twice the duration of the cyclic prefix and extends for a time corresponding to the duration of the preamble. The third pass searches over a search window corresponding to a range of cell radii from about 30 km to about 45 km. The search window for the third pass is delayed by a time corresponding to three times the duration of the cyclic prefix and extends for a time corresponding to the duration of the preamble. Although three passes are depicted inFIG. 6Apersons of ordinary skill in the art having benefit of the present disclosure should appreciate that the present invention is not limited to this number of passes and in alternative embodiments any number of passes may be performed.

FIG. 6Bdepicts a random access channel burst detection sequence for a random access channel burst format including a cyclic prefix, a preamble, and one repetition of the preamble. In the illustrated random access channel burst detection sequence, a receiver makes a first pass in which a received signal is searched over two approximately contiguous or adjacent search windows corresponding to a range of cell radii from 0 km to about 15 km. The first search window for the first pass is delayed by time corresponding to the duration of the cyclic prefix and the second search window for the first pass is delayed by the duration of the cyclic prefix plus the duration of the first search window. The receiver accumulates energy during the first and second search windows. Since the sequences in the preamble and the repetition of the preamble are the same, the energy accumulated during the first and second search windows adds constructively. The receiver may then combine the energy detected during the first and second search windows to determine whether or not the preamble of a random access channel burst has been detected.

The receiver may also make second and third passes in which the received signal is searched over two approximately contiguous or adjacent search windows corresponding to cell radii ranging from 15 km to about 30 km and two approximately contiguous or adjacent search windows corresponding to cell radii ranging from about 30 km to about 45 km. The first search window for each pass is delayed by times corresponding to twice and three times the duration of the cyclic prefix, respectively. The second search window for each pass is delayed by twice and three times the duration of the cyclic prefix, respectively, plus the duration of the first search window. The receiver accumulates energy during the first and second search windows for each pass and combines the energy detected during the first and second search windows. If the accumulated energy in the first and second search windows of either pass exceeds a threshold, then the receiver determines that preamble of a random access channel burst has been detected.

The detection performance for the four preamble formats discussed above has been analyzed. Theoretical detection performance for AWGN channel is shown inFIG. 7. The number of signatures is 64 and the search window size corresponds to maximum round trip delay (RTD) for the cell. The false alarm probability is assumed to be PFA=0.1% for all signatures and the entire delay hypothesis is adopted. As shown inFIG. 7, at miss detection probability PM=0.1%, detection performance improves by 2.3 dB by increasing the RPF from 1 to 2, and by 1.3 dB by further increasing the RPF to 3. Table 2 shows the summary of required Es/N0 for all RACH formats for static channel and TU channel. For TU channel, a 5.5 dB margin may be added to get required Es/N0.

A link-budget analysis may be performed using the extended COST-231 Okumura-Hata suburban path loss model. In the Okumura-Hata model, the path loss is modeled as

fC: carrier frequency in MHz

hb: Node-B antenna height in m

hm: UE antenna height in meters in m

d: distance between Node-B and UE in km.

C: constant factor. C=0 dB for medium-sized cities and suburban areas and C=3 dB for metropolitan areas.

System parameters used in link-budget analysis are shown in Table B.

Using this path loss model, the Es/N0at the antenna connector is obtained as
Es/N0=PL−Pnoise−IoT+Pmax−Pother+GNB+GUE
where PL denotes the propagation loss, Pnoisedenotes the equivalent noise power, IoT is the interference over thermal, Pmaxis UE maximum transmit power, Potheris other losses, and GNBand GUEdenote the NodeB and UE antenna gain. For thermal noise PSD of −174 dBm/Hz and uplink noise figure of 5 dB, the total effective noise PSD is −169 dBm/Hz. The noise power for RACH bandwidth of 1.08 MHz is obtained as −108.7 dBm. The values of IoT and other losses are assumed to be negligible.FIGS. 8 and 9show the Es/N0vs the distance between NodeB and UE for base station antenna heights 30 m, 90 m, and 90 m.FIG. 8is for carrier frequency 2.1 GHz. For 2.1 GHz carrier frequency, it is possible to cover the radius up to 12 km with RACH Format1and up to 14 km with RACH Format2for 90 m antenna height. By relaxing the PFArequirements or in benign channel conditions, coverage can be further extended. Es/N0for carrier frequency 900 MHz is shown inFIG. 9. Coverage for Formats1and2are 30 km and 38 km, respectively. Based on these observations, it is not always necessary to extend the preamble length for large cell operation.

The flexible RACH structure discussed herein may be used for range extension. The proposed structure uses fixed-length CP and a configurable number of sub-frames and preamble repetition factors. The proposed structure reduces the CP overhead for extended RACH and allows larger search window sizes. By configuring the number of sub-frames and the repetition factor, one can choose a RACH structure for a wide range of deployment scenarios. Cell based RACH structure configuration may also be used to simplify configuration so that mobile units in the same cell may be configured with the same RACH structure. The flexible RACH structure discussed herein may allow transmission of RACH structure with minimal cyclic prefix (CP) overhead. This increases the maximum cell size significantly. For example, the possible cell size for 2-subframe RACH structure is extended to 45 km, rather than 30 km, while having the same detection performance. This scheme allows the operator to configure the RACH structure optimally, depending on radio conditions and deployment scenario. In certain deployment scenarios, repeaters may be used for large cell operation. Also, there are deployment scenarios with remote radio heads. In these scenarios, RACH structure requires longer GT to cover large RTD, but it does not require preamble extension. In such cases, longer RACH structure without preamble repetition may be used.