Cellular wireless communication systems support wireless communication services in many populated areas of the world. While cellular wireless communication systems were initially constructed to service voice communications, they are now called upon to support data communications as well. The demand for data communication services has exploded with the acceptance and widespread use of the Internet. While data communications have historically been serviced via wired connections, cellular wireless users now demand that their wireless units also support data communications. Many wireless subscribers now expect to be able to “surf” the Internet, access their email, and perform other data communication activities using their cellular phones, wireless personal data assistants, wirelessly linked notebook computers, and/or other wireless devices. The demand for wireless communication system data communications will only increase with time. Thus, cellular wireless communication systems are currently being created/modified to service these burgeoning data communication demands.
Cellular wireless networks include a “network infrastructure” that wirelessly communicates with wireless terminals and/or mobile devices within a respective service coverage area. The network infrastructure typically includes a plurality of base stations dispersed throughout the service coverage area, each of which supports wireless communications within a respective cell (or set of sectors). The base stations couple to base station controllers (BSCs), with each BSC serving a plurality of base stations. Each BSC couples to a mobile switching center (MSC). Each BSC also typically directly or indirectly couples to the Internet.
In operation, each base station communicates with a plurality of wireless terminals operating in its cell/sectors. A BSC coupled to the base station routes voice communications between the MSC and a serving base station. The MSC routes voice communications to another MSC or to the PSTN. Typically, BSCs route data communications between a servicing base station and a packet data network that may include and/or couple to the Internet. Transmissions from base stations to wireless terminals are referred to as “forward link” transmissions while transmissions from wireless terminals to base stations are referred to as “reverse link” transmissions. The volume of data transmitted on the forward link typically exceeds the volume of data transmitted on the reverse link. Such is the case because data users typically issue commands to request data from data sources, e.g., web servers, and the web servers provide the data to the wireless terminals.
Wireless links between base stations and their serviced wireless terminals typically operate according to one (or more) of a plurality of operating standards. These operating standards define the manner in which the wireless link may be allocated, setup, serviced and torn down. One popular cellular standard is the Global System for Mobile telecommunications (GSM) standard. The GSM standard, or simply GSM, is predominant in Europe and is in use around the globe. While GSM originally serviced only voice communications, it has been modified to also service data communications. In GSM, wireless terminals are informed of the need to service incoming communications via pages from base stations to the wireless terminals. GSM General Packet Radio Service (GPRS) operations and the Enhanced Data rates for GSM (or Global) Evolution (EDGE) operations coexist with GSM by sharing the channel bandwidth, slot structure, and slot timing of the GSM standard. GPRS operations and EDGE operations may also serve as migration paths for other standards as well, e.g., IS-136 and Pacific Digital Cellular (PDC).
According to the GSM standard, a BSC transmits various signaling channels that facilitate communication with a wireless terminal, or mobile device. For example, Broadcast Channels can include a Broadcast Control Channel (BCCH), a Frequency Correction Channel (FCCH), and other channels as defined by the standard. These various channels facilitate the BSC and a mobile device to establish communications with one another. For example, the BCCH allows the BSC to broadcast information about the identity of a network to which it corresponds, such as a Mobile Network Code (MNC), Location Area Code (LAC), and other information. The FCCH includes an FCCH burst, which is an all-zero or all-one sequence that produces a fixed GMSK tone, which enables a mobile device to lock its local oscillator to the BSC clock.
Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.
Embodiments of the present disclosure are directed to algorithms that facilitate acquisition of control channels in the GSM standard, such as, but not limited to, a Frequency Correction Channel (FCCH). During wireless data communication, the base stations transmit data bursts to the mobile devices or wireless terminals in TDMA frames. Each TDMA frame has eight time slots corresponding to eight data bursts, equivalently to data bursts for each multiframe. The data bursts belong to frequency correction channels FCCH, synchronization channels SCH, broadcast control channels BCCH, or common control channels CCCH.
The frequency correction channel FCCH data bursts do not contain training sequences, and comprises data burst of “zeros” or “ones” so that the mobile station can correct the local oscillator frequency error. The synchronization channel SCH is a downlink channel comprising regular sequences of bits that enables the mobile stations to synchronize received frame boundaries with the base stations on registration. The common control channel CCCH transfers data bursts containing training sequences known to the mobile stations for timing synchronization, supporting common procedures to establish a dedicated link between the base station and the mobile station. In the GSM specification, eight training sequences for normal bursts are specified, each base station utilizes a fixed training sequence thereof on all channels. The training sequence in the CCCH data burst is shorter than that of the synchronization burst in SCH, thus the timing synchronization provided by the CCCH data is less accurate than that of the SCH data. In the GSM systems, the common control channel CCCH includes RACH (Random Access Channel) for initial access to the GSM network, PCH (Paging Channel) indicating incoming calls or messages on waiting for the mobile station, and AGCH (Access Grant Channel) assigning the GSM network resource to another mobile station requesting the network access. The broadcast control channel BCCH is a downlink channel containing specific parameters required by the mobile station to identify the base station and obtain network access through the base station.
Accordingly, embodiments of the disclosure are related to acquisition algorithms for control channel data that reduce computational complexity relative to other implementations. To begin, a general architecture of an example GSM environment is shown and discussed.
Each of the base stations 103-106 services a cell/set of sectors within which it supports wireless communications. Wireless links that include both forward link components and reverse link components support wireless communications between the base stations and their serviced wireless terminals. These wireless links support digital data communications, VoIP communications, and other digital multimedia communications. The cellular wireless communication system 100 may also be backward compatible in supporting analog operations as well. The cellular wireless communication system 100 supports the Global System for Mobile telecommunications (GSM) standard and also the Enhanced Data rates for GSM (or Global) Evolution (EDGE) extension thereof. The cellular wireless communication system 100 may also support the GSM General Packet Radio Service (GPRS) extension to GSM. However, the present invention is also applicable to other standards as well, e.g., TDMA standards, CDMA standards, etc. In general, the teachings of the present invention apply to digital communications that combine Automatic Repeat ReQuest (ARQ) operations at Layer 2, e.g., LINK/MAC layer with variable coding/decoding operations at Layer 1 (PHY).
Wireless terminals 116, 118, 120, 122, 124, 126, 128, and 130 couple to the cellular wireless communication system 100 via wireless links with the base stations 103-106. As illustrated, wireless terminals may include cellular telephones 116 and 118, laptop computers 120 and 122, desktop computers 124 and 126, and data terminals 128 and 130. However, the cellular wireless communication system 100 supports communications with other types of wireless terminals as well. As is generally known, devices such as laptop computers 120 and 122, desktop computers 124 and 126, data terminals 128 and 130, and cellular telephones 116 and 118, are enabled to “surf” the Internet 114, transmit and receive data communications such as email, transmit and receive files, and to perform other data operations. Many of these data operations have significant download data-rate requirements while the upload data-rate requirements are not as severe. Some or all of the wireless terminals 116-130 are therefore enabled to support the GPRS and/or EDGE operating standard as well as supporting the voice servicing portions the GSM standard.
The mobile device 200 can also include a battery 224 or other power source that can provide power to the various components in the terminal. The terminal can also include one or more Subscriber Identification Module (SIM) port 213, a flash RAM 216, an SRAM 218, or other system resources. The mobile device 200 can also include one or more ports 210, which can comprise a universal serial bus (USB) port and its variants (e.g., micro-USB, mini-USB, etc.), a proprietary port, or any other input/output ports that can provide for data operations as well as power supply that can facilitate charging of the battery 224.
Accordingly, reference is now made to
Reference is now made to
X
F(k)=FFT(XT(k))
Where:
X
T(k)={xT(k,0),xT(k,1), . . . , xT(k,N−1)}
X
F(k)={xF(k,0),xF(k,1), . . . , xF(k,N−1)}
In the above formulation, N can equal 32, or the number of samples of the GMSK symbols for which a fast Fourier transform is executed. A subset of the frequency bins can then be selected in box 403. In one embodiment, the fourth through the twelfth bins can be selected as shown below:
{circumflex over (x)}F(k,n)=xF(k,n+4)
Where:
n=0,1, . . . , 8
This subset of the frequency bins can be selected because it can be assumed that the FCCH signal frequency is
as well as that the maximum frequency error is
Accordingly, the energy of each selected bin can also be calculated as a square of its absolute value as shown below:
Ê
F(k,n)=|{circumflex over (x)}F(k,n)|2
n=0,1, . . . , 8
In box 405, each of the three last output vectors can be summed as shown below:
Ê
OUT(k,n)=ÊF(k,n)+ÊF(k−1,n)+ÊF(k−2,n)
n=0,1, . . . , 8
In box 407, a sliding window comprising sets of pairs of frequency bins can be generated as shown below:
E
OUT(k,n)=ÊOUT(k,n)+ÊOUT(k,n+1) n=0,1, . . . , 7
Such a sliding window 501 is further illustrated in
where, Pntr(k) represents a pointer to a frequency bin and n=0,1, . . . , 7, and
E
MAX(k)=EOUT(k,Pntr(k))
If (EMAX(k)>Threshold) and (EMAX(k)>EMAX(k+1)) as shown in boxes 415 and 417, the frequency bin corresponding to k is selected as the initial frequency estimation and a pointer to that bin is returned as shown in the following pseudocode:
if(ÊOUT(k,Pntr(k))>ÊOUT(k,Pntr(k)+1)) return Pntr(k)
else return Pntr(k)+1
The threshold to which the energy level associated with a respective bin is compared can represent a minimum energy level associated with an FCCH signal.
Reference is now made to
In box 601, the initial frequency estimation can be derotated as shown below:
Where Pntr is a pointer to a frequency bin associated with the initial frequency estimation. Next, in box 603, the derotated initial frequency estimation can be downsampled to reduce computational complexity of the FCCH acquisition algorithm. In one embodiment, the signal sampling rate can be reduced by a factor of four and potentially even more without performance degradation. In one example, the derotated initial frequency estimation can be downsampled by a factor of four using an accumulator as a decimater as shown below:
In box 603, the downsampled and derotated initial frequency estimation can be autocorrelated for the purpose of improving the timing of the initial frequency estimation. In one embodiment, the correlator can be implemented as a sliding window in which thirty samples in the middle of the FCCH burst can be employed.
Next, in box 605, the baseband processor 206 can execute a frequency estimation function on the result of the autocorrelation. The frequency estimation can be performed as shown below:
Where FEST is the first frequency estimation, FSYM=270.8333 KHz, Y is a first vector representing the autocorrected downsampled initial detection, and M and N are frequency estimation parameters. To generate the first frequency estimation, the frequency estimation parameter M can be set to 4 and N can be set to 26. However, it should be appreciated that other frequency estimation parameters can also be employed. In box 605, the baseband processor 206 can derotate the first frequency estimation and in box 607, the derotated frequency estimation can be used to generate a second frequency estimation by using frequency estimation parameters of M=20 and N=30. The second frequency estimation can be derotated to generate a refined frequency estimation.
Accordingly, as shown in box 305 of
Where X(n) is the downsampled initial detection, N is a number of samples in the downsampled initial detection, FrEstim is the refined initial estimation, and Thr is a threshold to which the value resulting from execution of the filter is compared. If the refined initial estimation when passed through the filter exceeds the threshold, then the baseband processor 206 can declare that a carrier frequency has been acquired from the FCCH.
Any logic or functionality illustrated herein, if embodied in software, each block may represent a module, segment, or portion of code that comprises program instructions to implement the specified logical function(s). The program instructions may be embodied in the form of source code that comprises human-readable statements written in a programming language or machine code that comprises numerical instructions recognizable by a suitable execution system such as a processor in a computer system or other system. The machine code may be converted from the source code, etc. If embodied in hardware, each block may represent a circuit or a number of interconnected circuits to implement the specified logical function(s).
Although the flowcharts show a specific order of execution, it is understood that the order of execution may differ from that which is depicted. For example, the order of execution of two or more blocks may be scrambled relative to the order shown. Also, two or more blocks shown in succession may be executed concurrently or with partial concurrence. Further, in some embodiments, one or more of the blocks shown may be skipped or omitted. In addition, any number of counters, state variables, warning semaphores, or messages might be added to the logical flow described herein, for purposes of enhanced utility, accounting, performance measurement, or providing troubleshooting aids, etc. It is understood that all such variations are within the scope of the present disclosure.
Also, any logic or application described herein that comprises software or code can be embodied in any non-transitory computer-readable medium for use by or in connection with an instruction execution system such as, for example, a processor 803 in a computer system or other system. In this sense, the logic may comprise, for example, statements including instructions and declarations that can be fetched from the computer-readable medium and executed by the instruction execution system. In the context of the present disclosure, a “computer-readable medium” can be any medium that can contain, store, or maintain the logic or application described herein for use by or in connection with the instruction execution system. The computer-readable medium can comprise any one of many physical media such as, for example, magnetic, optical, or semiconductor media. More specific examples of a suitable computer-readable medium would include, but are not limited to, magnetic tapes, magnetic floppy diskettes, magnetic hard drives, memory cards, solid-state drives, USB flash drives, or optical discs. Also, the computer-readable medium may be a random access memory (RAM) including, for example, static random access memory (SRAM) and dynamic random access memory (DRAM), or magnetic random access memory (MRAM). In addition, the computer-readable medium may be a read-only memory (ROM), a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), or other type of memory device.
It should be emphasized that the above-described embodiments of the present invention are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.
This application claims priority to co-pending U.S. provisional application Ser. No. 61/565,864, entitled “Cellular Baseband Processing,” filed Dec. 1, 2011, which is incorporated herein by reference in its entirety. This application also claims priority to co-pending U.S. Provisional application Ser. No. 61/568,868, entitled “Cellular Baseband Processing,” filed Dec. 9, 2011, which is incorporated herein by reference in its entirety.
Number | Date | Country | |
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61565864 | Dec 2011 | US | |
61568868 | Dec 2011 | US |