The disclosure relates to global system for mobile communication (GSM) networks. In particular, the disclosure relates to synchronization processes for a GSM device using unallocated frames in a half-rate sub-channel.
A key indicator of call performance on wireless systems is the drop call rate (DCR). Operators are continuously pushing for lower DCR on their network and may use this measurement as a commercial advantage.
The GSM device adjusts an automatic frequency control (AFC) with the estimation done on the FCH signal (and mixed with other cells), and opens a window (where timeslot synchronization has been done by decoding FCH) of one timeslot to acquire and decode the SCH. The GSM device then verifies the BSIC of the neighbor cell, at step 110, and hands off to the neighbor cell, at step 112.
The failure to hand-off to a new cell is a drop call contributor, thus the ability of the mobile to synchronize quickly and stay synchronized to the appropriate neighbor cells is important to ensure proper neighbor cells measurement reports to the network for handover procedure trigger.
For the mobile station (MS), there are two points on neighbor cells monitoring that are critical for handovers. The mobile should be able to get synchronized as fast as possible to a new neighbor cell that may appear in its surroundings. If the MS is not synchronized on the new cell, i.e. has not performed Base Station Identification Code (BSIC) verification, when the network requests a handover to this new cell, the MS will fail to perform the handover. As a consequence, the MS stays connected to its original cell and will likely drop the call
The mobile should be able to keep synchronized as often as possible with neighbor cells it is monitoring. Otherwise, there could be “BSIC confusion” on handover. This can happen when a mobile is moving forward from a first cell to a second cell, through a third cell. If the first cell and the second cell beacon channels are on same Absolute Radio Frequency Channel Number (ARFCN), and if the mobile has not synchronized to the second cell fast enough (to verify its BSIC), the network can request the mobile to hand-off from the third cell to the second cell, while the mobile is only synchronized with the first cell. The mobile would try to hand-off to the first cell which is no more the stronger cell, hence it will likely fail the handover, and finally drop the call.
One solution is to use adaptive multi-rate (AMR) Half-Rate channels. Despite its limitation in terms of voice quality, wide AMR Half-Rate use can benefit in regards to both blocking and to drop call rates by reducing the transitions from full-rate (FR) to half-rate (HR) and back again, and repeating the process. This limits intra-cell handovers and provides interference reduction that leads to significant improvements in drop call rates on the hopping layer for the same carried Erlangs. Use of AMR also provides easier Mobile Allocation Index Offset (MAIO) planning for frequency hopping and reduces the probability of blocking occurring.
Consequently, improving drop call rate in the Half-Rate case is particularly interesting for the customer. The need to speed up the acquisition of Frequency Channel (FCH) and Synchronization Channel (SCH) is not new. The actual technique is to first detect FCH in idle frames and then SCH. Another technique (published by Siemens) is the detection in parallel of FCH and SCH during idle frames. The gain is only 240 ms. Therefore, a need exists for a method that provides appreciable gain.
Embodiments of the present disclosure are now described, by way of example only, with reference to the accompanying figures in which:
The present disclosure is defined by the appended claims. This description summarizes some aspects of the present embodiments and should not be used to limit the claims. While the present disclosure may be embodied in various forms, there are shown in the drawings and will hereinafter be described some exemplary and non-limiting embodiments, with the understanding that the present disclosure is to be considered an exemplification of the disclosure and is not intended to limit the disclosure to the specific embodiments illustrated.
In this application, the use of the disjunctive is intended to include the conjunctive. The use of definite or indefinite articles is not intended to indicate cardinality. In particular, a reference to “the” object or “a and an” object is intended to denote also one of a possible plurality of such objects.
In an example embodiment, the mobile station (MS) synchronizes as fast as possible to a new neighbor cell that may appear in its surroundings. If the MS is not synchronized on new cell, i.e. has not performed BSIC verification, and if the network requests a handover to this new cell, the MS will fail perform the handover. As a consequence, the MS will stay connected to its original cell and will likely drop the call.
The MS 205 may be a cellular telephone configured to operate with the GSM protocol or other 3G protocol. The MS 205 may include other devices, such as MS 207, that transmit and receive data signals interoperable with the GSM protocol. The BS 210 contains radio frequency transmitters and receivers used to communicate directly with the MSs 205 and 207. In this type of cellular network, the MSs cannot communicate directly with each other but have to communicate with the BSs 210.
The GSM network 215 includes components that connect the MS 205 and the BS 210 with other components, such as the PSDN 220 and the PSTN 225. The GSM network 215 includes support nodes, servers, and gateways operable to transmit the data carried within the GSM network 215 and between the MS 205 and the PSDN 220 and/or the PSTN 225.
The MS 205 is configured to maintain synchronization as often as possible with neighbor cells it is monitoring. Otherwise, there could be a so-called “Base Station Identity Code (BSIC) confusion” on handover to a neighboring cell, such as when the MS 205 is moving forward from a first cell 211 in the GSM network 215 to a second cell 212 in the GSM network 215 through a third cell 213 in the GSM network 215. If the first cell 211 and second cell 212 beacon channels are on the same ARFCN, and if the MS has not yet synchronized to the second cell 212 fast enough (to verify its BSIC), the GSM network 215 can request the MS 205 to hand-off from the third cell 213 to the second cell 212, while the MS is only synchronized with the first cell 211. The MS 205 would try to hand-off to the first cell 211, which is no longer the stronger cell, and hence it will likely fail the handover, and finally drop the call.
In a half-rate (HR) transmission protocol, the MS 205 is allocated half of the time division multiple access (TDMA) traffic channel (TCH) frames in the 26 multi-frame structure. The remaining half of the TCH channels are allocated to another MS 207. Half-rate traffic channels, which are mostly used especially by North American operators when deploying AMR, free one TDMA frame out of every 2 in the 26-multiframe structure. The synchronization module 312 uses these TDMA frames of the unused half-rate sub-channel to start the process of acquisition of FCH/SCH of neighbor cells. This prevents waiting for idle frames occurrences.
During operation, the synchronization module 312 interacts with the power measurement module 314 to perform a power measurement of a neighbor cell. The sub-channel determination module 316 determines a GSM sub-channel associated with the half-rate TCH frames. The GSM sub-channel includes unallocated channels not used for transmission and reception by the GSM device. The synchronization module 312 then uses the unallocated channels to synchronize the MS 205 with the neighbor cell.
The synchronization module 312 is further configured to decode an FCH signal acquired by the MS 205 from the neighbor cell in the GSM network 115. The synchronization module 312 then decodes an SCH signal acquired by the GSM device from the neighbor cell. The synchronization module 312 verifies a base station identification code (BSIC) received by the GSM device from the neighbor cell. The MS 205 is further configured to hand off to the neighbor cell when the BSIC is verified.
Using the unallocated frames in a sub-channel allows more occurrences (unused half-rate sub-channel and idle frames) to acquire the FCH signal and the SCH signal during these slots, hence speeding up the acquisition time of a new neighbor cell. Moreover, it increases the synchronization rate as BSIC verification can be performed more often on neighbor cells.
By using the TDMA frames of unused half-rate sub-channel to start the acquisition/synchronization process, instead of waiting for idle frames, the average acquisition time of a new neighbor cell is decreased by a factor of 6. Also, the process allows more occurrences of SCH windows openings, hence reducing the risk for BSIC confusion. Moreover, idle frames that are not used for FCH/SCH acquisition could be used for other purposes like inter-RAT measurements (e.g. 3G RSSI measurements). In another exemplary embodiment, the network measurement module 318 is further configured to perform inter-radio access technology (inter-RAT) measurements. The network measurement module 318 may perform received signal strength indicator (RSSI) measurements, while the MS 205 is in communication with 2G cells in the GSM network 215.
The MS 205 uses the unallocated half-rate TCH frames in the determined sub-channel to acquire and decode the FCH signal from the neighbor cell, at step 412. The unallocated half-rate TCH frames allow the MS 205 to use frames that are not used for receiving and transmitting to acquire the FCH signal. When the MS 205 is established on a TCH/HS sub-channel, half of the TCH frames of the 26-multiframe are available for neighbor cells synchronization purpose. A user who is established on sub-channel #0, for example, only receives and transmits traffic frames on frames 0, 2, 4, 6, 8, 10, 13, 15, 17, 19, 21, 23. This means that frames 1, 3, 5, 7, 9, 11, 14, 16, 18, 20, 22, 24 are available for other purposes, such as acquiring the FCH signal. The MS 205 then acquires and decodes the SCH signal, at step 414, from the neighbor cell, using the unallocated frames of the HR TCH frames. The MS 205 verifies the BSIC received from the neighbor cell, at step 416. When the BSIC is verified, at step 418, the MS 205 hands off to the neighbor cell.
The use of the unallocated frames in half-rate traffic channels for FCH/SCH acquisition may reduce the minimum time to get synchronized on a new cell. The minimum time requires 2 frames, i.e. after 9.2 ms and requires 2 radio window openings at a best case. At worst, FCH and SCH acquisition takes 51 frames, i.e. 235.4 ms and needs 23 radio window openings. On average, the time required to decode both the FCH and the SCH is 28 TDMA frames, i.e. 129.2 ms and requires 9 radio window openings. With a conventional FCH/SCH acquisition procedure, in the best case, the time required to decode the FCH and the SCH is 240 ms and requires 2 radio window openings. In the worst case, the time required to decode the FCH and the SCH is 1560 ms and requires 12 radio window openings. On average, the time required to decode the FCH and the SCH is 900 ms and requires 7 radio window openings.
The methods shown in
A “computer-readable medium,” “machine-readable medium,” “propagated-signal” medium, and/or “signal-bearing medium” may comprise any module that contains, stores, communicates, propagates, or transports software for use by or in connection with an instruction executable system, apparatus, or device. The machine-readable medium may selectively be, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. A non-exhaustive list of examples of a machine-readable medium would include: an electrical connection having one or more wires, a portable magnetic or optical disk, a volatile memory such as a Random Access Memory “RAM” (electronic), a Read-Only Memory “ROM” (electronic), an Erasable Programmable Read-Only Memory (EPROM or Flash memory) (electronic), or an optical fiber (optical). A machine-readable medium may also include a tangible medium upon which software is printed, as the software may be electronically stored as an image or in another format (e.g., through an optical scan), then compiled, and/or interpreted or otherwise processed. The processed medium may then be stored in a computer and/or machine memory.
While the principles of the disclosure have been described above in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the disclosure. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this disclosure.