Frequency hopping
FH can also decrease the overall C/I value in the network and thus improve the Quality Of Service (QOS).
Frequency hopping behavior:
Lognormal fading and Rayleigh distributed fast fading can decrease the speech quality. Rayleigh fades are the sum of a lot of reflected and phase shifted signals.
The fading at different frequencies is not the same and become more and more independent when the difference in frequency increases. With frequencies spaced sufficiently apart they can be considered completely independent (no correlation). Thus Rayleigh fading does then not damage all the bursts containing the parts of one code word in the same way.
When the ms moves of high speed the difference between its positions during the reception of two successive bursts of the same channel (i.e. at least 4,615 ms) is sufficient to decorrelate Rayleigh fading variations on the signal. In this case FH does not help except if there is interference.
The worst case is when ms is stationary or moves at slow speeds because the interleaved coding does not bring any benefit to reception. In this case FH “simulates ms movement” and thus the reception quality. This phenomenon is called frequency diversity.
In the other hand frequency hopping averages the interference directed towards each base station. Instead of a continuous interferer there are several interferers that affect only a short time each and with different intensity. Methods like power control and DTX (discontinuous transmission) affect only a single interference source and benefits can be distributed to the whole network by using FH. The gain, which comes from interference averaging, is called interference diversity.
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Baseband hopping occurs between TRXs in BTS. The number of frequencies used in the hopping sequence is the same as the number of TRXs in the sector. Both random and cyclic hopping can be used.
The digital (baseband) and analogue (RF) parts of the TRX are separated from each other. The switching of TRXs is on a per timeslot basis ands enables a particular TCH to hop from one carrier to another.
Synthesized hopping
Synthesized hopping is available in configurations, which have at least 2 TRX per sector. It enables each TRX to change frequency on successive time slots, so that given carrier can hop quickly onto many different frequencies. The carrier on which the BCCH IS transmitted must remain at fixed frequency to enable the MS to measure correct signal strength. Both random and cyclic hopping can be used.
Discontinuous transmission (DTX)
The transmission is disconnected when no information flow happens in signal. This is done by lower speech encoding bit rate than when the user is effectively speaking. This low rate flow is sufficient to encode the background noise, which is generated for the listener to avoid him thinking that the connection is broken. The low rate encoding corresponds to a decreased effective radio transmission of one frame each 20 ms to one such frame each 480 ms. Typically transmission is effective 60% of the time, which decreases the interference.
In order to implement such a mechanism, the source must be able to indicate when transmission is required or not. In the case of speech, the coder must detect weather or not there is some vocal activity. This function is called Voice Activity Detection (VAD). At the reception side, the listener’s ear must not be disturbed by the sudden disappearance of noise and the decoder must therefore be able generate some Comfort noise when no signal is received.
DTX is an option controlled by the operator, and which may be used independently in the MS to BTS and in BTS to MS.
PARAMETER PLANNING
GSM radio path
GSM is system using time division multiple access (TDMA) frame structure. The TDMA frame has duration of 4.615 ms and consists of 8 timeslots. There is two types of logical channels carried over the timeslots: Common channels and dedicated channels.
Common channels
The common channels are used for signaling and ca be divided into broadcast channels (BCH), continuously sending information from BTS to MS, and common control channels (CCCH).
The Broadcast channels send information on the cell properties such as synchronization, frequency correction, used frequencies and power levels, neighboring cells. There are three different broadcast control channels (BCCH).
The common control channels are used when establishing a signaling connection between the MS and BTS. The paging channel (PCH) is used when BTS wants to contact the MS. The MS requests a signaling channel on a random access channel (RACH). The signaling channel is allocated to the MS by using Access grant channel (AGCH)
Dedicated channels
The dedicated channels are divided into dedicated control channels and traffic channels. Call set up signaling and location updating procedures are performed on stand-alone dedicated control channel (SDCCH). In case of a call setup the connection is transferred into a traffic channel (TCH).
Both SDCCH and TCH have a parallel slow associated control channel (SACCH) which is used for transfer of measurement results from MS to BTS and power control commands from BTS to MS. During the short messages are transmitted over SACCH channel, while the fast associated control channel (FACCH) is used to transmit the handover commands to the MS.
Radio path measurements
The radio path measurements are used to keep the connection in good quality and therefore to trigger power changes and handover if needed. Both MS and BTS measure signal level and quality (bit error ratio). In addition to that MS measures the signal levels of all adjacent BCCH frequencies even though it is able to report only six best measurements.
Power control and handover
The BTS sends the raw measurement results received from the MS (downlink) and the results of its own measurements (uplink) to the BSC every SACCH multiframe period. The BSC does not support the measurement pre-processing in the BTS.
The BSC does the pre-processing of the measurement samples namely the book keeping and the averaging. The BSC is able to maintain a table of maximum 32 measurements results for up to 32 adjacent cells per call. After the averaging the BSC makes comparisons with the thresholds related to both power control (PC) and hand over (HO) algorithms.
The BSC determines the RF output power of the MS and the BTS on the basis of the results received from the pc threshold comparison process.
The HO decision is based in signal strength (RXLEV), quality (RXQUAL) and distance measurements. Another possible criterion is the power budget (PBGT) or umbrella condition fulfillment from an adjacent cell. The HO command is given over FACCH, which uses TCH temporally. Handovers can be done to TCH and SDCCH. The intra BTS handover can occur either to a timeslot in a new carrier or to a different timeslot in the same carrier. The intra-BSC handover to performed autonomously by the BSC. If there is an inter-BSC handover to be performed, the BSC sends the list of performed cells to the MSC and MSC performs the handover according to that list.
Handover strategies and parameters
The HO decision process may be triggered in different situations. Similarly to the pc it is controlled by the level (RXLEV) and quality (RXQUAL) in both UL and DL. In addition to these it depends in the distance and some periodic checks (PGBT, UMBRELLA). Only one type of periodic can be used per cell. The main principle when making HO caused by radio criteria is that the new server should be better than the current one.
The parameters, averaging and threshold comparison for level, quality and distance are similar to PC but only one threshold associated. The periodic checks occur every power budget (HO period PGBT) or umbrella (HO period Umbrella) period. In order to be performed the periodic checks require some data for the neighboring cells: the comparison process uses the calculated PGBT (n) or AV_RXLEV_NCELL (n) for neighboring cells instead of fixed thresholds.
Like with PC it is possible by changing the HO related cell parameters to affect the HO algorithm at all stages: preprocessing, threshold comparison, decision making.
NETWORK VERIFICATION AND OPTIMIZATION
This is the last step of the network planning procedure. It can start during the network trial period and continues after opening the commercial service and during the network expansion.
The aim of this process is to evaluate and maximize the quality of service in the network with the corresponding set of quality criteria.
Network verification
The purpose of the network verification (NV) is to evaluate an independent and objective quality of service (QOS) inside a given service area. This is done with network measurement system. Some OMC traffic measurements are dine in parallel to provide a statistical data and to complete the network picture.
The network verification procedure consists of the following steps:
- Planning of the measurement resources (including tools), reference network, schedule and test route(s)
- Setting of the network performance objectives and quality criteria
- Measurement execution and analysis of the statistical results
- Agreement on possible corrective actions if the set quality criteria is not met
The field verification takes place after successful completion of site acceptance. It should be repeated before and after any major network hardware/software changes to verify their affect on the network quality.
The service area, or the part of the network to be verified, is defined as a group of cells giving continuous coverage. It is always connected with a selection of test routes; the all verification and optimization activities are based in recurrent measurements over the same routes.
Network quality criteria
The quality objectives are specified according to the capacity requirements and customer’s QOS strategy which se agreed with the customer.
Some examples
Number of successful mobile originating (MO)
Call attempts with normal cell releases :90-95%
Minimum overall downlink quality :value 0-5 in 90-95%
Minimum overall downlink level on street :>37dBuV/m in 90-95%
level for 2W mobile minimum
Minimum successful rate for handovers (HO) :90-905%
Maximum system response time :0-7 s in 90-95%
(Time for TCH assignment)
One basic requirement for Network quality survey is to monitor continuously QOS and Hp behavior, compare the network against other similar network and present the result in such a way that they are easy to understand for non-technical parts of the operator’s organization as well. To be capable of doing that requires additional metrics that are sensitive enough and very easy to understand.
Analysis of the Results
The main result of the verification procedure is the statistical quality sheet generated by the NPS/X or similar system
In addition to the statistical quality sheet the network element availability statistics from the OMC also form part of the network verification results. These results are collected either on a monthly or weekly basis and they concern the network elements as a whole or by unit basis. The statistics of unit restarts and the availability of transceiver units are reported.
The congestion level of each BTS is obtained from the OMC traffic statistics and reported together with other network Verification results as the Network Verification report
Network Optimization
Network Optimization can be defined as a continuous process of improving overall network quality. Looking at network quality two different views should be considered . The customers (subscribers) view and the more comprehensive operators view fig. 1 overall network quality is illustrating this.
Usually a subscriber is not interested in site leasing or maintenance socts. As long as his service is not affected things like spectrum efficiency and network traffic are of no interest to him. For the operator these fig are of fundamental importance.
BSS Default Parameter Assessments
Proper BSS default parameter settings are needed to ensure the best possible performance of the network. The parameter sets are based on experience from optimized networks.
BSS Configuration Analysis Module
Network configuration Analysis is the smallest possible service module of network optimization. With this service the system configuration as existing in the real network (system configuration network) is compared against the system configuration as provided by network planning. This task ensures consistency between different system configuration databases and therefore is the basis of all following tasks. Basic configuration analysis should be repeated on a regular basis.
Basic Network Optimization Module
Field-tests OMC measurements and customer complaints are the three main sources to provide a detailed network quality picture.(e.g. call and handover success rates, problems reported by customer and field test personnel) the network performance data analysis together with single quality improvement actions raise network quality on a case by case basis. These tasks are combined in the Basic Network Optimization Module.