The objective of the invention is to propose a kind of rational method of handover that has more, improve the overall performance of radio honeycomb communication system, comprise cutting off rate, blocking rate etc.
Mobile communication system is the same with many other communication systems, and the running status of system is all the time all among constantly changing.At each its corresponding state set V (t) is all arranged constantly: ... V
i(t), V
I+l(t) ... V
I+k(t) ... }.From the angle of operator, he may wish that the running status of system is to be limited in some comparatively desirable subsets of states V ' (t): ... V
i' (t), V
I+l' (t) ... V
I+k' (t) ... within.For mobile network, these states are and lower cutting off rate and blocking rate that higher communication quality or the like links together.
Major different is that the former terminal has mobility between mobile communication system and the fixed communications.Because the mobility of terminal if there is not the mechanism of handover, just has a large amount of users causes communication quality because of the motion of himself decline even call drop, the operation conditions of communication system will deviate from desirable subsets of states gradually.And handover is to change this mechanism that deviates from, and perhaps is called opposite strength.Once switching is exactly the transfer of a next state, and when several candidates' Target cell, system just is faced with once and selects: which state is the state of " good " more.
" good " is the comment of a subjectivity, reach more " good ", and the tolerance of " good " at first will be arranged.In the assessment of the mobile communication system of reality, we adopt and resemble performance statistics such as cutting off rate, call completing rate.And the state of system is the state sample of moment, is not possess statistical, and therefore the notion of a virtual statistic is necessary.Defining virtual statistic like this, it thinks that the correlation of system in time and space is so good, instantaneous property can be expressed eternal characteristic.Suppose that at a time system is in state V
i(t), the virtual perturbation r of a hypothesis is arranged: comprise that the virtual quality of wherein a certain conversation descends, perhaps virtual call request, perhaps a virtual handoff request does not perhaps take place whatever, or the like.The probability P that these virtual perturbations take place
r(P
r={ P
r| r ∈ Ω }) be that Ω is the set of the possible virtual perturbation of institute by network statistical property decision at that time.Through virtual perturbation, the state set that system is possible be V ' (t).The transition probability that virtual perturbation causes is P
j={ P
j| P:V (t) → V '
j(t) }, therefore can obtain the virtual assembly average E[x of arbitrary index x].
If think that the work of communication network is an ergodic process, then virtual statistic is the statistic of network momentary status.
The quality of passing judgment on a communication network often is not only to see an index, perhaps only pursues a target, but a multiobject problem.Need to suppose η in the investigation system
1, η
2, η
3... η
nThe characteristic of n index then can be set up a n dimension normed linear space X:{X, ‖ ‖ altogether }, each dimension is represented an index, and the definition of norm provides according to the actual needs.For a definite system, be not that the index of some representative all in the space all can actually reach.For an actual physical system, be without loss of generality, can think that all can realize that set a little is a convex set.Every bit (x in the set
1, x
2... x
n) all representing a group system state, the point on the state of system and the convex set is many-to-one mapping.Figure (3) has provided the example of a two dimension.Grid partly is meant the attainable convex set of mark.For mobile communication system, desirable index point is a no call drop naturally, and is clog-free, and the index that all high-quality all conversed are finished might as well the called after ideal point.Nearly all beyond convex set, this existing technical reason also has reason economically to ideal point.Actual operator can also provide the index point of its expectation according to the configuring condition of network.This is a desired point, and the point that correct expection draws had both dropped in the convex set, and it is ultimate attainment that the ability of system is performed to, and it should drop on the edge of convex set; The desired point of mistake or drop on the inside of convex set does not have the ability of abundant estimating system, or falls beyond the convex set, remains the index point that can not realize.
Switching is the strength of resisting the system performance index deterioration that causes because of terminal mobility as a whole.Switching each time all is the transfer of a kind of system mode of once realizing by the communication link that changes individual travelling carriage.This is a kind of system state transitions initiatively.According to top description, at t constantly, the state of system is V
i(t), after the virtual perturbation the possible state set of system be V ' (t), V ' (t) in one of existence be considered to " good " dbjective state collection V ' (t).Therefore, theoretically,, can make system all be operated in the dbjective state collection of " good " of prediction at any time, even can think, from V by handover
i(t) there is a shortest path to V ' between (t), can be by the minimum realization of switching several times.
Actual system can only once switch at every turn, and existing cellular system in fact also also can not predict which travelling carriage will switch next time, switch to which sub-district, therefore pursues the ability that optimum has exceeded system.Target cell queuing in the handover procedure in the reality is carried out in several candidate cells, in other words, the state one step transition probability that has only so several systems is greater than zero, and system selects and can't distinguish the Target cell in twice handover in the system, the each selection and selection next time and last the selection all without any related is fully independently.Therefore an admissible noninferior solution is to select a system mode, the index point of its representative in the index space X of virtual statistic and the index point " approaching " the most of " good " state set representative.Therefore the norm of index space, i.e. distance, selection most important.Therefore our problem can be summed up as the selection of index space norm.
It is that the expert puts into practice according to long-term work that the norm of index space is selected, and the analysis mass data obtains, and should be able to embody a concentrated reflection of expert's knowledge.The handoff algorithm of gsm system has defined an index (GSM Specification 05.08 Version 5.4.0) that is called as power budget (PBGT), from can be regarded as a kind of norm in form.
PBGT(n)=(Min(MS_TXPWR_MAX,P)-RXLEV_DL-PWR_C_D)-
(2)
Min (MS_TXPWR_MAX (n), P)-RXLEV_NCELL (n)) RXLEV_NCELL (n) is the average signal strength that travelling carriage receives adjacent area n broadcast channel, and the RXLEV_DL travelling carriage receives the average signal strength of the down channel of the current use of Serving cell.PWR_C_D is poor between the maximum radio frequency transmitting power that allows of cellular downlink and the actual down-link transmitting power that uses; MS_TXPWR_MAX is travelling carriage allows use on the Traffic Channel of current area a maximum radio frequency transmitting power; MS_TXPWR_MAX (n) is the maximum radio frequency transmitting power that allows use on the Traffic Channel of adjacent area n; P is the maximum power transmission ability of travelling carriage.Power budget has embodied a kind of thought of planning: at the cell boarder of making rational planning for (comprising uplink and downlink), the path loss between travelling carriage and the base station will equal the value of budget; If the path loss between travelling carriage and the Serving cell excesses budget, conversation just should switch on the contiguous sub-district, and in this inside, sub-district, the path loss between travelling carriage and the Serving cell will be less than budget.The topmost defective of power budget is the planning that it has only considered wireless aspect, and consider to switch other conversations beyond the current talking, and network traffic flow etc. is not then considered fully.Its other defective is: what it was considered is unalterable program budget, does not adjust according to the present case of network.
The design optimization of mobile communications network is a multiobject problem, and the state of some of them target when switching selected relevant.In the application of reality, should take all factors into consideration the significance level of these targets, the realized situation of reality and the relation between each target, select the combination of a part of target wherein.In fact, some indexs of using on mobile network's engineering are de facto standards of assessment cellular network performance, are well goal sets in the goal systems.Therefore, the present invention selects PCS, PDC, PHS, SQ as index space X.
The present invention selects the norm of network index factor as index space X, and its expression formula is:
NIF=K
1PCS+K
2(1-PDC)+K
3PHS+K
4SQ (3) NIF is named as the network index factor, and it is the weights combination of the network index that uses on the engineering in form.K
1, K
2, K
3And K
4Be constant weight, will give the user decision.To these indexs, the user has psychological approval degree to its importance, and they can use weights K
1~K
4Express.Its value is generally 0~100 integer.For example, 0: do not need to consider 10: inessential, 20: general, 50: important, 100: extremely important.Their existence makes the user that the inlet that participates in network management arranged, and makes the network operation more can embody user's demand.PCS is a call successful rate, is meant that the user successfully starts the probability of calling.PDC is a cutting off rate.PHS is a handover success rate, is meant the probability at sub-district, destination pilot scale handover success.SQ is the ratio of signal quality, refers to that all flashy signal qualitys in office (error rate) of conversing are better than the probability of a certain thresholding.The benefit of network index factor is directly from engineering objective, has more directly reflected the optimization direction of network.
Notice that if x, x ' and x " being to represent in the X space three points arbitrarily, no matter be the network index factor then, or power budget all satisfies ‖ x-x ' ‖=‖ x-x " ‖+‖ x "-x ' ‖.This is a kind of simplification to a certain extent, and it is making great efforts when ideal point is approaching also on equal extent ground near desired point system, and desired point and ideal point do not have difference optimizing on the direction.Its implicit thought is the notion of " gesture ", the switching each time of system select all seek " gesture " minimum a bit, so system is stable system, can resist abominable accident.
Switching is to finish in the moment in a moment, and the network index factor also can only be the value of moment, so it will adopt virtual statistic.Because the selection of Target cell is a scalar process relatively, its absolute value is unimportant, importantly its relative value.And its relative value is only relevant with the conversation during this switches, and is irrelevant with simultaneous other conversation in the system, therefore only need to calculate conversation switch before and after the relative value of network index factor.With Serving cell is the 0th sub-district, and Target cell is the 1st sub-district, below corresponding statistical indicator is discussed respectively.1. call successful rate (PCS)
Switching only is embodied on the blocking rate B of each sub-district the influence of call successful rate.Switch changing into of front and back PCS:
Δ PCS=(PCS
1'-PCS
1)+(PCS
0'-PCS
0) (4) PCS
j(z=0 or 1) is the PCS of single subdistrict, and that is with apostrophe is meant that virtual switching is later.The call successful rate of single subdistrict can be calculated as:
PCS
j=(1-B
i)
*(1-P
Cll) (5) P
CllBe because the link-quality reason is called out unsuccessful probability, can think zero (following letter is 0) generally speaking.Meet Poisson distribution if call out entering of (comprising switching) in the sub-district, the index of coincidence that leaves of calling distributes, and allocatable channel is C, and the Traffic Channel occupation rate is P
T, the virtual time is t, and μ is the cell call departure rate, and λ is the cell call admission rate, supposes that the t time is enough short, and the sub-district at t has n to call out arrival in the time probability is
, and the sub-district has at one time m to call out the probability that leaves and be
, Traffic Channel is blocked and to be meant that arriving calls in the sub-district adds and be the calls that the leaves Traffic Channel number greater than the sub-district, so the virtual Traffic Channel blocking rate of single subdistrict is:
Wherein
With formula (5), (6) substitution formula (4), and suppose in the virtual unit interval Serving cell and switching target small area the leaving of travelling carriage of all not conversing, therefore get m=0, and only keep n=C-CP
T, make t get the unit interval, (4) formula can be rewritten as:
As long as know the Traffic Channel occupation rate P of current switching target small area and Serving cell
T, can receive the value of current Δ PCS.2. cutting off rate (PDC)
Selecting consideration partly for call drop at switching target small area, mainly is whether Target cell can provide enough good wireless communication link.The detection of wireless reason call drop has nothing in common with each other in the different occasion of different systems, but all be unable to do without the detection of sampled point signal to noise ratio on principle.Might as well define a sampled point signal to noise ratio herein, is wireless reason call drop greater than given thresholding.
Suppose that the up-downgoing of communicating by letter in the sub-district is in a basic balance, consider, can calculate switch sampling point probability of call according to one of them direction.The virtual cutting off rate PDC that Serving cell is total
0Can comprehensively be expressed as:
The virtual cutting off rate PDC that Target cell is total
1Can comprehensively be expressed as:
Switch changing into of front and back PDC:
ΔPDC=PDC
1-PDC。Wherein L is that the later average cutting off rate of switch reasons call drop is being deducted in single conversation in the sub-district, r is the threshold level that causes call drop, i is the interference level (Jin Si consideration i value is roughly stable herein) of the channel of distribution, T switches required level threshold, s is the level value of measuring, f is the distribution function of sampled point actual reception level, and its expression formula is generally
The subscript of each parameter is the mark of sub-district.Under most of situation, L is negligible, has only when the covering quality of some sub-district is poor especially, just considers this value.3. handover success rate (PHS)
Here Ding Yi handover success rate be meant can normal talking in the Target cell that switches at the sampled point travelling carriage probability.It requires the signal to noise ratio of up-downgoing to satisfy the requirement of communicating by letter simultaneously.Because our sampled point that defines can obtain the information of Serving cell and Target cell simultaneously, see from the angle of sampled point and think that what difference both do not have that Serving cell and Target cell can be thought the Target cell that switches here.If think that uplink downlink is a balance, can calculate (descending) line link handover success rate P on Serving cell and switching target small area virtual respectively
0, P
1 Therefore, if think that being retained in that former channel once do not switch also is a kind of switching, the handover success rate before and after switching is:
Δ PHS=P
1P
1-P
0P
0(12) the interference level i of switching target small area is the interference level value of Target cell with allocated channel.4. the ratio of signal quality (SQ)
Signal quality and signal to noise ratio are one to one.Therefore the variation of signal quality is exactly the variation of signal to noise ratio.If think that noise is constant, so the variation of signal quality also is the variation of signal level intensity.The rational communication quality of supposing the system setting is S, according to the up-downgoing balance, can obtain the ratio of the signal quality of Serving cell and switching target small area:
SQ's changes into before and after switching
ΔSQ=SQ
1-SQ
0。
With above-mentioned Δ PCS, Δ PDC, Δ PHS, Δ SQ value, replace PCS, PDC, PHS, SQ in (3) formula, calculate and obtain network index factor NIF.
Among the present invention, directly with this network index factor NIF as handover trigger mechanism, realize that better cell switches, be called NIF and switch.
NIF among the present invention switches, and requires to exist an adjacent cell n, satisfies:
NIF(n)>NIF-MARGIN(n),
Here NIF_MARGIN is a network index factor hysteresis.It is at weights K
1, K
2, K
3, K
4After determining, specifically in engineering, adjust.Generally between 0.3~0.5.
The present invention adopts the Target cell selection algorithm of network index factor can successfully prevent the traffic of local sub-district to block, and reduces call blocking rate greatly, has improved the calling cutting off rate.
Specify the present invention and effect thereof with simulation example below.
We compare research to power budget and two criterions of the network index factor in the simulation study.Emulation is carried out at gsm system, and it is the system that a frequency division adds the time-division, and up-downgoing can not be balanced fully, because the measurement data that system can provide is limited, the calculating of network index factor is simplified through certain hypothesis.In addition, in emulation, the least interference level of sub-district allocatable channel is initial random given, no longer changes later on.
The mode of mainly taking in the calculating of NIF to table look-up realizes.Notice and when the network index factor calculates, extensively adopted function
As long as therefore set up a table and just can use by all Q (a) of computation of table lookup.(7) formula is directly adopted in the calculating of Δ PCS, and it all is given in advance calling out admission rate and calling out departure rate, according to P
TCan set up the another one form.Because a large amount of calculating can realize that the complexity of NIF algorithm is not high with the mode of tabling look-up, the complexity of calculating with power budget is in an order of magnitude.
Ground Fig. 4 (GSM Specification 05.22 version 5.0.0) is adopted in emulation, synchronization have a plurality of travelling carriages in the street the enterprising words that work, the motion of travelling carriage is from standing fast, and in the employing of each corner any optional path wherein, its probability equates.The VELOCITY DISTRIBUTION of travelling carriage is: 70 percent motion velocity of mobile station is lower than a metre per second (m/s), and they evenly distribute therein, and it is average with ten metre per second (m/s) speed that 30 percent motion velocity of mobile station is one, and five metre per second (m/s)s are the Gaussian Profile of variance.A travelling carriage is exactly a calling, and the generation of calling meets Poisson distribution, and the place of calling out beginning evenly distributes on whole map, and statistics begins later on to carry out running to stable state.
The level of the desirable switching point between each sub-district is-95dBm, switch level thresholding T is-98dBm, the level threshold T of call drop is-105dBm that communication quality thresholding S adopts the coding 4 (in gsm system, the communication quality value is divided into eight ranks of 0-7 by the error rate) of gsm system.Four local sub-districts telephone traffic (calling admission rate λ=2/ (0.85 of local sub-district that is higher than one times of other sub-district is set
*24)/and 0.48s, call out departure rate μ=0.0025/0.48s; Calling admission rate λ=1/ (0.85 of other sub-district
*24)/and 0.48s, calling departure rate μ=0.0025/0.48s), and the performance of two kinds of switching target small areas queuing norms of comparing cell index factor and power budget under this environment, particularly different handoff algorithms are to the resistivity of local traffic congestion.Table 1 is the result of emulation.
The analog result that table 1 switching target small area is selected
The Target cell selection mode | Cutting off rate | Blocking rate | Average communication quality
* |
Power budget | ????0.0245 | ????0.0115 | ????1.57 |
The network index factor
** | ????0.0233 | ????0.0052 | ????1.82 |
NIF switches
*** | ????0.0182 | ????0.0000 | ????2.25 |
*Be meant that least interference level that average signal level of arbitrary moment is lower than allocatable channel adds the conversation of 5dBm
Number
*K1=25, K2=50, K3=25, K4=10,
* *NIF_MARGIN (n)=0.3
Though because emulation is the working condition of system under the better environment, can obtain comparatively satisfied running index in the Target cell selection algorithm of power budget mode, but can find that from table the Target cell of network index factor mode selects to be queued in all to have obtained certain improvement on cutting off rate and the blocking rate.Average speech quality slightly descends, but compared with because call drop or to block be very little.
Simulation result shows that the object cell selection method of network index factor is effectively, can successfully prevent local cell telephone traffic congested, greatly reduces call blocking rate, and has improved the calling cutting off rate to a certain extent, and improve few on computational complexity.