1、Review of Last Class,2、Frequency reuse and mobility management,1、Fundamentals of Cellular Communications,3、Cell cluster concept,Chapter 4 - Lecture 2,5、Call blocking and delay at the cell-site,4、Cochannel and adjacent channel interference,6、Other mechanisms for capacity increase,7、Channel assignment
2、 strategies,2、 Frequency reuse and mobility management,1、Introduction,3、 Cell cluster concept,4、 Cochannel and adjacent channel interference,Cochannel InterferenceAdjacent channel interference,4.4 Cochannel and adjacent channel interference,Deployment of frequency reuse is necessary to enlarge the s
3、ystem capacity. On the other hand, frequency reuse will introduce cochannel interference from cells using the same set of frequencies. Therefore, frequency needs to be carefully planned so that cochannel interference is kept at an acceptable level.,Cochannel Interference,4.4 Cochannel and adjacent c
4、hannel interference,Interference from other mobiles at the cell-site receiver in the same cell is intracell interference, interference from other cells is intercell interference.,Cochannel Interference,4.4 Cochannel and adjacent channel interference,Base Station,Forward channel or downlink,Reverse c
5、hannel or uplink,Intercell interference in the downlink that affects the reception at the individual mobile hosts may be more of a problem than uplink interference at the cell-site receiver,Cochannel Interference,4.4 Cochannel and adjacent channel interference,For simplicity in the following analysi
6、s, we will consider only the average channel quality as a function of the distance dependent path loss, without going into details of channel statistics due to propagation shadowing and multipath fading.,Cochannel Interference,4.4 Cochannel and adjacent channel interference,Cochannel Interference,Th
7、e average received signal strength at any point decays as a power law of the distance between the transmitter and the receiver.,4.4 Cochannel and adjacent channel interference,Assuming,S - the power of the desired signal,I - the power of the cochannel interference,NI - the number of cochannel interf
8、ering cells,Ii - the interference caused by transmissions from the i-th interfering cochannel cell base station,Cochannel Interference,4.4 Cochannel and adjacent channel interference,The signal-to-cochannel interference ratio (S/I) at the desired mobile receiver is given by,Cochannel Interference,4.
9、4 Cochannel and adjacent channel interference,Assuming,r - the distance between the moblie and the serving BS,Di - the distance between the ith interfer and the mobile,k - the path loss exponent,When the transmit powers from all base stations are equal, and the path loss exponent is the same:,Cochan
10、nel Interference,4.4 Cochannel and adjacent channel interference,The worst case cochannel interference occurs as the power of the desired signal is minimum:,We neglect cochannel interference from the second and other higher tiers,Cochannel Interference,4.4 Cochannel and adjacent channel interference
11、,Cochannel Interference,4.4 Cochannel and adjacent channel interference,For the U.S. AMPS analog FM system, a value of S/I=18 dB or greater is acceptable. With a path loss exponent of k=4, the frequecny reuse ratio q is determined as:,Example 4.3,4.4 Cochannel and adjacent channel interference,Suppo
12、se the acceptable signal-to-interference ratio in a certain cellular communications situation is S/I = 20 dB or 100. Also, from measurements, it is determined that k = 4. what is the minimum cluster size?,Example 4.4,4.4 Cochannel and adjacent channel interference,If we use a better approximation of
13、 the distance between the mobile and the first interfering base station as illustrated in figure, then the S/I ratio can be expressed as:,Cochannel Interference,4.4 Cochannel and adjacent channel interference,Consider a cellular system that requires an S/I ratio of 18 dB a. For a frequency reuse fac
14、tor of 7, calculate the worst-case signal-to-cochannel interference ratio, assuming a path loss exponent of k=4. b. Is a frequency reuse factor of 7 acceptable in terms of cochannel interference? If not, what would be a better choice of frequency reuse factor?,Example 4.5,4.4 Cochannel and adjacent
15、channel interference,Adjacent channel interference (ACI) results from signals which are adjacent in frequency to the desired signal. ACI is mainly due to imperfect receiver filters which allows nearby frequencies to leak into the passband.,Adjacent Channel Interference,4.4 Cochannel and adjacent cha
16、nnel interference,Near-far effect,Adjacent Channel Interference,4.4 Cochannel and adjacent channel interference,To reduce ACI, we should:Use modulation schemes which have low out-of- band radiation (e.g. GMSK)Carefully design the bandpass filterUse proper channel interleaving Avoid using adjacent ch
17、annels in adjacent cellsSeparate the uplink and downlink properly,Adjacent Channel Interference,5、 Call blocking and delay at the cell-site,ErlangProbability of Call BolckingErlang-B Formula (LCC Model)Erlang-C Formula (LCH Model),4.5 Call blocking and delay at the cell-site,From the users perspecti
18、ve, quality of service (QoS) is more than an acceptable transmission accuracy. In fact, there are two crucial question:How successfully can a new user get a connection established?After connection establishment, how successfully will the connection be maintained as the user moves from one cell to an
19、other?,The performance measure is the probability that a call (new or handoff) is blocked,4.5 Call blocking and delay at the cell-site,One Erlang represents the amount of traffic load carried by a channel that is completely occupied, such as one call-hour per hour. If a channel is busy for 30 minute
20、s during a one hour period, then the channel is said to carry 0.5 Erlang of traffic.,Erlang,4.5 Call blocking and delay at the cell-site,To formulate the probability of call blocking, consider a radio cell which has been allocated J channels, and assume that The aggregate arrival traffic is Poisson
21、distributed with rate ; The channel holding time is exponentially distributed with parameter . That is, the mean channel holding time of the call is , correspond -ing to a mean service rate of for the call.,Probability of Call Bolcking,4.5 Call blocking and delay at the cell-site,Probability of Call
22、 Bolcking,The duration of a call is exponentially distributed with parameter ; The residence time of each user in a cell is exponentially distributed with parameter . As an exponential random variable is memory less, the channel holding time is the minimum of the call duration and the cell residence
23、 time, which is also exponential distributed with parameter,4.5 Call blocking and delay at the cell-site,When the system is in state j=J, all J servers are engaged and new requests will be blocked. In the bufferless J-server system, the delay for any request is zero. The request is either granted or
24、 blocked.,Erlang-B Formula (LCC Model),4.5 Call blocking and delay at the cell-site,Erlang-B Formula (LCC Model),4.5 Call blocking and delay at the cell-site,Erlang-B Formula (LCC Model),4.5 Call blocking and delay at the cell-site,Erlang-B Formula (LCC Model),Where is defined as traffic intensity，
25、and the call blocking probability is considered to be the grade of service (GoS) parameter for the Erlang-B system.,4.5 Call blocking and delay at the cell-site,Erlang-C Formula (LCH Model),In this case, the queuing delay would be nonzero. The probability of nonzero delay that can be tolerated is al
26、so considered to be the GoS parameter for the Erlang-C system.,4.5 Call blocking and delay at the cell-site,Erlang-C Formula (LCH Model),6、 Other mechanisms for capacity increase,Cell splittingAntenna sectoring,4.6 Other mechanisms for capacity increase,The capacity of a cellular system can be enlar
27、ged through frequency reuse. The capacity can also be improved based on cellular layout and antenna design using: Cell splitting Antenna sectoring,4.6 Other mechanisms for capacity increase,Cell splitting,4.6 Other mechanisms for capacity increase,Cell splitting is to subdivide a congested cell into
28、 smaller cells, each with its own base station :Reduction in antenna height and transmit powerCell splitting increases the capacity of a cellular system, and reduces the call blocking probability in the areaIncreases the frequency with which mobile hand off from cell to cell,Cell splitting,4.6 Other
29、 mechanisms for capacity increase,Reduction in the transmit power,R2=R1/2,Cell splitting,4.6 Other mechanisms for capacity increase,Increases the capacity,With more cells, there will be more clusters in the same coverage area, this is equivalent to replication a cell cluster more times. Hence, cells
30、 splitting increasing the capacity of a cellular system since it increase the number of times that channels are reused.,Cell splitting,Consider a cellular system, where the original cells have radius R. These cells are split into smaller cells, each with radius R/2. Suppose each cell is allocated 60
31、 channels regardless of the cell size. Find the number of channels contained in a 33 km2 area for the following cases: a. Without cell splitting b. With cell splitting,4.6 Other mechanisms for capacity increase,Example 4.6,Directional antennas can increase the system capacity relative to that of omn
32、idirectional antennas:,4.6 Other mechanisms for capacity increase,Directional Antennas (Sectoring),4.6 Other mechanisms for capacity increase,Directional Antennas (Sectoring),Consider a cellular system, a value of S/I=18 dB or greater is acceptable. With a path loss exponent of k=4, the frequecny re
33、use ratio q is determined as: a. Omnidirectional b. 1200 sectorization,4.6 Other mechanisms for capacity increase,Example 4.7,4.6 Other mechanisms for capacity increase,Smart Antennas,4.6 Other mechanisms for capacity increase,SDMA,7、 Channel assignment strategies,Fixed Channel Assignment (FCA)Dynam
34、ic Channel Assignment (DCA),4.7 Channel assignment strategies,In FCA, each cell is allocated a predetermined set of voice channels. Any call attempt within the cell can only be served by the unused channels in the particular cell A borrowing option may be considered, if all of a cells own channels a
35、re already occupied and the neighboring cell has spare channels.,Fixed Channel Assignment (FCA),4.7 Channel assignment strategies,In DCA, voice channels are not allocated to different cells on a permanent basis. Each time a call request is made, the serving base station requests a channel from the MSC.,Dynamic Channel Assignment (DCA),Conclusions,5、Call blocking and delay at the cell-site,4、Cochannel and adjacent channel interference,6、Other mechanisms for capacity increase,7、Channel assignment strategies,Problems,P 5.5，P 5.11,
