Contents: • • INTRODUCTION TO CELLULAR CONCEPT INTERFERENCE • • • • Trunking and Grade of Service(GoS) Introduction To Wireless Communication ATM(Asynchronous Transfer Mode) Internet Protocol INTRODUCTION TO CELLULAR CONCEPT: Cellular concept was a major breakthrough in solving the problem of spectral congestion and user capacity. It offered very high capacity in a limited spectrum allocation without any major technological changes. It is a system level idea which calls for replacing a single, high power transmitter into many low power transmitter. Each neighboring base station is assigned with different frequency of channel so that interference will not occur. INTERFERENCE It is a major limiting factor in the performance of cellular radio systems.(In comparison with wired comm. Systems, the amount and sources of interferences in Wireless Systems are greater. Sources of interference are: 1. Mobile Stations 2.Neighboring Cells 3. The same frequency cells 4.Non-cellular signals in the same spectrum Interference in Voice Channels: Cross-Talk & Interference in Control Channels: missed/blocked calls. Urban areas usually have more interference, because of: Greater RF Noise Floor & More Number of Mobiles. TYPES OF INTERFERENCE 1. Co-Channel Interference (CCI) 2. Adjacent Channel Interference (ACI) Co-Channel Interference CCI: The cells that use the same set of frequencies are called co-channel cells. The interference between signals from these cells is called Co-Channel Interference (CCI). It cannot be controlled by increasing RF power. Rather, this will increase CCI. It depends on minimum distance between co-channels. The yellow cells use the same set of frequency channels, and hence, interfere with each other. In the cellular system there are 6 first- layer co-channels In constant cell size and RF power, CCI is a function of Distance between the co-channel cells (D), and the size of each cell (R). Increasing ratio D/R, CCI also decreases. Define Channel Reuse Ratio = Q = D/R=√3𝑁 For hexagonal geometry, D/R can be calculated: Smaller Q provides larger capacity, since that would mean smaller N. (Capacity ∝ 1/N). Larger Q improves quality, owing to less CCI. N=3, Q=3
N=7, Q=4.58N=12, Q=6 N=13, Q=6.24 Adjacent Channel Interference ACI: • • • • • Interference resulting from signals which are adjacent in frequency to the desired signal is called adjacent channel interference (ACI). The primary reason for that is Imperfect Receive Filters which cause the adjacent channel energy to leak into your spectrum. Problem is severer if the user of adjacent channel is in close proximity Near-Far Effect Near-Far Effect: The other transmitter (who may or may not be of the same type) captures the receiver of the subscriber. Also, when a Mobile Station close to the Base Station transmits on a channel close to the one being used by a weaker mobile: The BS faces difficulty in discriminating the desired mobile user from the “bleed over” of the adjacent channel mobile. NEAR-FAR EFFECT: CASE-I The mobile receiver is captured by the unintended, unknown transmitter instead of the desired base station. NEAR-FAR EFFECT: CASE-II The base station faces difficulty in recognizing the actual user when the adjacent channel bleed over is too high. Minimization of ACI: 1. Careful Filtering-----minimum leakage or sharp transition 2. Better Channel Assignment Strategies • Channels in a cell need not be adjacent: For channels within a cell, keep frequency separation as large as possible. • Sequentially assigning cells the successive frequency channels. • Also, secondary level of interference can be reduced by not assigning adjacent channels to neighboring cells. • For tolerable ACI, we either need to increase the frequency separation or reduce the passband BW. Trunking and Grade of Service(GoS): • • It is related to the accommodation to a large number of user in a limited radio spectrum. Trunking allows a large number of user to share the relatively small number of channels in a cell by providing access to each user, on demand, from a pool of available channels. • Each user is allocated a channel on a per call basis and upon termination of the call, the previously occupied channel is immediately returned to the pool of available channels. • In trunked mobile radio system, when a particular user requests service and all of the radio channels are already in use, the user is blocked or denied access to the system. • A queue may be used to hold the requesting users until a channel is becomes available. Terminology: • Erlang: Unit of Traffic intensity
• • • • • o One Erlang: traffic in a channel completely occupied. o 0.5 Erlang: channel occupied 30 minutes in an hour. Set-Up Time: time to allocate a channel. Blocked Call: Call that cannot be completed at time of request due to congestion. Also referred to as Lost Call. Holding Time: (H) average duration of typical call. Load: Traffic intensity across the whole system. Request Rate: (λ) average number of call requests per unit time. Grade of Service (GoS): • • • The grade of service (GoS) is the measure of the ability of a user to access a trunked system during the busiest hour. it is a benchmark used to define the desired performance of a particular trunked system by specifying a desired likelihood of a user obtaining channel access given a specific number of channels available in the system. GoS is typically given as the likelihood that a call is blocked or the likelihood of a call experiencing a delay greater than a certain queuing time. There are two types of Trunked System: ❖ Blocked Calls Cleared ❖ Blocked Calls Delayed Blocked Calls Cleared: It provide no queuing for call request to Every user who requests service, it is assumed there is no setup time and the user is given immediate access to a channel if one is available. If no channel is available, the requesting user is blocked without access and is free to try later. Assumption in Trunked System: Memoryless arrival of requests: all users plus blocked users may request a channel at a time. The probability of a user occupying a channel is exponentially distributed i.e. longer calls are less likely to occur there are finite number of channels available in the trunking pool. This is known as M/M/m queue and it leads to Erlang formula known as Blocked calls clear formula or Erlang B Formula. It determines the probability that a call is blocked and is a measure of the GoS for a trunked system which provides no queuing for blocked calls. Where C=the no of trunked channel A=total offered traffic Blocked Calls Delayed: • • • Here queue is provided to hold calls which are blocked. If channel is not available immediately, the call request may be delayed until a channel becomes available. The measures of GoS is defined as the probability that a call is blocked after waiting a specific length of time in queue. To find GoS, it is necessary to find the likelihood that a call is initially denied access to the system.
• • • • The likelihood of a call not having immediate access to a channel is determined by the Erlang C formula. If no channels are immediately available the call is delayed and the probability that the delayed call is forced to wait more than t second is given by the probability that a call is delayed, multiplied by the condition probability that the delay is greater than t. Trunking efficiency is a measure of the number of users which can be offered a particular GoS with a particular configuration of fixed channels. The 10 channels trunked together support 60% more traffic at a specific GoS than two 5 channel trunks. Improving capacity in cellular Systems Different cellular design techniques are used to provide more channels per unit coverage area. The techniques are: 1. Cell splitting 2. Sectoring 3. Coverage zone approaches/ microcell zone concept The cell splitting increases the number of base stations in order to increase capacity. The sectoring and zone microcells rely on base station antenna placements to improve capacity by reducing co-channel interference. Cell splitting: ✓ Cell splitting is the process of subdividing a congested cell into smaller cells, each with its own base station and a corresponding reduction in antenna height and transmitted power. ✓ It increases the capacity of a cellular system as it increases the number of time that channels are reused. ✓ The smaller cells called microcells are installed in between the existing cells and hence the capacity increases due to additional number of channels per unit area. ✓ Here each microcell is having half the radius of the original cell. ✓ The cells are splitted in such that it will preserve the frequency reuse plan of the system. ✓ For new smaller cell, the transmitted power of these cells must be reduced. ✓ It is necessary to ensure that the frequency reuse plan for the new microcells behaves exactly as for the original cells. Sectoring: ❖ In this approach, the capacity is increased keeping the cell radius unchanged and decrease the D/R ratio. ❖ The capacity improvement is achieved by reducing the number of cells in a cluster and thus increasing the frequency reuse ratio. ❖ To achieve this it is necessary to reduce the relative interference without decreasing the transmit power. ❖ The co-channel interference in a cellular system may be decreased by replacing a single Omni-directional antenna at the base station by several directional antennas, each radiating within a specified sector. ❖ Illustration of cell splitting within a 3k by 3km square centered on base station. ❖ The technique for decreasing co-channel interference and thus increasing system capacity by using directional antennas is called as sectoring.