Base Station Subsystem

CHAPTER 3 BASE STATION SUB SYSTEM The base station subsystem (BSS) is the section of a traditional cellular telephone network which is responsible for handling traffic and signaling between a mobile phone and the network switching subsystem (mobile switching centre). The BSS carries out transcoding of speech channels, allocation of radio channels to mobile phones, paging, quality management of transmission and reception over the air interface and many other tasks related to the radio network. It comprises of two main components Base station transceiver (BTS) Base station controller (BSC)

3.1 Base transceiver station Base Transceiver Station (BTS) The BTS is the Mobile Station's access point to the network. The base transceiver station, or BTS, contains the equipment for transmitting and receiving of radio signals (transceivers), antennas, and equipment for encrypting and decrypting communications with the base station controller (BSC). It is responsible for carrying out radio communications between the network and the MS. It handles speech encoding, encryption, multiplexing (TDMA), and modulation/demodulation of the radio signals. It is also capable of frequency hopping.

Typically a BTS will have several transceivers (TRXs) which allow it to serve several different frequencies and different sectors of the cell. A BTS is controlled by a parent BSC via the base station control function (BCF). The BCF is implemented as a discrete unit or even incorporated in a TRX in compact base stations. The BCF provides an operations and maintenance (O&M) connection to the network management system (NMS), and manages operational states of each TRX, as well as software handling and alarm collection. . Frequency hopping is often used to increase overall BTS performance; this involves the rapid switching of voice traffic between TRXs in a sector. A hopping sequence is followed by the TRXs and handsets using the sector. Several hopping sequences are available, and the sequence in use for a particular cell is continually broadcast by that cell so that it is known to the handsets.

3.1.1 Sector antenna

By using directional antennae on a base station, each pointing in different directions, it is possible to sectorise the base station so that several different cells are served from the same location. The standard antenna for a three-sector site has a horizontal beam width of 65°.This means that the gain at 32.5 ° is 3 dB less than the maximum gain. At 60° it is suppressed typically 10 dB. Broader antennas with 90° and 105° horizontal beam widths are alternatives. However, the difference in shape of the coverage area is small but the gain is slightly lower for a certain antenna length.

THREE SECTORS OF BTS

Typically these directional antennas have a beam width of 65 to 85 degrees. This increases the traffic capacity of the base station (each frequency can carry eight voice channels) whilst not greatly increasing the interference caused to neighboring cells (in any given direction, only a small number of frequencies are being broadcast). Typically two antennas are used per sector, at spacing of ten or more wavelengths apart. This allows the operator to overcome the effects of fading due to physical phenomena such as multipath reception. Some amplification of the received signal as it leaves the antenna is often used to preserve the balance between uplink and downlink signal Antenna combiners are implemented to use the same antenna for several TRXs (carriers), the more TRXs are combined the greater the combiner loss will be 3.1.2Transceiver

RF Transceiver Block Diagram

Radio transceiver both sends and receives radio signals. In order to be classified as a transceiver, the transmitter and the receiver must use the same set of wiring or be located within the same device.A TRX transmits and receives according to the GSM standards, which specify eight TDMA timeslots per radio frequency.

3.2 Base Station Controller (BSC) Base Station Controller (BSC) - The BSC controls multiple BTSs. It handles allocation of radio channels, frequency administration, power and signal measurements from the MS, and handovers from one BTS to another (if both BTSs are controlled by the same BSC). A key function of the BSC is to act as a concentrator where many different low capacity connections to BTSs (with relatively low utilization) become reduced to a smaller number of connections towards the mobile switching center (MSC) (with a high level of utilization). It reduces the number of connections to the Mobile Switching Center (MSC). The databases for all the sites, including information such as carrier frequencies, frequency hopping lists, power reduction levels, receiving levels for cell border calculation, are stored in the BSC. A BSC may be collocated with a BTS or it may be geographically separate. It may even be collocated with the Mobile Switching Center (MSC).

Base Station Controller

3.3 BSS interfaces The interface between the MS and the BTS is known as the Um Interface or the Air Interface.

Absolute Radio Frequency Channel Number (ARFCN) The ARFCN is a number that describes a pair of frequencies, one uplink and one downlink. The uplink and downlink frequencies each have a bandwidth of 200 kHz. The uplink and downlink have a specific offset that varies for each band. The offset is the frequency separation of the uplink from the downlink. Every time the ARFCN increases, the uplink will increase by 200 khz and the downlink also increases by 200 khz.

The interface between the BTS and the BSC is known as the Abis Interface

3.4 Time Division Multiple Access GSM uses Time Division Multiple Access (TDMA) as its access scheme. This is how the MS interfaces with the network. TDMA is the protocol used on the Air (Um) Link. GSM uses Gaussian Minimum-Shift Keying (GMSK) as its modulation method. Time Division means that the frequency is divided up into blocks of time and only certain logical channels are transmitted at certain times. The time divisions in TDMA are known as Time Slots.

3.4.1 Time Slots A frequency is divided up into 8 time slots, numbered 0 to 7.

Each time slot lasts 576.9 µs. A time slot is the basic radio resource used to facilitate communication between the MS and the BTS.

GSM divides up each ARFCN into 8 time slots. These 8 timeslots are further broken up into logical channels. Logical channels can be thought of as just different types of data that is transmitted only on certain frames in a certain timeslot.Different time slots will carry different logical channels, depending on the structure the BSS uses. There are two main categories of logical channels in GSM: 1.Signaling Channels 2.Traffic Channels (TCH)

3.4.2 Signaling Channels These are the main types of signaling Channels: Broadcast Channels (BCH) - Transmitted by the BTS to the MS. This channel carries system parameters needed to identify the network, synchronize time and frequency with the network, and gain access to the network. Common Control Channels (CCH) - Used for signaling between the BTS and the MS

and to request and grant access to the network. Standalone Dedicated Control Channels (SDCCH) - Used for call setup. Associated Control Channels (ACCH) - Used for signaling associated with calls and call-setup. An ACCH is always allocated in conjunction with a TCH or a SDCCH. The above categories can be divided into the following logical channels: Broadcast Channels (BCH) Broadcast Control Channel (BCCH) Frequency Correction Channel (FCCH) Synchronization Channel (SCH) Cell Broadcast Channel (CBCH) Common Control Channels (CCCH) Paging Channel (PCH) Random Access Channel (RACH) Access Grant Channel (AGCH) Standalone Dedicated Control Channel (SDCCH) Associated Control Channel (ACCH) Fast Associated Control Channel (FACCH) Slow Associated Control Channel (SACCH) 3.4.2.1 Broadcast Channels (BCH) Broadcast Control Channel (BCCH) - DOWNLINK - This channel contains system parameters needed to identify the network and gain access. These parameters include the Location Area Code (LAC), the Mobile Network Code (MNC), the frequencies of neighboring cells, and access parameters. Frequency Correction Channel (FCCH) - DOWNLINK - This channel is used by the MS as a frequency reference. This channel contains frequency correction bursts. Synchronization Channel (SCH) - DOWNLINK - This channel is used by the MS to learn the Base Station Information Code (BSIC) as well as the TDMA frame number (FN). This lets the MS know what TDMA frame they are on within the hyperframe. Cell Broadcast Channel (CBCH) - DOWNLINK - This channel is not truly its own type of logical channel. The CBCH is for point-to-omni point messages. It is used to broadcast specific information to network subscribers; such as weather, traffic, sports, stocks, etc. Messages can be of any nature depending on what service is provided. Messages are normally public service type messages or announcements. The CBCH is not allocated a slot for itself, it is assigned to an SDCCH. It only occurs on the downlink. The CBCH usually occupies the second sub slot of the SDCCH. The mobile will not acknowledge

any of the messages.

3.4.2.2 Common Control Channels (CCCH) Paging Channel (PCH) - DOWNLINK - This channel is used to inform the MS that it has incoming traffic. The traffic could be a voice call, SMS, or some other form of traffic. Random Access Channel (RACH) - UPLINK This channel is used by a MS to request an initial dedicated channel from the BTS. This would be the first transmission made by a MS to access the network and request radio resources. The MS sends an Access Burst on this channel in order to request access. Access Grant Channel (AGCH) - DOWNLINK - This channel is used by a BTS to notify the MS of the assignement of an initial SDCCH for initial signaling.

3.4.2.3 Standalone Dedicated Control Channel (SDCCH) UPLINK/DOWNLINK - This channel is used for signaling and call setup between the MS and the BTS.

3.4.2.4 Associated Control Channels (ACCH) Fast Associated Control Channel (FACCH) - UPLINK/DOWNLINK - This channel is used for control requirements such as handoffs. There is no Timeslot and frame allocation dedicated to a FAACH. The FAACH is a burst-stealing channel, it steals a Timeslot from a Traffic Channel (TCH). Slow Associated Control Channel (SACCH) - UPLINK/DOWNLINK - This channel is a continuous stream channel that is used for control and supervisory signals associated with the traffic channels.

3.5 Signaling Channel Mapping Normally the first two timeslots are allocated to signaling channels. Control Channel is composed of 51 TDMA frames. On a time slot within the multiframe, the 51 TDMA frames are divided up and allocated to the various logical channels. There are several channel combinations allowed in GSM. Some of the more common ones are: FCCH + SCH + BCCH + CCCH BCCH + CCCH

FCCH + SCH + BCCH + CCCH + SDCCH/4(0..3) + SACCH/C4(0..3) SDCCH/8(0 .7) + SACCH/C8(0 . 7)

FCCH + SCH + BCCH + CCCH

Downlink

Uplink

BCCH + CCCH

Downlink

Uplink FCCH + SCH + BCCH + CCCH + SDCCH/4(0.3) + SACCH/C4(0.3) The SACCH that is associated with each SDCCH is only transmitted every other multiframe. Each SACCH only gets half of the transmit time as the SDCCH that it is associated with. So, in one multiframe, SACCH0 and SACCH1 would be transmitted, and in the next multiframe, SACCH2 and SACCH3 would be transmitted. The two sequential multiframes would look like this:

Downlink

Uplink The downlink and uplink multiframes do not align with each other. This is done so that if the BTS sends an information request to the MS, it does not have to wait an entire multiframe to receive the needed information. The uplink is transmitted 15 TDMA frames behind the downlink. For example, the BTS might send an authentication request to the MS on SDCCH0 (downlink) which corresponds to TDMA frames 22-25. The MS then has enough time to process the request and reply on SDCCH0 (uplink) which immediately follows it on TDMA frames 37-40.

SDCCH/8(0 .7) + SACCH/C8(0 . 7) Once again, the SACCH that is associated with an SDCCH is only transmitted every other multiframe. Two consecutive multiframes would look like this:

Downlink

Uplink

3.6 Traffic Channels (TCH) Traffic Channels are used to carry two types of information to and from the user: 1. Encoded Speech 2. Data There are two basic types of Encoded Speech channels:

Encoded Speech - Encoded speech is voice audio that is converted into digital form and compressed. Full Rate Speech TCH (TCH/FS) - 13 kb/s Half Rate Speech TCH (TCH/HS) - 5.6 kb/s

Data - Data refers to user data such as text messages, picture messages, internet browsing, etc. It includes pretty much everything except speech. Full rate Data TCH (TCH/F14.1) - 14.4 kb/s Full rate Data TCH (TCH/F9.6) - 9.6 kb/s Full rate Data TCH (TCH/F4.8) - 4.8 kb/s Half rate Data TCH (TCH/F4.8) - 4.8 kb/s Full rate Data TCH (TCH/F2.4) - ≤2.4 kb/s Half rate Data TCH (TCH/H2.4) - ≤2.4 kb/s

3.6.1Traffic Channel Mapping Time slots 2 through 7 are normally used for Traffic Channels (TCH) Traffic Channel Multiframes are composed of only 26 TDMA frames. On each multiframe, there are 24 frames for Traffic Channels, 1 frame for a SACCH, and the last frame is Idle. A MS (or other device) only gets one time slot per TDMA frame to transmit, so in the following diagrams we are looking at a single time slot.

Full Rate Traffic Channel (TCH/FS)

When using Half-Rate Speech Encoding (TCH/HS), the speech encoding bit rate is 5.6

kb/s, so one time slot can handle two half-rate channels. In this case, one channel will transmit every other TDMA frame, and the other channel would be transmitted on the other frames. The final frame (25), which is normally used as an Idle frame, is now used as a SACCH for the second half-rate channel.

Half Rate Traffic Channel (TCH/HS)

3.6.2 ARFCN Mapping This diagram shows a sample Multiframe with logical channels mapped to time slots and TDMA frames. This is just one possible configuration for an ARFCN. *For illustrative purposes, half of the traffic channels are full-rate and the other half are half-rate

TS0

TS1

TS2

TS3

TS4

TS5

TS6

TS7 *Remember that CCH Multiframes have 51 frames and TCH Multiframes only have 26. Their sequences will synchronize every superframe.

Offset Even though GSM uses a full duplex radio channel, the MS and the BTS do not transmit at the exact same time. If a MS is assigned a given time slot, both the MS and the BTS will transmit during that given time slot, but their timing is offset. The uplink is exactly 3 time slots behind the downlink. For example, if the MS was allocated a TCH on TS3, the BTS would transmit when the downlink is on TS3 and the MS is set to receive on TS3. At this point, the uplink is only on TS0. Once the uplink reaches TS3, the MS would begin to transmit, and the BTS is set to receive on TS3. At this point, the downlink would be at TS6. When the MS is not transmitting or receiving, it switches frequencies to monitor the BCCH of adjacent cells.

3.6.2 Speech Data Throughput When looking at a Time slot allocated to a TCH, you will notice that TCH does not occur on every single frame within a time slot. There is one reserved for a SACCH and one that is Idle. So, in a TCH Multiframe, only 24 of the 26 frames are used for traffic (voice/data). This leaves us with a data throughput of 22.8 kb/s. Here is the math: 1. Calculate bits per TCH Multiframe: We know that there are 114 bits of data on a single burst, and we know that only 24 of the 26 frames in a TCH multiframe are used to send user data. 114 bits × 24 frames = 2736 bits per TCH multiframe So, we know that on a single timeslot over the duration of one TCH multiframe, the data throughput is 2736 bits. 2. Calculate bits per millisecond (ms): From step one above, we know that the throughput of a single TCH multiframe is 2736 bits. We also know that the duration of a TCH multiframe is 120ms. 2736 bits / 120 ms = 22.8 bits per millisecond 3. Convert milliseconds (ms) to seconds: Now we need to put the value into terms of seconds. There are 1000 milliseconds in a second, so we simply multiply the value by 1000.

22.8 bits/millisecond × 1000 = 22,800 bits per second (22.8 kb/s) 4. Convert bits to kilobits: Finally, we want to put it into terms of kilobits per second, wich is the most common term for referring to data throughput. We know a kilobit is 1000 bits, so we simply divide the term by 1000. 22,800 bits/s ÷ 1000 = 22.8 kb/s So now we see why the data throughput of a single allocated timeslot is 22.8 kb/s.

A single BTS may have several Transceivers (TRX) assigned to it, each having its own ARFCN, each ARFCN having 8 time slots. The logical channels that support signaling will normally only be on one ARFCN. All of the other ARFCNs assigned to a BTS will allocate all 8 time slots to Traffic Channels, to support multiple users. The following diagram is an example of how a medium-sized cell might be set up with 4 TRX (ARFCNs).

Sample Medium-Size Cell

3.6.3 Frequency Hopping Each radio frequency Channel (ARFCN) is influenced differently by propagation conditions. What affects channel 23 may not affect channel 78 at all. Within a given cell, some frequencies will have good propagation in a certain area and some will have poor

propagation in that area. In order to take advantage of the good propagation and to defeat the poor propagation, GSM utilizes frequency hopping. Frequency hopping means that a transceiver hops from one frequency to another in a predetermined sequence. If a transceiver hops through all of the available frequencies in a cell then it will average out the propagation. GSM uses Slow Frequency Hopping (SFH). It is considered slow because the system hops relatively slow, compared with other frequency hopping systems. In GSM, the operating frequency is changed every TDMA frame. The main reason for using slow frequency hopping is because the MS must also change its frequency often in order to monitor adjacent cells. The device in a transceiver that generates the frequency is called a frequency synthesizer. On a MS, a synthesizer must be able to change its frequency within the time frame of one time slot, which is equal to 577 µs. GSM does not require the BTS to utilize frequency hopping. However, a MS must be capable of utilizing frequency hopping when told to do so. The frequency hopping and timing sequence is known as the hopping algorithm. There are two types of hopping algorithms available to a MS. · ·

Cyclic Hopping - The transceiver hops through a predefined list of frequencies in sequential order. Random Hopping - The transceiver hops through the list of frequencies in a random manner. The sequence appears random but it is actually a set order.

There are a total of 63 different hopping algorithms available in GSM. When the MS is told to switch to frequency hopping mode, the BTS will assign it a list of channels and the Hopping Sequence Number (HSN), which corresponds to the particular hopping algorithm that will be used. The base channel on the BTS does not frequency hop. This channel, located in time slot 0, holds the Broadcast Control Channels which the MS needs to monitor to determine strength measurements, determine access parameters, and synchronize with the system. If a BTS uses multiple transceivers (TRX) then only one TRX will hold the the Broadcast Channels on time slot 0. All of the other TRXs may use time slot 0 for traffic or signaling and may take part in the frequency hopping. There are two types of frequency hopping method available for the BTS: synthesizer hopping and baseband hopping. ·

·

Synthesizer Hopping - This requires the TRX itself to change frequencies according to the hopping sequence. So, one TRX would hop between multiple frequencies on the same sequence that the MS is required to. Baseband Hopping - In this method there are several TRX and each one stays on a fixed frequency within the hopping frequency plan. Each TRX would be assigned a single time slot within a TDMA frame. For example, time slot 1 might be

assigned to TRX 2 in one TDMA frame and in the next TDMA frame it would be assigned to TRX 3, and the next frame would be TRX 3. So, the data on each time slot would be sent on a different frequency each frame, but the TRXs on the BTS do not need to change frequency. The BTS simply routes the data to the appropriate TRX, and the MS knows which TRX to be on for any given TDMA frame.

Baseband Frequency Hopping

3.7 TRANSMISSION A BTS is usually geographically separated form a Mobile Switching Center (MSC).Whatever is send by a mobile station (phone) is to be transmitted to the Mobile Switching Center for routing to other networks. For example a call made by a subscriber a in Bulawayo to an Econet subscriber in Harare, data is to be transmitted from a remote BTS to BSC and MSC at Telecel headquarters(Harare) then to econet. For transmission optical fibre or microwave transmission can be used.

3.7.1Microwave transmission

3.8 Activities carried out v Preparation of daily reports Sample of daily report

v Site survey at Peterhouse (Marondera) The author paid a visit to Peterhouse to identify a new site and wrote a report . The following consideration were made (i) (ii) (iii) (iv) (v)

Environmental checking-power, accessibility and settlement pattern Checking terrain of proposed microwave path Use of binoculars to verify line of sight Use of GPS –recording of grid reference Soil type (when constructing a tower this is necessary)

v BSS network optimization (i) (ii) (iii)

Analyzing traffic statistics Drive testing and maintaince of drive test log Carrying out follow up actions 1. Antenna down-tilting to control coverage 2. Transmit power control 3. Frequency coordination

v

Project implementation

(i) (ii) (iii) (iv)

Supervision of installations of BTS, BSC by contractors Decommissioning of existing BTSs, for relocation to new sites Commissioning of the newly installed sites Transportation of recovered BTSs to new sites

v Beatrice network proposal report

Um The air interface between the mobile station (MS) and the BTS. This interface uses LAPDm protocol for signaling, to conduct call control, measurement reporting, handover, power control, authentication, authorization, location update and so on. Traffic and signaling are sent in bursts of 0.577 ms at intervals of 4.615 ms, to form data blocks each 20 ms.

Abis The interface between the BTS and BSC. Generally carried by a E1 TDM circuit. Uses TDM subchannels for traffic (TCH), LAPD protocol for BTS supervision and telecom signaling, and carries synchronization from the BSC to the BTS and MS. A The interface between the BSC and MSC. It is used for carrying traffic channels and the BSSAP user part of the SS7 stack. Although there are usually transcoding units between BSC and MSC, the signaling communication takes place between these two ending points and the transcoder unit doesn't touch the SS7 information, only the voice or CS data are transcoded or rate adapted.

Ater The interface between the BSC and transcoder. It is a proprietary interface whose name depends on the vendor (for example Ater by Nokia), it carries the A interface information from the BSC leaving it untouched. Gb