Transmission Technique for PLMN (c) Manzur Ashraf 1 Transmission techniques for PLMN © Manzur...

Transmission Technique for PLMN (c) Manzur Ashraf

Transmission techniques for PLMN

Introduction

• Mobile networks mainly use two types of transmission technique:

• A) Cellular radio for subscriber access • B) Point-to-point systems (including radio

links) for all communication above the base station level.

• C) A new trend is the use of point-to-multipoint systems (omnidirectional radio) for the connection of base stations.

• In the PLMN, the access network of digital systems handles voice channels that are coded using low bit rates, which enables several traffic channels to be transmitted over a 64 kbit/s connection (four 13 kbit/s connections in GSM). Conversion between 13 and 64 kbit/s is performed by the BSC.

• Mobile Network owners often make use of leased lines to interconnect the various elements of the access and core networks. Alternatively, they can build their own radio links for the communication between base stations and BSCs.

Mobile transmission frequency range

• Low frequencies (below 30 MHz) are able to propagate around the earth using the ionosphere as a reflector. This can be utilised for long-distance communication with ships and aircraft. On the other hand, it is difficult to reuse these frequencies - which happens to be a prerequisite for mobile communications. They are therefore unfit for use in cellular architectures

• Frequencies between 30 and 300 MHz are especially suitable for nationwide radio broadcasting. They cannot be reflected by the ionosphere, are only slightly affected by attenuation and are relatively insensitive to large obstacles, such as buildings and terrain formations.

• Frequencies in the band between 300 and 2000 MHz are more suited to mobile telephony

Time dispersion

• A problem caused by reflections is time dispersion.• The previous Figure illustrates the transmission and reception of a

bit sequence (here, a one followed by two zeros). The mobile will receive two signals; one of them is a reflection that occurred a few kilometres from the mobile. The bit rate of GSM frequency channels is 270 kbit/s, which is equivalent to 3.7 microseconds per bit - a time during which the signal travels 1.1 kilometres.

• If the difference in distance between the two signals is close on 2 kilometres, the mobile will detect a 0 from the direct signal (the third bit) and a 1 from the reflected signal (the first bit). This phenomenon is referred to as inter-symbol interferenceinter-symbol interference. If the reflected signal is of sufficiently high power, such interference will cause the mobile difficulties in determining whether it received a 1 or a 0.

Time alignment

• The figure illustrates two mobiles which are located at approximately the same distance from a base station and which have been assigned time slots 3 and 4. This means that the interval between the instants at which they send is approximately 600 microseconds (the length of one time slot). As mobile M2 moves away from the base station, the time slot it uses (time slot 4) will be received by the base station later and later. There is then a risk that time slots 4 and 5 will start overlapping one another Thus, the mobiles' sending instants must be adjusted at regular intervals, which is controlled via signals from the base station.

Time alignment in GSM & D-AMPS network

• To avoid a need for frequent adjustment, GSMGSM has been designed to include extra space equivalent to just over an eight-bit sequence (30 microseconds) in its time slots. This space is used by the base station to balance time delays between different mobiles. Repeated adjustment is needed only when the signal delay from a given mobile is close to 30 microseconds (approximately 8 30 microseconds (approximately 8 kilometers difference in distance)kilometers difference in distance) in relation to the last adjustment.

• The base stations of a D-AMPSD-AMPS network can continuously order adjustment of the sending instants up to a distance of 92 kilometers.

Transmission technique for Radio-path

Digital and analog mobile networks both require the following:

• suitable antennas; • a modulation method; • frequency and channel multiplexing; and • some sort of error handling (both digital and analog

mobile networks employ error-correction techniques for signaling and control information; digital systems also include functions for error correction on traffic channels).

Additional requirements applicable to digital mobile networks:

• the need for voice coding• encryption across the air interface.

Transmission technique for the radio path in GSM.

• Voice is analyzed in blocks 20 milliseconds long; in other words, 50 times per second.

• Including protection bits, voice blocks are represented by a total of 456 bits arranged into eight "payload bit sequences" of 57 bits each.

• Mobiles send in bursts every fifth millisecond. Between the bursts sent by one mobile, seven other mobiles (at peak load) send over the same frequency employing time-division multiplexing (TDM).

• Each burst contains 25% of the number of bits representing a block, that is, 114 "payload bits". The length of a burst corresponds to a time slot.

Time division multiplexing of channels in GSM

• With TDM, eight time slots in a frame are carried by a single frequency channel. At any particular instant, several mobiles use the same time slot but on different frequency channels. On one of the frequency channels, two time slots in each cell are reserved for signaling. The technique for allocating a time slot to a call is called time division multiple access (TDMA)

Error handling on GSM traffic channels

• The process includes two phases. In the first phase - called channel coding - redundant bits are added to the information. In the second phase, the bits are distributed over a number of bursts in accordance with a predetermined pattern (interleaving).

• The majority of the bits delivered by the voice coder to the channel coder are first block-coded, which means that parity bits are added for error detection. Additional bits are then added (convolution coding) for error correction. The original 260-bit sample has now almost doubled in size (456 bits).

• The use of channel coding makes it possible to detect and correct single-bit errors, but one cannot guard against disturbances involving bursts of erroneous bits - a situation that frequently occurs. Interleaving is a technique that may be able to solve this problem. The interleaving process is performed in two stages and does not add any bits. The first stage involves breaking up the 456 bits into groups of 57 bits. A burst can only carry two such groups , so that if portions of a burst are lost, only stray-bit losses will occur and these losses will be evenly distributed over the voice block. The lost bits can ordinarily be recovered through channel coding.

• If a complete burst is lost, 25% of the total number of bits will be missing, and such situations cannot be corrected through channel coding. To guard against this happening, the second stage produces bursts that are a mixture of 57-bit groups belonging to consecutive voice blocks. The maximum loss will then be reduced to 12.5% of the total number of bits, which can be corrected by channel coding. However, this technique increases the delay.

Burst management

• Every burst of this sort is capable of carrying 2•57 = 114 encrypted payload bits. 26 known bits26 known bits, located between the two blocks of payload bits, are used to cope with any time dispersion and the resultant inter-symbol interference that can have arisen .By comparing the known bits with the received signal sequence, it is possible to draw conclusions about the time dispersion. In GSMIn GSM, this applies to differences of up to five kilometers between the path traveled by the direct signal and that traveled by the reflected one. The known bit pattern, along with a method referred to as equalising, is used to calculate what was actually transmitted. The tail bits (T-bits) mark the beginning and end of a burst.

• The burst format also takes into account the need for time alignment .A buffer space 8.25 bits in length (approximately 30 microseconds) is allocated at the end of the bursts, corresponding to about eight kilometers' difference in distance. Thanks to this buffer, there is no longer any need for mobiles to constantly adjust their relative sending instants when approaching or moving away from base stations.

Channel multiplexing: Physical channels in TDMA

• A GSM mobile sends in bursts every fifth millisecond. Every burst contains 25% of the number of bits representing a voice block. The length of a burst corresponds to one time slot which can be regarded as a physical channel in the air interface.

• A physical channelA physical channel - for example, a time slot on a particular frequency channel in a TDMA system - may be said to be a carrier of a logical channel, such as a traffic channel (sometimes abbreviated TCH), or several logical channels, such as various control channels in a multiframe structure

Mapping process

• In digital systems based on TDMA, such as GSM and D-AMPS, the mapping process means that a time slot and a frequency are allocated to the traffic burst. If frequency hopping is applied, the frequency of a traffic channel also changes - in GSM slightly more than 200 times a second

• The frequency channel separation in GSM is 200 kHz; in D-AMPS 30 kHz. In GSMGSM, each cell is provided with a frequency channel (C0) for control channels. Normally, time slots 0 and 1 (TS0 and TS1) on C0 are used for this purpose. The remaining six time slots on C0 and all eight time slots on other frequencies are used for traffic channels.

• In D-AMPSD-AMPS, each frequency channel is normally used for three traffic channels.

Uplink & Downlink

• The physical channel in GSM - that is, the time slot - has a length of 0.577 milliseconds. This length is sufficient for one traffic channel burst plus a guard period. For the frequency channels between the mobile and base station (uplink), the frequency range 890-915 MHz is used, while the downlink utilises the range 935-960 MHz.

• The total number of duplex frequency channels is 124, resulting in 992 physical channels (compared to 823 in AMPS and D-AMPS).

The principle of time division multiple access

Physical channels in FDMA-based systems (such as NMT and AMPS) • The frequency range assigned to the cell consists of one

uplink (mobile to base station) and one downlink (base station to mobile). The frequency separation between these links must be sufficiently large - normally 45 MHz (duplex separation) - so that no interference between them will arise at the mobile.

• Each link is divided into an equal number of unidirectional channels. One channel should be sufficiently wide (25-30 kHz) to be able to transmit telephone-quality voice (approximately 3 kHz). To be able to make use of duplex telephony, the mobile must have access to one uplink and one downlink channel - two channels combined to form a traffic channel pair.

• However, a cell consists not only of traffic channels; each cell contains a number of channels that are predefined as control channels. Such channels can use the downlink for the distribution of cell and network information and to transfer call requests arriving from the network and use the uplink frequency for call signaling generated by mobiles. One or more other channels may have been defined as control channels for two-way signaling between the mobile and the network.

• The total number of FDMA system channels is standardized and is specified by the government agency that allocates frequencies. The uplink frequency range of the AMPS system is 824-849 MHz and its downlink range is 869-894 MHz. Every link contains a total of 823 unidirectional channels

Frequency division multiple access principle

Transmission Power

• Base station transmission power is 1-50 W, depending on the type of system and the desired cell size: a macrocell, a microcell or a picocell. Mobile output power is 0.6-20 W, depending on the type of mobile (hand-held or car-mounted). Mobiles must be capable of adjusting their output power in steps if ordered to do so by the base station (power regulation).

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Transmission Technique for PLMN (c) Manzur Ashraf 1 Transmission techniques for PLMN © Manzur...

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