Second-generation, digital,wireless systems

• North-American TDMA-based IS-136 or D-AMPS

• GSM, the TDMA-based pan-European system, which is also being deployed in the USA and elsewhere in the world; and the CDMA-based IS-95.

• A given base station (BS) will typically control multiple mobile stations (MS)

• Multiple base stations are, in turn, controlled by a mobile switching center (MSC), responsible for handling inter-cellular handoff, as well as mobile location, paging, and other mobile management and control functions.

• The home location register (HLR) contains reference and profile information for all mobile subscribers registered with this MSC as their “home location.”

• Visitors to a “foreign” location register with the MSC in that area. The visitors’ reference and profile information are then stored in the visitors’ location register VLR associated with that MSC, after communicating with the mobile’s HLR.

• Registration and authentication of a mobile turning itself on, preparatory to either sending or receiving calls, is done by sending appropriate control messages across the air interface between the mobile and BS. These messages are then forwarded to the MSC for authentication.

• If the mobile is in its home location, the MSC queries its HLR to verify and approve registration of the mobile. If the mobile is in a foreign location, the local MSC/VLR combination will forward the requested registration of the mobile to its HLR.

GSM

• GSM (Global System for Mobile Telecommunications) operates in the 890– 915 MHz band uplink (MS to BS) and the 935–960 MHz band downlink (BS to MS).

• The 25 MHz of bandwidth in each direction is divided into 200 kHz frequency channels, with guard bands of 200 kHz left unused at the lower end of each band.

• There are thus 124, 200 kHz channels available in each direction of transmission. Each frequency channeluses 8-slot repetitive frames

• A 26-bit training sequence in the middle of the slot is used to provide an estimate of the radio channel, to be used in training an adaptive equalizer at the receiver to help overcome the multipath fading that may be encountered.

• The two one-bit flag (F) bits indicate whether the data fields carry user or control traffic.

• On powering on in any cell, the mobile must first lock onto or acquire the frequencies used in that cell, and then synchronize to the framing-time slot structure

• it first searches for a specific control channel, the frequency correction channel, FCCH, broadcast by the local BS, which enables it to adjust or synchronize its frequency characteristic to that of the base station

• The FCCH message is always followed by the synchronization channel, SCH, message, which identifies the BS and provides frame, hence time, synchronization to the mobile.

• Once frequency and timing information is acquired, the mobile listens to a channel called the broadcast control channel, BCCH, which provides information needed to set up the call: the cell configuration, the network to which it belongs, access information, and control channel information, among other items.

• The mobile terminal is ready to initiate a call. It does this by sending a random access request message over the random access channel, RACH, which carries a 5-bit random number plus a 3-bit purpose indicator.

• If the access attempt is successful, the BS acknowledges receipt of the RACH message with an access grant channel, AGCH, message.

• This message repeats the 8-bit request message, and directs the terminal to a specified stand-alone channel, SDCCH, over which the mobile transmits the signaling information required for authentication, as well as to make the desired call connections.

• If the call setup is approved, the BS replies to the MS with SDCCH messages directing the mobile to the frequency/time slot traffic channel (TCH) to use for actually beginning the call and sending the desired user information.

• The control channels are grouped into three categories: broadcast channels, common control channels, and dedicated control channels.

• The paging channel, as the name indicates, is used by a base station to locate a mobile for an incoming call.

• The random access channel, is directed uplink, from MS to BS.

• The dedicated control channels are bi-directional, allowing mobile signaling, management, and supervisory information to be transmitted in either direction.

• and broadcast channel measurement results from neighboring cells to be used for mobileassisted

• handoffs. The fast associated control channel, FACCH, is used to send handoff

• requests and other urgent signaling messages. It is sent on a normal traffic channel, TCH,

• or an SDCCH, interrupting that channel for this purpose.

• In particular, the slow associated control channel, SACCH, is used by the BS, in the downlink direction, to send transmitter power level and timing advance instructions to the MS.

• The mobile uses this channel, in the reverse, uplink direction, to send the base station indications of received signal strength.

• The fast associated control channel, FACCH, is used to send handoff requests and other urgent signaling messages

• The stand-alone dedicated control channel, SDCCH, used in both directions on a temporary basis before assigning dedicated TCHs to a mobile-base station duplex (two-way) connection.

• GSM uses three different ways of allocating time slots to the other control channels: It sets aside a prescribed number of frequency channels of the 124 available in each direction and “robs” one time slot per frame

• It uses a regular traffic channel time slot, when needed, by setting the TCH flag bit.

• The allocation of one time slot in 13, in a sequence of traffic channels, to a control channel is carried out by defining a repetitive 26-frame multiframe structure 120 msec long.

• Traffic channels occupy frames 0–11 and 13–24. Frame 12, labeled S is assigned as the dedicated, slow associated control channel, SACCH.

• Frame 25 may also be so assigned, if desired. If not, it is left as an idle frame, labeled I.

• Since each such slot carries 114 information bits, the SACCH bit rate, with one slot per

• 26 used, is 114 bits/120 msec, or 950 bps.

In a sequence of traffic channels, to a controlchannel is carried out by defining a repetitive 26-frame multiframe structure 120 msec long.

• Traffic channels occupy frames 0–11 and 13–24. Frame 12, labeled S is assigned as the dedicated, slow associated control channel, SACCH.

• Frame 25 may also be so assigned, if desired. If not, it is left as an idle frame, labeled I.

• This 51-frame multi frame structure is diagrammed.

• The 51 frames, repeating every 235 msec, are organized into five 10-frame groups, the first channel in each group, labelled F corresponding to the frequency correction channel, FCCH.

• Each FCCH is immediately followed by the synchronization channel, SCH, used by a mobile to establish frame, hence time, synchronization. This channel is labelled S.

• The broadcast control channel, BCCH, labelled B in Fig. 8.5, used to provide information needed to set up a call.

• The remaining channels indicated by the letter C, correspond to either access channels (ACH) or paging channels (PCH).

• Fig. 8.6(a). Figure 8.6(b) show the 78-bit data field is actually generated from a 25-bit information message.

• This message is protected against errors, first, by a 10-bit cyclic redundancy code (CRC) which adds ten parity bits, and then by a rate-1/2 convolutional code.

• Figure 8.7 portrays the generation of the 456 bits.

• These 456 bits are themselves generated from a 184-bit information message that is protected by an error-correcting code with 40 parity bits followed by rate-1/2 convolutional coding

Random access channel, RACH: GSM

• A paging channel, as the name indicates, is used to locate a mobile in a particular cell for an incoming call.

• The traffic channel slots were shown there to contain 148 bits, followed by a 30.5 sec guard time.

• Figure shows how the 78-bit data field is actually generated from a 25-bit information message.

• This message is protected against errors, first, by a 10-bit cyclic redundancy code (CRC) which adds ten parity bits, and then by a rate-1/2 convolutional code

• CRC provides the outer coding • The convolutional coder carries out the inner

coding.

IS-136 (D-AMPS)

• As deployed in North America, occupies the 25 MHz bands from 824–849 MHz uplink, and 869–894 MHz downlink. Within these bands frequency channels are spaced 30 kHz apart, a frequency channel containing repetitive TDMA frames, carrying six time slots each.

• Two slots per frame are allocated to each full-rate traffic user.

• The system transmission rate is 48.6 kbps: 1944 bits/frame (324 bits per slot) are transmitted in 40 msec.

• Occupies the 25 MHz bands from 824–849 MHz uplink, and 869–894 MHz downlink. Within these bands frequency channels are spaced 30 kHz apart, a frequency channel containing repetitive TDMA frames, carrying six time slots each.

• The system transmission rate is 48.6 kbps: 1944 bits/frame (324 bits per slot) are transmitted in 40 msec.

• The 6-bit guard time G in the uplink direction is needed because mobiles in a given cell may be moving with respect to the base station. It prevents terminals initiating communication at the same time from interfering with one another.

• The power ramp-up time R is needed to accommodate terminals that may not be on.

• The 12-bit CDVCC field, or coded digital verification color code, consists of an 8-bit DVCC number plus four parity-check bits to protect it.---- This field is used as a continuing handshake: the BS transmits the number; the MS replies with the same number. If no reply or an incorrect reply is received, the slot is relinquished.

• Finally, the 12-bit SACCH fields in each direction carry the slow associated control channel.

• The downlink broadcast control channels, BCCH, consist of the fast broadcast control channel, F-BCCH, used to carry time-critical information.

• Extended BCCH, E-BCCH, carrying less time-critical information

• SMS BCCH --S-BCCH, used to control a broadcast short message service defined for IS-136 systems.

• The SMS point-to-point, paging, and access response channel, SPACH, is a logical channel designed, to carry paging and access response control information, as well as point-to-point messages concerning the SMS service.

• Shared control feedback, SCF, channel is used to carry downlink information, from BS to MS.

•

Repetitive super frame structure, IS-136

• The broadcast channels are transmitted sequentially using a repetitive super frame structure.

• Each super frame corresponds to 16 consecutive TDMA frames, for a total time interval of 640 msec

• The slot format of control channels appears in Fig. 8.11.

• Each slot carries 28-bit synchronization field and a total of 260 bits of data.

• The SCF channel is used to respond to the mobile’s random access attempt.

• CSFP field is used to indicate the location of a TDMA block within a super frame.

• Random access messages, used for call setup or origination, as well as mobile registration and authentication, are carried over the one uplink channel, the random access channel, RACH.

• 12-bit CSFP field (for coded super frame phase) is used to indicate the location of a TDMA block within a super frame.

• This field carries an 8-bit SFP (superframe phase) number and four parity-check bits to protect this number against errors.

• This is the layer-3 data field carrying one of the various F-BCCH messages.

• The 8-bit parameter L3LI is the length indicator

• Figure 8.14(a) represents the frame format used when an F-BCCH message can be transmitted completely within one frame. If a message requires more than one frame for completion, the Begin and Continue formats of Figs. 8.14(b) and (c) are used.

• The one-bit EI flag set to 1 indicates that filler (all 0s) has been used to pad out the frame; EI = 0 says that a new message follows.

• EC is used to designate a change in the E-BCCH.

• The 7-bit CLI, or Continuation Length Indicator, in the Continue frame of Fig. 8.14(c), is used to indicate the number of bits belonging to a Continue message.

• The IS-136 access procedure is diagrammed in Fig. 8.15• The mobile station listens to a downlink SCF channel to

determine a specific future time slot to use to send its RACH message

• It then sends the RACH message• A later SCF message, carried in a specified time slot, will

indicate whether the RACH message has been correctly received and access granted.

• If access is granted, an ARCH message carried on the SPACH channel will follow, indicating the specific digital traffic channel the mobile is to use for communication. If the access is not successful (other mobiles might be attempting access at the same time) the access attempt will be retried a random time later.

IS-95

• IS-95 is a CDMA-based system. Its traffic and control channels are defined as specified codes rather than time slots as in the case of GSM and IS-136.

• A binary information stream is “multiplied” by a pseudo-noise (PN) chip spreading sequence, the resulting output shaped by an appropriate low-pass shaping filter, and then fed to a high-frequency transmitter.

• For full-rate traffic transmission of 8.6 kbps• IS-95 defines consecutive 172-bit traffic

frames, 20 msec long. This obviously equates to a traffic input rate of 8.6 kbps. Twelve forward error-correction bits per frame are then added.

• The convolutional encoder is of the rate-1/2 type.

• The 28.8 kbps convolutional encoder output is then fed into a block inter leaver to reduce the effect of burst errors

• This block inter leaver operates consecutively on each frame of 576 bits.

• The block inter leaver may be visualized as being a 32-row by 18-column array

• The inter leaver is filled each frame, one column at a time. It is then read out bit by bit, each row at a time: bits 1, 33, 65,97, . . . , 545, 2, 34, 66, . . . , 546, . . . in succession. This procedure reduces the possible adverse effect of a burst of errors.

• The block inter leaver output at the 28.8 kbps rate is now fed into a 64-ary Walsh encoder.

• The Walsh encoder acts as an orthogonal modulator.

• The 64-aryWalsh encoder is generated as shown by the matrix representation following.

• We start by defining the Walsh matrix W2 as being given by the two-by-two matrix

• L × L Walsh matrix WL be defined in terms of the L/2 × L/2 Walsh matrix

• Walsh code W4 is given,

• Each row has half (L/2) 0s and half 1s. If each 0 is converted to the equivalent −1, the 1s remaining unchanged, it is clear from (8.4) that multiplying elements of the same column in two different rows together and summing over all columns, one gets 0 as the resultant sum.

• The required CDMA spreading of the traffic signal is now carried out by mod-2 addition

• Consider the system block diagram for forward, BS to mobile, traffic channels shown in Fig. 8.19.

• The convolutional encoder is of the rate 1/2 type, rather than the rate 1/3 type in the reverse direction.

• The control channels used in IS-95• Forward, downlink, direction has pilot, sync,

and paging control channels defined• Traffic channels in this direction carry the

power control sub-channel

Mobile management: handoff, location, and paging procedures

• The control required to handle the movement of mobiles is referred to as mobile management.

• The movement of mobiles involves essentially three functions: handoff control, location managment, and paging. Handoff control is required as a mobile, involved in an on-going call, moves from one cell to an adjacent one, or from the jurisdiction of one system to another.

• Location management is required to handle the registration of a mobile in areas or regions outside its home area, to enable it to be located and paged in the event of an incoming call.

Mobile-assisted handoff, IS-136

• As it reaches the boundary (generally ill-defined) of that cell, the power received from the cell base station with which the mobile has been in communication across the air interface between them will drop below a pre-defined threshold. In contrast, assume the power received from a neighbouring base station exceeds a threshold.

• A decision to handoff to the neighbouring base station and enter the new cell associated with that base station would then be made by the MSC controlling both base stations

• The MSC notifies the base station that channel quality measurements are to be carried out.

• The base station responds by transmitting to the mobile a Measurement Order.

• Channel quality measurements consist of received signal strength measurements for the current and neighbouring traffic channels, and bit error rate measurements for the current traffic channel.

• The results of these measurements are reported back to the base station by the mobile, when they are completed, in a Channel Quality Measurement message carried on the SACCH

• The base station, in turn, forwards the measurement results to the MSC, which then issues a stop measurements command, sent on to the mobile by the base station as a Stop Measurement Order message.

• If, on analysis of the measurements, handoff is deemed necessary, the MSC so orders, with the base station then signalling the mobile to which new channel to tune.

• Handoff of a mobile to a new base station thus results in the immediate need to allocate to the mobile a channel within the new cell. Should a channel not be available, the ongoing call would have to be dropped.

Inter-system handoffs

• This type of handoff is encountered when a mobile roams, moving from an area controlled by one MSC to that controlled by another.

• A mobile m moving from one region controlled by an MSC, labelled MSC-A here, to a region under the control of MSC-B. This procedure is called handoff-forward.

• As the mobile moves through region A, as shown in Fig. 8.35(a), it eventually comes into an overlap region between A and B

• (Fig. 8.35(b)). It is here that MSC-A makes the decision, based on measurements made, to hand the call over to MSC-B

• Once the call is handed over, the mobile is under the control of MSC-B, as shown in Fig. 8.35(c).

Inter-system handoff forward

• MSC-A, initially serving mobile m’s call, is the switch that decides a handoff to the neighbouring MSC-B is appropriate.

• It then sends the Handoff Measurement Request INVOKE message, carrying the id of the serving cell in MSC-A, to MSC-B.

• MSC-B replies with a list of one or more of its cells, with the signal quality of each.

• MSC-A then makes the determination to hand off or not.

• If it decides affirmatively, it sets up a circuit between the two MSCs and then sends a Facilities Directive INVOKE to MSC-B.

• If MSC-B finds an available voice channel in the designated cell, it replies with a RETURN RESULT message.

• MSC-A then commands mobile m to switch to that voice channel.

• With the mobile on that channel, MSC-B sends a Facilities Release INVOKE, releasing the inter-MSC circuit. Handoff is now complete.

Location management

• Location management refers to the requirement that roaming mobiles register in any new area into which they cross.

• They can then be paged in the event of incoming calls. An area consisting of multiple cells is controlled by an MSC

• Visitor Location Register, VLR, which maintains the data base of foreign mobiles registered in that area.

• Base stations within a given area periodically broadcast the area id. A roaming mobile, on entering a new area, senses, by listening to the base station broadcast, that it has crossed into a new area and begins the registration process.

• Consider call delivery to an idle mobile terminal located outside of the area in which the call originated, roaming beyond its home area.

• The Call Initiation message from the call-originating terminal (this could be a mobile itself, or could be a stationary phone within the wired network), is directed to the nearest MSC, based on the dialled destination mobile digits carried in the message.

• The MSC, in turn, forwards the message to the HLR of the destination mobile, using a Location Request INVOKE(here abbreviated to LOCREQ).

• The HLR then sends a Routing Request INVOKE (ROUTREQ) to the VLR identified as the last VLR with which the destination mobile registered. The VLR forwards the message to the current serving MSC.

• The MSC assigns a Temporary Local Directory Number (TLDN) to the intended call and includes this number in a Routing Request RESPONSE (RSP) returned to the HLR, via its VLR.

• The HLR then sends a Location Request Response (RSP) to the originating MSC.

• The originating MSC now sets up an end-to-end voice connection to the MSC serving the mobile. That MSC then notifies all base stations in its area via a paging message to page the destination mobile. Each base station, in turn, broadcasts a paging message to all mobile terminals in its cell.

• The idle roaming mobile being paged, recognizing its id, replies to the base station page using its random access channel

• Consider a location area of area A which contains within it N cells, each of area a.

• “Radius” of the area=R

•

•Say there are m uniformly distributed mobiles within this area. •The average velocity of a mobile is taken to be V m/sec, uniformly distributed over all directions.•Border-crossings/sec. by a mobile is •O -----order of•A more detailed analysis, using a fluid-flow model with mobiles represented as infinitesimally small particles offluid, shows that the average rate of crossing an area of size S is V L/π S, with L the perimeter.

• For a circle or square (Fig. 8.39) of radius R, this becomes 2V/πR. For a hexagon, this is 2.3V/πR

• The average rate of location area border crossings per mobile=

• If each border crossing results in l location messages being transmitted, the average rate of transmitting location messages=

• A typical roaming mobile terminal is called, and is hence paged, on the average, λp times per second. Let each page require p messages to be transmitted

NrlV /2

• The average number of paging messages transmitted per unit time is

• N is a continuously varying variable

Voice signal processing and coding

• Voice signals are transmitted at considerably reduced rates compared with the rates used in wired digital telephone systems.

• This is necessary because of the relatively low bandwidths available in wireless cellular systems.

• The harsh transmission environment involving fading and interference from mobile terminals requires strong error protection through coding as well.

• Two steps are therefore necessary in transmitting voice messages over the wireless air interface.

• The voice signals must first be compressed significantly to reduce the bit rate required for transmission.

• Coding techniques must then be used to provide the error protection needed.

• Both steps must clearly result in voice signals that are acceptable to a receiving user.

• The excitation waveforms appearing in the LPC model of Fig. 8.40 would thus be a combination of periodic pulses and a noise-like (random) signal.

• “long-term”= 3–15 msec• “Short-term”= 1 msec.• Fig. 8.41.--- The linear predictor model may be

written as

• ----------(A)

• The are weighting factors • Taking the z-transform of (A)

• Transfer function is of all-pole linear filter given by

• This system compares the output of the model with the actual speech samples and attempts to minimize the difference (error) signal by adjusting the excitation and filter parameters periodically.

• It shows both a coder at the speech generating side

• and a decoder at the receiving side .The combined system is normally called a speech codec. Consider the coder first. Quantized input speech samples labeled s(n) are generated every 125 sec.

• The difference ε(n) between these and the speech model output is minimized by adjusting the excitation generator and filter parameters.

• The resultant parameters are then transmitted at sample intervals to the receiving system. The receiving system, the decoder, then carries out the inverse process, using the parameters received to adjust the excitation generator and filters.