CA2350879C - Radiotelephone system for groups of remote subscribers - Google Patents

Abstract

A power-conserving time division multiple access (TDMA) radiotelephone system is disclosed in which a cluster of subscriber stations, remote from a base station, employs a common pool of frequency-agile modems each of which digitally synthesizes, on a time slot-by-time slot basis, the different channel-identifying intermediate needed to support communications between several of the subscriber stations and the base station. Power conservation is facilitated inter alia by controlling the assignment of modems to calls, maintaining unassigned modems in a powered-down state and by controlling the number of calls using the same time slot. Delay in assigning a powered-down modem to a call is eliminated by making available to all modems the highest quality synchronization information obtained by any of the active modems.

Description

RADIOTELEPHONE S''STE!"I FOR. GROUPS OF REMOTE SUBSCRIBERS
Field of the Invention This invention relates to radiotelephone systems for serving a plurality of remote subscriber stations and, more particularly, to a radiotelephone system in which certain of said subscriber stations are located in a physically adjacent group.
Backcrround of the Prior Art A radiotelephone system including a base station for serving remote subscriber stations is described in U.S.
patent 5,119,375. In that system each subscriber station was equipped with a radio that could be instructed by the base station to tune to a particular channel and to employ a particular time slot for the duration of a given conversation. Time division multiplex (TDM) radio channel transmission was employed from the base station to the subscriber stations and time division multiple access (TDMA) transmission from the individual subscriber stations to the base station. The time division of each radio channel into time slots and the compression of speech signals permitted each radio frequency channel to support a number of voice paths equal to the number of time slots.
Analog, voice signals to and from the public switched telephone network were first converted to 64 kbps ,u-law companded pulse coded modulation (PCM) digital samples.
Before transmission over the radio channel the digital samples were subjected to voice-compression to reduce the voice information rate from 64 kbps to 14.6 kbps using residual excited linear predictive (KELP) coding. A voice codec and modem were required to be dedicated to a specific frequency and time slot for the duration of a call.
While the foregoing system operated in a highly satisfactory manner in allowing telephone service to be provided especially to areas where wire lines are =2-impractical, the unforeseen growth of such telephone service has given rise to sitii~tions iii which several subscriber stations are found to :Lie in close proximity with one another. Initial efforts to lower the per-line cost of serving a group of~such closely situated subscriber stations were focused on consolidating the~installation and maintenance costs of individual subscriber stations~through the sharing of common equipment such as the enclosure, power supply, RF power amplifier and antenna. Thus, in a closely situated group of subscriber stations, each of which could access an RF channel, a single broadband RF
power amplifier could be employed to serve the group.
However such efforts still required each subscriber line to have its own modem anc~ radio transceiver. ' ~'he individual transceiver outputs were fed, to .the common RF power amplifier, which had to be designed to handle a peak power equal to the sum of the power of ell of the. transceivers in the group of adjacent eubecriber~ stations~~that could simultaneously be active on the~.eame time slot. It is 2o apparent that further consolidation over that possible in the '375 patent system and a reduction~~in the~peak and average power required would be..desirable, especially in remote areas recxuired to be served by~solar cell power.
Summary of the Invention 2S - In accordance with the principles of our invention, per-line costs are reduced for a physically adjacent group of subscriber lines by pexznitting~ the lines within such a group to share not only a common power~supply and RF power amplifier, but modem, synchroniaation,~ IF, up-and down-30 conversion and controller functions as well,' so that significant concentration is achieved. In our system, a small number of modems is provided to serve the multiple subscribers in a physicallyadjacent group, hereinafter referred to as clust2r or, more part:icUlarly, as a modular 35 cluster. In an illustrative embodiment, subscriber line circuits and modems are modularized printed circuit cards which plug into a frame employing backmlane wiring to dvst~-ibtrt~ t~mir.g _n~orna~i~:~ and :iota amor_g ~.~-:e a~:i~s .
Ar_y o= the modems may be seized to handle a call for any of the subscribers and each modem may handle calls for several subscribers on successive time slots. The same or a different freauer_cy may be used to support communicatior_s for each subscriber on successive time slots.
It is a feature of our invention that the selection from the common pool of frequency-agile modems of the modem to be used to handle a call is controlled to conserve Dower consumption in two ways. First, a new modem is preferably not seized fog use to handle a call until all of the time slots on active modems have been assigned to calls, thereby allowing all not-yet-selected modems to remain in a power-conserving, "powered-down" state.
Second, the number of calls using the same time slot (on different frequencies) is controlled to reduce the peak power demand on the RF power amplifier.
It is a further feature oz our invention to avoid synchronization delay when it is necessary to seize a powered-down modem for use on a call. Once time slot synchronization with the base station has been established for the first modem of the pool at the cluster, synchronization infornation is made available to the remaining modems, advantageously over backplane wiring, under control of a microprocessor-based cluster controller.
Accordingly, all powered-down modems remain instantly assignable to handle calls without undergoing any delay to become synchronized with the base station's time division frame.
It is a further festur' of our invention to classify modem synchronization states according to several sv-ichronizaticn parameters and to deri~r4 a confidence factor ,-__or each active modem that reflects the reliability of the synchronization parameters and to distribute s~nchtonizatiori information trorti the modem haling the best confidence factor.
Briez Description of the Drawings The foregoing and other objects and features of our S invention may become more appareht by referring now to the drawing in which Fig. 1 is a block diag~-aM of a Modular cluster having a common pool of frequency agile modems for handling a group of subscriber stations;
Fig. 2A shows the association'of subscriber line circuits and Modems at the time slot iriterchanger;
Fig. 2H shows the TDMA RF frame allocated for 15PSK
time slot ; ~ ' Fig. 2C shows the TDMA RF frame allocated for QPSK
time slots;
Fig, 2D shows the task scheduling between the TDMA
time slots and the PCM buffers;
Fig. 3 shows the principle circuit elements of a frequency agile modem module;

2~ Fig. 4 shows the IF portion afwthe frequency agile modem;
Fig. 5 is a block diagram of the block synthesizer, up/down converter;
_" Fig.~6 shows the frequency synthesis and noise shaper for the receiver portion of the modem;
Fig. 7 shows the frequency synthesis, modulation and noise shaper circuitry for the IF'transmitter portion of the modem; arid Fig . B shows the system clock generation circuitry for the modular cluster.
General Description Fig. 1 is a block diagram of a modular subscriber cluster that is located rAmotely from a base station (not _S-shown). The subscriber cluster is termed "modular" because the line circuits 100 and the modens 400 are comp=iced of plue-in units . accord-ng'~y, the ruT~er cf p 1 ugaed-subscriber line circuits 100 will depend on the number o=
subscribers in the local i ty ar_d the r_umber of pl ugged-in modems 400 may be traffic-engineered to handle the amount of traffic expected to be generated by the number oz line circuits 100. Line circuits 100 are contained on quad line module cards 101-108, each of which serves four subscriber lines. Eight such quad lire module,; provide loop control functions to a line group of 32 subscriber lines and circuits 100 may cor_tain multiple line groups.
Each lire circuit on each quad line module 101-108 is given a dedicated PCM time slot appearance in PCM speech 1S highway 200 and in signaling highway 201. The quad lln~
modules 101-lOB include voice codecs (not shown) to encode subscriber loop analog voice onto PCM data highway 200.
Subscriber loop signaling information is applied to sigr_aling highway 201 by a subscriber line interface circuit SLIC (not shown). Either ~C-law or A-law PCM coding may be used.
The connection of a particular one of modems 400 to handle a call from or to a particular one of the line circuits on one of quad lire modules ? O1-108 is made via 2~ time slot interchangers 310 and 32.0, as instructed by cluster controller 300. PCM data time slot interchanger 320 conveys speech samples between the PCM speech highway 200 serving Line modules 101-108 and the Pr'M speech highway 220 serving modem pool 400. Signaling time slot ir_terchanger 310 conveys signalling information between signalling highway 201 serving the modules 100 and signalling highway 221 sensing modem pool 400.
Two RF channels are required for a telephone conversation, one for transmissions from the base station ~o the subscriber (the 'forward' channel) and one from the subscriber to the base station (the 'reverse' channal).
The forward and reverse channel frequencies are assigned by the telecoinmun~catione authox'ity and in a typical example may be separated from each other by 5 MHz. The path of tine forward channel radio 9igr_al received at the cluster from the base station may be traced ftoni c:ltister antenna 900 and duplexer B00 to block synthesizer up/down converter (BSUD) 600: In block converter 600 the RF signal is limited, band-pass filtered and down-converted from the 450 MHz, 900 MHz off- other high, or ultra-high frequency RF band to an IF
signal iri the 26 - 28 MI-~z range . The IF signal is l0 delivered to modems 400 which process the signal for delivery to the subscriber line circuits via the time slot interchangers in the cluster controller 300:
. The modems each include a baeebarid~digital signal processor (see Fig. 3, DSP/HH) and a rridderri' ~rocesaor (gee Fig: 3, DSP/MDM?: In the for4iard channel direction; modem processor DSP/MDM demodulates the IF signal received from bloclt Converter 600 and transfers the data to baseband prdcesaor DSP/BB which expands the demodulated data into ~e-law or A~law eilcoded signals for transmission through time slot interchanges 320 to the line modules: The modem's baseband processor DSP/BB interfaces to modem processor DSP/MDM vie ~ direct memory access (DMA) interface (see Fig. 3) and to the PCM highways through the processor's serial port. In the reverse channel direction, baseband processor DSP/BB con~tterts the u-laai or A-law coded PCM
inforc~tation received from PCM highway 500 into linear form, compresses the linear data using'~RELP coding and DMA
transfers the compressed data to digital signal processor DSP/MDM which modulates the sigilal far transmission on the radio channel time slot:
As shown W Fig. 2A, each -of inodecris 400 and each of line modules 100 has four dedicated time slot appearances in PCM data time slot interchanger-320 for non-blocking access. Each modern is assigned two adjacent PCM slots in PCM time slots 0-15 and two adjacent PCM time slots in PCM
time slots 16-31. As an example, for a particular call, TSI 320 connects line circuit 0 of line'module '101 to _7_ channel 1 of modem 1, and line circuit 1 of line module 101 is connected to channel 0 of modem 1, and so on. Time slot ir_t°-rchancers 31C; anc 320 p~ov=de a repetitive 125 ~.r,S
sampling period containing 32 time slots operating at a S rate of 2.o4s Mbits/sec. During each I25 ~S PCM interval, the line modules may send thirty-two, 8-bit bytes of data to time slot interchanges 320 and each modem may receive four of the 8-bit bytes at its baseband processor serial port, packed together as two 16-bit words. Each 16-bit to word causes a aerial port interrupt on the baseband processor. When the interrupt is received, the baseband processor determines whether the pair of PCM samples contained in the l~-bit word correspond to slots 0 and 1 or to slots 2 a_nd 3. Similarly, during each 125 ACS PCM
15 interval, four voice channels of PCM data, packed together as two I6-bit words, may be sent. from each baseband processor's serial port to time slot intercha_nger 320 for delivery to the line modules.
The TDM (RF) frame at the base station is shown in 20 Figs. 2B and 2C, each having a duration, illustratively, of 45 ms. The 16PSK frame of Fig. 2B has four time slots, each of duration 7, each time slot capable of carrying the different frequencies assigned to the forward and reverse channels of the call. In Fig. 2C the RF frame of the same 25 duration is capable of accommodating the forward and reverse channels of two QPSK modulated calls. It can be appreciated that, alternatively, the TDM frame can carry four 16PSK calls or two QPSK modulated calls.
Fig. 2D illustrates the timing of the tasks performed 3o at the cluster in conveying information between an illustrative TDMA frame carrying QPSK modulated calls and the DCM highway frames. Line (1) represents the buffers for recAiving the two QPSK modulated forward channel time slots, R:{1 and Rx2, of the TDh~ frame. Demodulation is 3J begun as soon as the receive buffer has received the rust half, Rxla, of the time slot. Line (2) represents the buffers preparing to transmit in the. two reverse charnel QPSK time slots, TxI and Tx2, of a TDMA frame. Note that, at the cluster, the reverse channel time slots are offset from the forward channel time slots so that the eubscz~iber station may avoid the expense and bulk of a duplexer. In addition, the subscriber unit's the reverse channel c.till be offset so that it twill be received at the bade station at the proper time taking into accotiht 'the' distance between the subscriber station and the base Station, Lines (3) and .. , (4) of Fig, 2D represent the buffers in the .Stmt (Fig. 3) IO of the modem which store the PCM iaords to and from speech time slot interchanger TSI 320 (Fig. 1).
In normal voice operation, the modem processor DSP/MDM
demodulates received forward channel symbols, packs them into a buffer ~n SRAM/MDrt ~.nd sends the contents of the buffer to the baseband procesaof DSP/BB for KELP synthesis (expansion): The baseband processor encodes the expanded data to ~1-law or A-laW and puts it on the PCM bus for delivery to the line motiuleg. Voice code words are transmitted iii every frame during active vbice operation.
The code t~ord r~s~des at the beginning of the burst between the preamble and voice data on both the forward and reverse channels. The forward channel voice code words contain information that may be used to adjust transmit power and timing. Local loop control information (i.e.,~ onhook, ' 25 offhook, ring, forward disconnect) is also embedded in these code ~iords. The reverse channel code words contain subscriber station local loop control and forward channel link quality information.
The fortdard voice codewotd i5 decoded by the modem processor DSP/MDM. The forward voice codeword contains transmit fractional timing contfol,'transmit power level control and local lbop control information: The fractional timing and power level control irifox-mation is averaged out over a frame and the average adjustment made at the end of the frame. The local loop control information i9 stored locally and changes in loop state are detected and reported to the cluster controller. The local loon control also _g_ causes the modem to Send out line circuit control over the signallir_g bus. The reverse voice codeword contains local loop status that i~~ used by the cluster cone-oiler ar_c base station to monitor call progress.
The modem processor DSP/MDM performs receive FIR
filtering and automatic gain control of the received samples during a receive symbol interrupt service routine.
The demodulator routine in the modem processor is called when half a slot of baseband information has been received in the receive buffer. The demodulator operates on the half slot of data and passes the packed output data to the baseband processor DSP/BB for RELP synthesis. Data transfer to and from the baseband processor is controlled so that the ?2ELP input queues are filled before tine corresponding synthesis data is requ~.red, and RELP output quet:es are emptied before r_ew analysis (compression) output data arrives. During demodulation, automatic frequency control (AFC), automatic gain control (AGC) and bit tracking processes are performed to maintain close 2o syr_chronization with the base station.
It should be appreciated that mixed mode operation is possibl a whereby some time sl ots in the RF may employ lo'PSK
modulation while the remaining slots employ QPSK
modulation.
Synchronization to the Base Station Before an RF channel can be used for communication between the base station and the cluster, the cluster must be synchronized to the RF time slot scheme used by the base station (not shown). In accordance with our invention, one or more of modems 4oo will be ordered by cluster controller 300 to acquire synchronization with the base station RF
frame timing by searching for the channel frequency carT-ying the radio control ciian~wel (RCC) being used by the base station. Clu;>ter controller 300 includes a master

3~ control microprocessor 330, illustratively, one employing a Motorola 58000 series processor, which ~eendg control information over ~.he CP bus to the micz'oprocessors in modems 400. On power up, cluster controller 300 down-loads appropriate software and initialization data to modems 400.
S After the channel frequency is found; the modem must synchronize with the base station time slot by decoding the RCC unique word: As described in the aforementioned '375 patent, the RCC channel i9 distinguished from other channels in that it has an extended guard interval: during its time slot and includes a DBPSK modulated'unique word of 8 bits. In order to minimize the possibility of aborting a call if the modem with the active RCC time slot fails and it becomes necessary to assign the RCC time slot to a different modem, time slots are assigned within an active modem Bo that the synchronization (RCC) tirrie~ slot (referred to as Rx0 where the four time slots axe numbered . Rx0 through Rx3, or Rxl. where. the ' time slots are number Rxl through Rx4); is the last to be'filled:
At start-up, all of fiodecris 400 are assumed to be out of synchronization with the base station's RF 45 rris frame.
During tune slot zero of the RF frsme,~the base station transmits an RCC message on some RF channel which;'when received at the modular cltister,~will be decoded to~put the cluster into synchronization with the base station's RF
time slot frame for all RF channels. Until synchronization with the base station is achieved, each modem generates its own local RF frame sync. Cluster controller 300 next commands one or more moderns to hunt for the RCC transmitted by the base station on different RF dhannel~ until the RCC
is found or all channels have- been sea~th~d. If all channels have been searched and the Rc:C has not been found, the controller orders the search to begin again.' When a modem finds the RCC, the controller designates it as the RCC modem and distributes its sync inforni~tion to the remairiing modems vii the frame sync signal over the backpiane:

when the RCC slot search is undertaken, the channel number is used by the modem to digitally sweep a d=rct digital rrequency s;rnthes~.s (DDFS; local oscillator , illustratively over a 2 N~:z range. There are two stages to a modem's acquisition ef the RCC channel, coarsely identifying the center frequency and finding the "AM hole", a portion o= the RCC time slot where the number of symbols transmitted by the base station does not fill up the entire slot time- Coarse frequency acquisition is based on performing a Hilbert transform of the spectrum of the RCC
channel which yields a frequency correction for the local oscillator. This continues until the energy in the upper half of the spectrum approximates that in the lower half.
After coarse frequency acquisition is obtained, illustratively to within an accuracy of 300 Hz of the channel center frequency, a search is made for the AM hole.
A number of null signals are transmitted prior to the RCC
data. The AM hole is identified by monitoring tile amplitude of consecutive received symbols. when twelve consecutive null symbols are detected, an AM strobe signal is output by the modem to indicate the start of an RCC slot and the start of a TDMA frame. This coarsely synchronizes the baseband modem timing to the base station timing.
Synchronization need only be performed once since the radio link is shared by all baseband modems in the modular cluster. The frame sync signal is sourced by one modem to all other modems in the cluster via a signal on the backplane wiring. During the search for the RCC if the AM
hole is found to within 3 symbol periods of the start of frame marker, coarse acquisition is complete. The location of the unique word within the frame provides the modem with timing information that is used to bring the modem's local frame timing to within one symbol timing of the base station. 'he modem is said to be in receive sync, Rx RCC, 3S as long as it continues to receive and decode the u_niaue word correctly. Once synchronization is achieved, 16PSK
modulation cerresoonding to 4 bits per symbol, QPS~C

modulation cotresponding to 2 bits per symbol, or combinations of botfl may be employed.
While all modems are capable of receiving and synchronizing to the base station's radio control channel RCC, only one modem need do this since the modem which is selected by the cluster controller can share its timing with the other modems via the Frame Sync signal over the l~ackplane wiring. The selected modem will source the Frame Sync Out signal and all other modems will accept this signal as the Frame Sync In signal.
When a modem goes on line, its modem processor DSP/MDM
instructs its DDF 4S0 (Fig. 3) to try to synchronize its local frame timing to the backplane signal. Each modem's DDF 450 timing is at this moment independent of every other modem's timing. DDF 45o will initially be instructed by its DSP%MDM to look at the backplane signal for its -. synchronisation. If a backplan~ synchronization signal is present, the DDF will synchronixe its frame sync signal to the backplane signal and then disconnect from the backplane signal. The backplane signal thus does not feed directly into the modem's timing circuitry but merely aligns the modem's internal start of recef~re frame Hignal. If a backplane synchronization signal was not present, it is assumed that the modem is the first one that has been ~ activated by the cluster controller, in which case the cluster controller 300 will instruct the modem procpseor DSP~MDM to look for the RCC and fend the modem's timing to the cluster controller. ' Cluster cont oiler 300 next instructs the modem processor D5?~~MDM to demodulate the DHPSK signal on the RCC
cha=rnel. The path for demodulation of the ~IF signal received from block converter 500 may be traced to the modem IF modtt?a where it is again band-pass filtered and down-converted to a 16 kilosymbol peg second information stream. The DHPSK modulation that is employed on the RCC
channel is a one bit per symbol modulation- The RCC
messages that are received from the base station must be 1 _W
demodulated and decoded before being sent to the cluster controller. Only messages that are addressed to the cluster controller, have a va l id CRC a~: a=a a burst -yr~e message or an acknowledgment message are forwarded to the Controller. A~._L other messages are discarded.
acknowledgment message signifies the correct reception of the previous RCC message. A message is addressed to the cluster controller. if the Subscriber Identification number (SID) contained in the message matches the SID of the IO cluster.
Referring to Fig. 3, the 16 kilosyrtbol per second IF
sigr_al from the fF circuitry of Fig. 4 is entered into A/D
converter 804, which is sampled at a 64 KF-~z rate by a clock sigr_al received from DDF chip X50. A/D converter 804 15 performs quadrature band-pass sampling at a 64 kiiz sampling rate. Quadrature band-pass sampling is described, inter alia, in US patent 4,764,940. At its output, converter 80a provides a sequence of complex signals which contains a certain amount of temporal distortion. The output of 20 converter 804 (Fig_ 8) is entered into R.~cFIFO in DDF chip 450. Modem processor DSP/MDM reads the contents of R~cFIFO
and performs a complex FIR filtering operation, which removes the temporal distortion introduced by the quadrature band--pass sampling. After tine removal oz 25 temporal distortion, the signals are demodulated by procssor DSP/MDM.
During the demodulation of RCC messages, AFC, AGC and bit tracking processes are perfomed by modem processor DSP/MDM to maintain the cluster in close synchronization 30 with the base station. Transmit timing and power level adjustments are made according to information received in the RCC message. Processor DSP/MDrt examines the demodulated data and detects the RCC message, a message ~.vhich includes link status bits, and 96 bits of data that 3includes the subscriber ID. Modem pz-ocessor DSP/MDM also recognizes whether the subscriber ID belongs to one of the subscrii~er line circuits in the cluster.

_, tf,.th~ m~s~age is fo,f this clust~r,"'th~ me~g~ge is passed to cluster controllef 300, which. interprets the RCC
command. Forward RCC messages include page message, a call connect, clear indication and self=test. Reverse RCC
S messages include call accept, clear request,'test results and tall request: If the RCC triessage is ~a pale message;
the cluster controller for which it is designated will formulate a call accepted message to be~ transmitted back to the base station: From the call accepted in2g9age the base station determines the timing offset between the cluster and the base station and the base station sends symbol timing update information to the cluster in the next RCC
message, iahich is the call connect message:
When the RCC message is a call Connect m~ss~:ge, the ' information therein instructs the cltiatez- controller what adjustment to make in symbol timing,' whether to adjust power level , fractional timing, and what channel to use for the remainder of the call (chaninel htimber, TDM slot number, whether QPSK or 16PSK mod~ilation wi~l be employed~and what the subscriber line type ie): ~ ' The first modem which hag found the RCC is designated the RCC modem and its frequency offsetj~ receive gain control Rx AGC, and start offrame information i9 coilsidered valid and may be dist~ibiited to the other modems. The cluster controller recei~es'the channel riumber information and decides which modem is to be in~t.nlcted to tune ui~ to the designated channel to handle the~remainder of the call.
The firial~~tep toward total s~nchxonization is the s~icce~sful establishment of a voice Channel. When a lroice chanriel is established the last' two synchronization parameters become valid' the tranazriit,symbol timing and transmit symbol fractional timing, At this point; should another modem be activated by the cluster controller all of the necessary synchronization irifc~-trixtion is aliailable to be provided to the modem, making the establishment of a voice channel much. easier and clicker. A confidence level is calculated to evaluate the synchronization infoz-~nation of each modem. The cluster controller upda~es the COnLl~°_::C°_ lE?Vel LOr eaC:_ IT:Cd°,T:
W~1~°_n:yt~or t;:ero 1S 3 Cr:aI-'_Ce in sync status, link cruality, or receive AGC. The cluster controller finds the modem with the highest confidence level and distributes its synchronization parameters to the remaining modems.
When a modem slot is commanded to enter the voice mode by tile cl uster controller, the modem first atterrn_ is to perform refinement. Refinement is t=he process of finely synchronizing the modem's transmit timing and power level to the base station's receive timing. The refinement process is controlled by the base station. The base station and the modem exchange special refinement bursts until the base station terminates the refinement process when the predetermined degree of synchronization has been achieved. The modem then goes into normal voice operation.
If the base station aborts the refinement process, the modem will abort the call, go into the idle state and inform the cluster controller. Refinement bursts are DBPSK
bursts formatted like RCC bursts. Refinement bursts are detected by the presence of a unique refinement word. The modem is said to be in voice synchronization when the refinement unique word is detected with zero offset. The forward and reverse voice codewords have a voice codeword check byte attached for error detection. The modem will report a loss of sync if 9 consecutive frames are received with voice codeword errors, at which time the cluster controller enters the recovery mode until a good codeword is found or until the modem is commanded out of this mode and placed into idle mode.
Based upon the synchronization state, cluster controller 300 determines the validity of the syr:chror_ization parameters provided by the modem. The table below shows which parameters a_-e valid, based upon the cure=nt synchronization state of a modem. An °X" in the box indicates that the parameter i.s valid.

-is-Sync State Freq. Sy bol Fract. TxPLC RxAGC SORF
Offset Time Time No sync px syr~c~~cc) X ', x x Tx Sync (pCC) X X X X

Voice sync X X X X X X

A 12-bit confidence factor word ig computed by the modem to reflect the reliability of the synchronization parameters ascertained by the modem. The confidence factor word is assembled by concatenating the bits representing the voice and receive sync statee.of the modem with bits identifying the link quality and receive AGC parameters,. as set forth in the following table:. .
Bit Allocation 1 t 10 9.:8 7..0 Field Voice SyncRz SytiC(RCC)Link Quality~tiAGC

The single bits 11 and 10 identify, respectively, whether or not the modem is in voice sync and receive sync .
The two bits 9 and 8 udentify ' foU.r ' grsdations or - link quality, while the 8 bits allocated to receive ~GC level indicate the level of gain required:
~2 0 MODEM MODUL)J ; FIG : 3 The principle components of the ctiodem module are shown in Fig. 3. The modem module ~dan support up to four simultaneous full duplex voice channels. The processing to dynamically handle all functions required by an active channel is partitioned betWedn the Bluster controller processor 320, (Fig: 1}, and proce~~sors D5P/MDM and DSP/BB
in each modem f'~ig: 3). The clu~teY controller handles higher level functions incli.idiizg call set=Lip, channel allocation and system control: '~ Modem processor DSP/MDM

handles filtering, demodulation and routing or the incoming radio signals, fo=-matting of data oe~o~e t-ans;nissio;z over the radio channe ~ , and managemer_t o= data =low ~,etween itself and baseband processor DSP/BB. Basebar_d processor DSP/BB performs the computationally intensive tasks o=
voice compression and expansion and, in addition, handles the PCM bus interface. Tn normal voice operation, modem processor DSP/MDM demodulates received symbols, packs them into a receive buffer and sends the voice data buffer to baseband processor DSP/BB for RELP synthesis and transmission to the subscriber lire c~'_rcuit over the PCM
bus. The modem processor DSP/MDM also accepts compressed speech from baseband processor DSP/BB, formats it into TDMA
bursts and sends it to the transmit pulse shaping filter FIR contained in DDF 450 for transmission over the radio link. The modem operates on both QPSK and 16PSK
modulations (and DBPSK during refinement) under control of the cluster controller.
Processors DSP/BB and DSP/MDM each have a dedicated 2o random access memory, SRAM/MDM and SRAM/BB, respectively.
However, modem processor DSP/MDM may request access to the random access memory SRAM/BB by activating its DMA HOLD
output and obtains such access using the data and address bus when the baseband processor DSP/BB activates its DM.A
ACK output signal.
Assignment of Time Slots As described in the '375 patent, the RPU in the base station keeps track of , the radio channels and time slots that are in use and assigns both the frequency and the time slot to be used on any ca?1_ A slot is selected which is in use by the least number of calls so that the call traf f is can be more evenly distributed acr oss aI l s 1 ots .
HoweveY, in accordance with that aspect of the present invention which is- concerned with minimizing the power 3s expended at the remote modular cluster, calls are assigned so as to (a) minimize the number of active modems and (b) control the number of conversations simultaneously using the same time slots. ;urther, while it is desirable to employ 16PSK modulation in every ts.me slot of a TDMA frame so that four complete calls can be accommodated, it i9 also important to permit QPSK calls to be made and to keen an alternate RCC slot available for synchronization purposes.
Accordingly, the cluster and the base station must cooperate in the assignment of tittle slots to achieve these goals. The cluster keeps track of available time slots and the type of modulation being employed on each slot . The cluster then assigns priority levels to each available slot and maintains a matrix of priority values which takes into account the factors that (a) an alternate receive time slot (generally the first time slot) on.some channel must be allocated for RCC synchronization, (b) adjacent time slots should be left available as long as possible so that QPSK
calls can be handled if necessary, and (c) time slots should be assigned to handle calls without, iz possible, activating a powered-down modem or assigning a slot that is already in use by a large number of other calls: The routine (in pseudo code) for achieving these goals is as follows:
Prioritize Siot Routine 2 S List 1 = alt idle time slots available on already active modems for 16f5K
calls and QPSK calls;
List 1 A = ail idle modems;
List 2 = Llst time slots whose use wilt not exceed the thi~eshhold number of calls using the same time slot In the cluster, 3 0 ~ List 2A = List 1 minus Llst 2;
List 3 = List 2 minus time slots on modems having adjacent time slots available (for pPSK caNs);
List 3A = List 2 minus time slots on modems nol having adJacent time slots available (tor OPSK calls);

9_ List 4 = List 3 minus time slots on modems not having a synchronization time slot available (slot 0 for the RCC);
List 4A = List d minus time slots on modems having a synchronization time slot available;
Mark list 4 as first choice;
Mar',t list 4A as second choice;
Mark list 3 as third choice;
Mark list 3A as fourth choice;
Mark list 2 as fifth choice;
~ Mark list 2A as sixth choice;
Mark list 1 as seventh choice;
Mark list 1 A as eighth choice.
The above Prioritize Slot Routine is called whenever the cluster receives an RCC page message from the base station or is about to formulate a call request message to the base station. When the base station responds with a call conr_ect message containing the frequency, type of modulation and time slot to be used, the cluster once again performs the Prioritize Slot Routine to see i= the slot 2Q selected by the RPU is still available. If still available, the slot is assigned to the call. I-iowever, if in the meantime the slot assignments have changed, the call will be blocked.
An example of how the Prioritize Slot Routine is executed under light and heavier traffic conditions may be helpful. Consider first the following table, which illustrates a possible condition of the modems and assigned time slots under light traffic conditions, just before one of the subscribers served by the modular cluster initiates a request for service:

CJ -Modem Time Slot ' 0 1 2 _ _ 1 16PSK ~PSK OPSK

Z IDLE IDLa= IDLE IDLE

The above table indicates that modem 0 has slots 2 and 3 available, that modem 1 has slot 1 available and that modems 2, 3 , 4 and S are powezed-down, ail of their : time slots being idle. 'rhe cluster executed the Prioritize'Slot Routine which determines that slots 1, 2 and 3, in that order, az~~ the preferred slots to be assigned to handle the next 15PSK call and that for QPSK calls the preferred slots are 2 and 0, iri that order. The cluster then sends ~ "call request" signal to the base station using the RCC word and iriforzns tile b~.se station of Ehis pr~~erence . In the table belo:v the rationale for etch of the priorities is set forth 2 0 Slot PriorityRaiional8 Slot PriorityRationale ltSPSK t7PSK

1 No new modems to power 2 (Same reason up; as no (hcraas~ iri in3x ~ iSPSK for slot activity; slots OPSK sots 2.3 kept available;~ . 2,3}

RCC slot available.

2 New ~PSK call requires 0 Requires new new modem power up. ~ modern power up ' 3 2 S D flequires new modem power up.

Another example may be helpful. Consider the status of time slots among modems 0-5 under somewhat heavier traffic conditions, as shown in the following table, wherein empty boxes indicate idle time slots:

Modem Tirne Slot 0 - ~ ~

3 I c.3PSK ~ QPSK QPSK (~PSK

4 16PSK 16PSK 16PSK

The slots to be assigned set forth are i:~
the following table together i with t'r?

e rat onale:

Slot PriorityRationale Slot PriorityRationale 3 No new modems to power 2 only choice up;

max slot activity avoided;

OPSK slots 2.3 kept available;

RCC slot kepi available.

2 No new modems to power up;

max slot activity avoided;

RCC slot kept avaiiabie, BUT, new OPSK call requires new modem power up.

1 No new rnadems to power up;

QPSK slots 2.3 kepi available;

RCC slot kept available, BUT

max slot activity exceeded.

0 No new modem power Up;

QPSK slots 2.3 keptavaitable;

SUT both max slot activity exceeded and RCC slot not kept available.

Up/Down Converter 600 In Fig. S, Torward channel radio signals from the base stati on are received in uD/down converter ~00 from the base station via duolexer 800_ The received R~' signal is passed through low-noise amplifier SOZ, band-pass filterAd in _22_ filter 503, subjected to attenuation in attenuator 504 and applied to mixer 505, where it is subjected to a first down-conversion from the 450 I~iz RF band or the 900 hiHz RF
band to an IF signal in the 26 - 28 fi~fHHz range . The IF
S signal ie pa6sed through amplifier 506, bandpass 'filter 507, amplifier 508 and attenuator 509 and applied to splitter circuit 510 for delivery to the common pool of modems. ' The reverse channel modulated IF signals from the common pool of modems are applied to combines 520 of block up/down converter 600 at the upper left-hind corner of Fig.
S., subjected to attenuation in attenuator 521,-band-pass filtered in band-pass filter 522, amplified in amplifier 523 and applied to mixer 525; where the signal is up converted to an RF -signal .in either, the 450 M~i~ RF band or the 900 MHz RF band. The RF signal i9 then~.eiibjected to attenilation in attenuator 526, band=~agg filtered in band pass filter 527, amplified in amplifier 528 and applied to broadbarid highpower amplifier 700 ivh~ch 'ends the signal on to duplexes 800.
Mixers 505 and 525 receive their reference frec~lencies from RxPLL phase locked loop circuit 540 arid TxPLL phase lock loop circuit 550, respect3vel~%; phase locked loop 540 generates a 1 . 36 MHz recei-1ie ~ l~ca1 oscillator 'signal from the signal provided by 21.76 MHz master. clock 550, divided by 2 and then by a . The 1: 36 ~LFi~ ~ di~nal furnishes the reference input to phase comparator'PCm~The other input to the phase comparator is providac~.b~ a feadback loop which divides the output of circuit 540 by 2'and then by 177.
Feeding back this signal to the phase.comparator causes the output of circuit 540 to have a frequency that is 354 times that of the reference input, or 481.44 MHt. The 481.44 NIF-iz output of receive phase locked loop RxPLL 540 is applied as the local oscillator input to down-conversion mixer 505.
The 481.44 f~-iz output of circuit 540 is also applied as the reference input for circuit 550, so that circuit 550 is frequency slaved to circuit 540. Circuit S50 generates the transmit local oscillator signal, which has a _re~ae:rcy of 481.44 M::z + 5.44 MHz, ~.. e. it has a _;equency that is cffset 5.44 ~~~?z 'richer trap t_~:e receive local oscillator. For circuit 550, t:_~e 21.76 hiT-~z sienal from master clock 560 is d;~vided by 2, t:ner. by 2 again , to make a signal having a frequency of 5.44 MHz, which is presented to the reference input o~ phase comparator FC of circuit 550. The other input of phase comparator PC of circuit 5>0 is the low pass filtered difference frequency provided by mixer 542. Mixer 542 provides a frequency which is the difference between the receive local oscillator signal from circuit 540 and the VCO output signal of circuit 550. The output of circuit 550, taken from its ir_terna? VCO is a frequency of x61.44 MHz + 5.44 M_~iz .
Fig. 4 IF Portion of Modem Fig. 4 shows the details of the IF portion of tile modem board in relation to tile digital portions (whose details are shown in Fig. 3). At the lower right hand side of Fig. 4, the receive IF signal from BSUD 600 (Fig. 1) is applied through the lower terminal of loopback switch 402 to 4-pole band-pass filter 404 whose a passband extends from 20' to 28.3 MHz. The output of filter 404 is then amplified by amplifier 406 and down-converted in mixer x08 which uses a receive local oscillator signal having a frequency of between 15.1 MHz and 17.4 MHz. The output of mixer 408 is amplified by amplifier 410, and filtered by 8-pole crystal filter 412 whose center frequency is 10.864 3C MHz_ The amplitude of the signal at the output of filter 422 is controlled by AGC circuit 47_4. The gain o~ AGC
circuit 414 is controlled by the VAGC' signal from DDF ASIC
450 of Fig. 3. The output of AGC circuit 414 is then down-converted by mixer 416, using a reference Yrequency oz 3J 10.85 ~tHz, to produce a 16 kilosymbol per second secr~ence of IF data, which passes through amplifier 418 and is delivered to the Rx IF input port of the circuitry of Fig.
3.
Still referring to Fig. 4, the circuitry of Fig. 3 generates a receive local oscillator signal, Rx DDFS, which is filtered by '7-pole filter 432; then amplified by amplifier 434. The output of amplifier 434 is again low pass filtered by 7-pole filter 436, whbse output is amplified by amplifier 438, then mixed with the received IF
radio signal in mixer 408.
At the right hand side of Fig. 4,~ amplifier 420 receives a master oscillator signal having a frequiency of 21.76 Mliz and applies the 21.'76 MHz signal to splitter 422.
Orie output of splitter 422 ig doubled in frequency by frequency doublet 424, ~hoee t~utpttt~ is clipped in clipper 426 and shaped to TTL by gate 428, and inverted again by gate 430. The output of gate 430 is applied to the inset circuitry of Fig. 3 as a 4352 MHz refereilce cloc.l~ signal.
The other output of splitter 422 is passed through amplifier 454 and attenuator 455 and applied~to the local oscillator (L) input of mixes 444. Mixer 444 up=Converts the modulated IF signal, Tx DIF,'from inset~Fig. 3 after it has been iosJ pass filtered by filter 440 and attenuated by attenuator 442.
The output of gate 428 also connects to the input of inverter 460, whose output is frequency divided by 4 by divider 462 and then used as a local oscillator to down convert the output o~ AGC block 414 in mixer 416.
A loopback function is~ pro~lided by the serial combination of sviitches 450 and 402 and dilmm~ load 458 so 9o that signals from the Tx DID' oupu~. ~ of the inset reference to the circuitry of Fig. 3 may be looped back to its Rx IF input for test purposes when training sequences are applied to compensate for signal distortions; such as that occuring within crystal filter ~~12.
Still referring to Fig. 4, the circuitry of rig. 3 provides a modulated IF output, at a frequency of 4.64 to 6.94 f~-Iz, which is filtered by 7-pole filtez 440 and attenuated by attenuator 442. The output of attenuator 442 enters mixer 444, where it is up-converted to a rreguency ir_ the range of 2 6 . _ t~~ ~ ~.0 2 3 . 7 t4~ ~: . The output of ~:,'_xe;
a4a entors amplifier 440', whose output is filtered by 4-pole bandpass filter 448 and applied to switch 450, which is controlled by the loop-back ena:ole output LBE of the inset circuitry of Fig. 3. when loop-back testing is conducted lead LBE is energized causing switche 450 to connect the output or filter 44B to the top of dummy load 458 and energizing switch 402 to connect the bottom of dummy load 358 to bandpass filets 404 for loop back testing. Loop-back testing is used with modem trair_ing secuences to compensate for signal distortions within crystal filter 412 and in other parts of modem circuitry.
When loop-back testing is not being conducted, the output of switch 450 is applied to programmable attenuator 452 which may be programmed to one of lo' different attenuation levels by the transmit power level control signal, Tx PLC, from the inset circuitry of Fig. 3. The output of attenuator 452 comprises tile Tx IF PORT signal that is applied to the upper left-hand side of the HSL'D, rFig. 5.
Fig. 6, RxDDS - Generation of Dic~~tal IF for Receive Channels The exact intermediate frequency to tune to to for a receive time slot is determined when the cluster controller CC (Fig. 1? tells the modem which RF channel to search for the RCC message. During reception of the RCC message, fire tuning of frequency and timing is performed. The fine tuning is accomplished at the IF level using phase accumulator circuitry in the RxDDS circuit of the mocem~s DDF (Fig. 3), shown in retail in Fig. 6. The IF
freauencies are generated by repetitively accumulating, at the frequency of a digital IF master clock, a number that represents a phase step in the phase accumulator. Modem processor DSP/~M, via DSP/MDM data bus (Fig. 3) ; initially furnishes ~ 2d- _bit number F to the F~xDDS circuitry. This number is related (as will hereinafter be ~e~cribed) to the desired IF frequency required to demodulate ~ particular incoming signal cn a slot by slot b~eis. The 2~-bit number _F is loaded into one of the four registers R16-Ft46 at the lefthand side of Fig. 6. Iri the illustrative embodiment where a 16-bit processor is employed, the 24-bit frequency number F is supplied in 16-bit and e-bit segments, however, to simplify the drawing, .the ~24-bit number is Shown as being entered into a composite 24-bit register: Each of registers R16-R46 ie dedicated to one of the receive time slots. Since the RCC message is expected in the first Rx time Slot, the 24-bit number is loaded into the corresponding one of the four i~~g~sters ~t16-R46, e.g., register R16. At the appropr~~te slot~count for the.first Rx tune slot , register R16' s ~contertt~ ' are presented to synchronization register 602, irhose output.' is then presented to the upper input of adder s04: The~output of adder 604 is connected to the input o~ ecct~md~.ator register 606. The lower input of adder fio4 receives the output of register 506 . Register 606 is clocked by the 21..75 MHz DDS
clock and its contents are, accordingly, periodically re-entered into adder 504:
The periodic reentry of the contents of register 606 into adder 604 causes adder 604 to count up from the number F first received from register R16.~' Eventually, adder 606 reaches the msximum number that'it~cari hold, it overflows, and the count recommences ftom a lbcJ residual va111e . This has the effect of multiplying the DDS master clock frequency by a fractional value, to make a receive IF local oscillator signal having that fractionally multiplied frequency, with a "sawtooth" wave~orm. Since Yegister 606 is a 24-bir_ register, it overflows when its contents reaches 2". Register 606 therefore effectively divides the frequency of the DDS clock by 2'j and simultaneously rnuitiplies it by F. The circuit is termed a ''phase accumulator" because the instantaneous output number in register 606 indicates the instantaneous phase of the IF
frec~t:ency.

The accumulated phase from register 606 is applied ~o sine approximation circuit 622, which is more fully described in U. S. Patent No. S,OOE3,900, "Subscriber Unit for P7ireless Digital Subscriber Communication System."

Circuit 622 converts the sawtooth waveform oz register 606 into a sinusoidal waveform. The output of circuit 622 is 1o resynchronized by register 524 and then applied to one input of adder 634, in a noise shaper consisting of adder 634 and noise shaper filter 632. The output of filter 632 is applied to the other input of adder o'34. The output of adder 634 is connected to the data input of filter 632 and 1S to the input of resynchronizing register 636. This variable coefficient noise shaper filter 632 is more fully described in U. S. Patent 5,008,900. Tile noise shaper characteristics are controlled, on a slot by slot basis, by a 7-bit noise shaper control field which is combined with 2o the least significant byte of tile frequency number field received from the DSP/M17M BUS. The noise shaper may be er_abled or disabled, up to 16 filter coefficients may be chosen, rounding may be enabled or disabled, and feedback characteristics within the noise shaper may be altered to 25 allow the use oz an 8 bit output DAC (as shown in Fig. 6) i or a 10 bit output DAC (not shown) by asserting the appropriate fields in the noise shaper control field for each slot, in the four registers RN16-RN46. Multiplexes MPX66 selects one of the four registers RN16-RN46 for each 30 slot, and the resulting information is resynchronized by register 630 and presented to the control input of noise shaper filter 632.

Fia. 7, DDF - D~ gital IF Modal anon Tine exact IF frequency for any of the transmit 35 channels is generated on a slot by slot basis by the TxDIF

circuitry in the modem DDF b.~ock (r ig, 3 ) ; ' 'Which is shown in detail iri Fig: 7. On a sjot by got basis, an FIR
transmit filter (not qhown) shapes the 16 kilb~ymbol per second complex (I, Q) information signal data stream received from the modem DSP that will modulate each of the generated IF frequencies, The informatioiz'sig~al data stream must be shaped so that it can be transmitted in the limited bandwidth permitted in the assigned RF channel.
The initial processing of the information signal includes l0 FIR pulse shaping to reduce the bandwidth to +/- Zp FIR pulse shaping produces in-phase and qvadrature components to be used in modulating the generated IF.
After pulse shaping; several stages of linear interpolation are employed. Initial interpolation is 1S . performed to increase the eample~rate of the baseband signal, followed by additional ~interpolations;~ which ultimately increase the sample rate and' ttie ~ frequency at Which the main spectral replicationsl~occux~to 21.76 MHz.
Suitable int~rpolative techriiqtide -are - described, for 2d example, in ~'Multirate Digital Signal processing" by Crochiere and Rabiner; Prentice-Hall 1993. The iii-phase arid quadrattlre components of the shaped and interpolated modulating signal are applied td~"the' I wild Q inputs of mixers MXI arid MXQ of the modulator' pottioil of the 25 circuitry shbc~m in Fig ~ 7 . ~ ' At the left-hand side o~ Fi~v 7'is the circuitry for digit~.lly g~rierating the transmit IF fz-eqtiehcy: The exact intermediate frequency to be generated is determined when the base station tells cluster coiitrbllet~CC (Fig. 1) which 30 slot number and RF channel to asaigri~td a time slot supporting a. particular conversation, A 24=bit 'number which identifies the IF frequency ~to a high' degree of resolution (illustratively +/- 1.3 Hz), is supplied by processor DSP/l~7hf ('Fig. 3)' over the DSP/MDM data bug. The 35 24-bit frequency number is registered in a respective one of 24-bit registers R17-R47. Registers R17-R47 are each dedicated to a pa.rtictllar one of the four Tx time slots .

A slot counter (not shown) generates a repetitive two-bit time slot count derived from t'ne synchronizatior_ si~~-rals available ove= the backoiane, as Drevlcusly described. Th' time slot court signal occurs every 11.25 ms, regardless of whether the time slot is used for DPSK, QPSK or 16PSK modulation. P7her_ the time slot to which the frequency will be assigned is reached by the slot counter, the slot count selects the corresponding one of registers R17-R47, using multiplexes MPX71, to deliver its contents l0 to ;esynchronizing register 702 and ultimately, the upper input of adder 704. Accordingly, a different (or the same) 24-bit IF frequency can be used for each successive time slot. The 24-bit frequency number is used as the phase step for a conventional phase accumulator circuit 1~ comprising adder 704 and register 706. The complex c«rrier is generated by converting the sawtooth accumulated phase information in register 706 to sinusoidal and cosinusoidal waveforzns using cosine approximation circuit 708 and sine approximation circuit 722. Sine and cosine approximation 20 circuits 708 and 722 are more fully described in U. S.
Patent No. 5,008,900.
The outputs of circuits 708 and 722 are resynchronized by registers 710 and 724, respectively, and applied to mixers 712 and 726, respectively. The outputs of mixers 25 712 and 714 are applied to resynchronizing registers 714 and 728, respectively. Mixers 712 and 714 together with adder 716 comprise a conventional complex (I, Q) modulator.
The output of adder 716 is multiplexed with the cosine IF
reference by multiplexes 718, which is controlled by signal 30 DIF_CW_MODE from an internal register (not shown) of DDF
ASIC 450 (Fig. 3). The output of multiplexes 718 is resynchronized by register 720, whose output is connected to a variable coefficient noise shaper circuit, of a type as previously described in connection with Fig. 6, 35 consisting oz adder 734 and filter 732, with associated control registers RN17-RN47, control multiolexer MaX76, and resynchronizing registers 73o and 736.

This noise shaper compensates for the quantization noise caused by the finite resolution (illustratively +/-one-half of the least significant bit) of the digital to analog conversion. Since quaritization noise id l.iniformly distributed, its spectral characteristics ~pp~ar similar to white Gaussian noige. The noise power that falls within the transmitted signal bandwidth, which is relatively narrow compared to the sampling rate, can be redciced in the same ratio as the desired bandwidth bears to the sampling rate. For,- example, assuming the modulating signal has a kHz bandwidth and the dairipling rate is 20 i~-iz,~ the signal to noise ratio improvement woltld be 1000:1 or 60 dB.
The noise shaper characteristics are controlled, on a slot by slot basis, by a 7-bit noise shaper control field as 15 described in connection with Fig.~6.
Fig. a System Chock Generation It is an important aspect of Qur invention that voice quality is maintained despite the physical ~geparation between the base station and the recriote cluster. Timing 20 variations between the base station and the cluster, a9 . well a~ timing variations in the decoding and encoding of speech signals, will lead to various forms of voice duality degradation, heard as extraneous pops~and clicks in the voice signal. In accordance with our invention, strict congruency of timing is assured by syxlchronizing all timing signals, especially those used to clock the A/D converter, the voice codecs on quad line modules 101-108, as well as PCM highways 200 and 500, to the forward radio channel.
Referring to Fig. 8, the principal clocks used in the system are derived from a 21.75 MHz oscillator (not shown), which provides its signal at the lsfthand side of Fig. 8.
The 21.75 MHz signal is used to syncflronize a 64 k~iz sample clock to symbol transition tirries ix~ the received radio signal. More particularly, the 21:75 i~-iz signal is first 3S divided by 6.8 by fractional clock divider circuit 802, r.

which accomplishes this f=actional division by dividing the 21..76 Mhz clock by five diZferent ratios in a repetitive sequence of 5 , B , 6 , 8 , 6 , to p-c;c'uoe a coc:1 wi=;-~ an average freat:ency of 3 . 2 f~iHz .
Programmable clock divider 806 is of a conventional type and is employed to divide the 3.2 h~iz clock by a divisor whose exact magnitude is determined by the DSP/MDM.
Normally, programmable clock divider 806 uses a divisoL of SO to produce a 64 kHz sampling clock signal at its output.
l0 The 64 kHz sampling clock output of divider 806 is used to strobe receive channel A/D convertor 804 (also shown in Fig. 3). A/~7 converter 804 converts the received IF
samples into digital form, for use by the DsP/MDM
processor.
15 Still referring to Fig. 8, the DSP/1~M processor acts as a phase/frequency comparator to calculate the phase error in the received symbols from their ideal phase values , using the 64 kHz sampl ing cl ock to determine the moments when the phase error is measured. The DSP/i~M
20 processor determines the fractional timing correction output ftc. Fractional timing correction output ftc is applied to programmable divider 806 to determine its divide ratio. If the 64 kHz sampling clock is at a slightly higher frequency than the symbol phase transitions in the received 25 IF signal, the DSP/MDM processor outputs a fractional timing correction that momentarily increases the divisor of divider 806, thus extending the phase and lowering the average freauency of the 64 kHz sampling clock output of divider 806. Similarly, if the 64 kHZ sampling clock 30 frequency is lower than the frequency of the received symbol phase transitions, the divide ratio or divider 806 is momentarily reduced.
The 64 kHz sampling clock at the output oz Drogrammable clock divide= 806 is =requency-multiplied by 35 a Lacto= o= 64, using a convention~:i analog phase locked multiplier circuit 808, to make a .095 Ng~2 clock. The 4 .096 (~-~z clock is. delivered to time s:loc interchanger~~ 31o and 320 (see Fig. 1). Time slot int~rchangers 3I0 and 320 divide the 4 . 096 i~'i~iz clock by two, to form two 2 _ 048 MHz clocks, which are used by tire voice codecs on line modules 101-108 (Fig. 1) to sample and convert analog voice inputs to PCM voice. Providing a commonly derived 2.048 MHz clock to the voi ce codecy which is in eyncflroilism with the radio-derived 64 kHz sampling clock assures that there will be no slips between the two clocks. As mentioned, such slips would other~tise result in audible voice quality degrad~tions, heard as extraneous pons and clicks in the voice signal.
The foregoing has described an illustrative embodiment of our invention. Further and other embodiments may be devised by those skilled in the art without, however, departing from the spirit and acdpe of our invention.
Among such variations, for example,. would be increasing the sampling rate on the PCM buses to make~possible the handling of both PCM speech and signalling on the same time slot interchanges without degrading the quality of the PCM
speech coding. In addition, the circuitry of the ASIC
transmit pulse shaping may be modified to permit forms of modulatioin dther than PSK, such as QAM and FM, to be employed. It should be undetstood that although the illustrative embodiment has described the use of a common pool of frequency~'agile modems for serving ~a group of t~mote subscriber Stations iri a modular cluster, a similar group of frequency agile modernd'may be employed at the base station to shpport communications between the cluster and any number of remote subscribe Stations. 'Lastly, it should be apprciated that a transmission fiedium other than over the aii radio, such as coaxial cable or fiber optic cable, may be employed.

Claims (21)

CLAIMS:

1. In a radio telephone system having a central office base station, a plurality of remote subscriber stations, means defining a repetitive set of time slots for supporting radio telephone calls between said subscriber stations and said central office station, a group of modems, each of said modems being capable of handling a plurality of said telephone calls on successive ones of said time slots, the process of assigning said time slots to said calls, comprising the steps of:
(a) ascertaining which active modems have idle time slots;
(b) assigning preference ratings to said idle time slots at said group of modems;
(c) ascertaining which of said time slots has the highest of said preference ratings;
(d) assigning the time slot corresponding to said ascertained highest preference rating to the next one of said calls.

2. In a radio telephone system having a central office base station, a plurality of remote subscriber stations, means defining a repetitive set of time slots for supporting radio telephone calls between said subscriber stations and said central office station, a group of modems, each of said modems being capable of handling a plurality of said telephone calls on successive ones of said time slots, the process of assigning said time slots to said calls comprising the steps of:
(a) ascertaining which active modems have idle time slots;
(b) assigning preference ratings to said idle time slots at said group of modems;
(c) ascertaining which of said time slots has the highest of said preference ratings;
(d) assigning the time slot corresponding to said ascertained highest preference rating to the next one of said calls;
wherein said step of assigning said preference ratings includes the steps of:
i. ascertaining which time slots are in use by more than one modem:
ii ascertaining which modems have adjacent time slots available to handle a call;
iii. ascertaining whether a modem has a time slot available to handle the synchronization task.

3.The process of :assigning time slots to calls according to claim 2 wherein said step of assigning said preference ratings assigns the highest priority to a slot which is (a) available on an active modem, (b) is not a slot available for the synchronization task, (c) leaves adjacent slots available for QPSK
calls and (d) does not increase the number of modems using a slot beyond a predetermined threshold.

4. The process of assigning time slots to calls according to claim 3 wherein said step of assigning said preference ratings assigns the second highest priority to a slot which is (a) available on an active modem, (b) leaves adjacent slots available for QPSK calls, and (c) does not increase the number of modems using a slot beyond a predetermined threshold.

5. The process of assigning time slots to calls according to claim 4 wherein said step of assigning said preference ratings assigns the third highest priority to a slot which is (a) available on an active modem and (b) leaves adjacent slots available for QPSK calls.

6. The process of assigning time slots to calls according to claim 5 wherein said step of assigning said preference rating assigns the fourth highest priority to a slot which is available on an active modem.

7. A method of minimizing synchronization delay in a radiotelephone system between a modular subscriber cluster communicating with a common base station, the modular subscriber cluster receiving repetitive time slots from the base station and having a plurality of frequency-agile modems, the method comprising the steps of:
synchronising a selected one of the modems to a selected time slot of the received time slots;
generating a frame sync signal from said selected modem; and distributing said frame sync signal to remaining ones of the plurality of modems.

8. The method according to claim 7 wherein said synchronizing step further comprises the steps of:
receiving a plurality of channel-identifying frequencies from the common base station, each of said channels containing a synchronization time slot;
instructing the plurality of modems to search said channels for said synchronization time slot;
locating within one of said channels said synchronization time slot by one of the plurality of modems; and assigning said one of the plurality of modems as said selected modem.

9. The method according to claim 7 wherein said distributing step further comprises the steps of:
sourcing said frame sync signal over a common bus coupled to all of the plurality of modems; and aligning each of the plurality of modems start frame with said frame sync signal.

10. The method according to claim a further comprising the steps of:
determining synchronization parameters for each active modem from the plurality of modems;
ascertaining reliability from said synchronization parameters;
identifying from said synchronization parameters the modem with the highest reliability; and designating said modem with the highest reliability as said selected modem.

11. A radio telephone system for supporting communications between a base station and a plurality of remote subscriber stations on repetitive time slots of high frequency radio channels comprising:
a group of modems assignable to said subscriber stations, each of said modems being capable of digitally synthesizing, modulating and demodulating a plurality of channel-identifying intermediate frequencies on successive ones of said repetitive time slots to simultaneously handle a number of communications during successive time slots;

means for sending and receiving said channel-identifying intermediate frequencies between said base station and said subscriber stations;

means for upwardly block converting all of said intermediate frequencies produced by said group of modems to said high frequency radio channels; and means for downwardly block converting said high radio frequency channels to a plurality of modulated channel-identifying intermediate frequencies.

12. A radio telephone system according to claim 11 wherein said modems are each capable of digitally synthesizing a different one of said channel identifying intermediate frequencies on a plurality of successive time slots.

13. A method of supporting communications in a radio telephone system between a base station and a plurality of remote subscriber stations on repetitive time slots of high frequency radio channels comprising the steps of:

assigning a group of modems to said subscriber stations;
digitally synthesizing, modulating, and demodulating of channel-identifying intermediate frequencies on successive ones of said repetitive time slots by anyone of said group of modems;

simultaneously handling a number of communications between said base station and said subscriber stations by using successive time slots;

sending and receiving said channel-identifying intermediate frequencies between said base station and said subscriber stations;

upwardly block converting all of said intermediate frequencies by said group of modems to said high frequency radio channels; and downwardly block converting said high radio frequency channels by said group of modems to a plurality of modulated channel-identifying intermediate frequencies.

14. A method according to claim 13 wherein anyone of said group of modems has the capacity of switching to and using a different one of said channel identifying intermediate frequencies on a plurality of successive time slots.

15. A radio telephone system having a base station and a modular subscriber cluster, where a plurality of physically adjacent subscribers sharing a common pool of frequency agile modems, comprising:

means for defining a repetitive set of time slots for signal transmission;
said cluster tracking and prioritizing all available time slots of said, common pool modems;
said cluster selecting one of available time slots based on assigned priority;
and said cluster through a plurality of channel identifying frequencies synchronizing said selected time slot of a selected one of said common pool modems to a selected time slot of the received time slots.

16. A radiotelephone system according to claim 15 wherein said means for prioritizing by setting higher priority to all of said set of time slots to one of said common pool modems before assigning a time slot to any remaining ones of said modems.

17. A radiotelephone system according to claim 16 wherein said remaining ones of said modems reside in a powered-down state until assigned to a time slot by said assigning means.

18. A radiotelephone system according to claim 15 wherein said means for synchronizing includes means for synchronizing said modems with said base station.

19. A radiotelephone system according to claim 18 wherein said means for synchronizing includes means for sequentially directing certain of said plurality of modems to search through said channel-identifying frequencies during one of said time slots.

20. A radiotelephone system according to claim 16 wherein said one of said modems assigned by said assigning means provides synchronization information to said remaining ones of said modems.

21. A radiotelephone system according to claim 20 wherein certain of said modems compute a respective set of synchronization parameters, wherein said assigning means ascertains the reliability of said respective sets of synchronization parameters, and wherein said assigning means identifies said one of said modems to deliver said synchronising information to the remaining ones of said modems.