Explain the some of the important terms of CDMA digital cellular systems.

CDMA development started in early 1989 after the NA-TDMA standard (IS-54) was established. A CDMA demonstration to test its feasibility for digital cellular systems was held in November 1989. The CDMA “Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System” was issued as IS-95 (PN-3118, Dec. 9, 1992). CDMA uses the idea of tolerating interference by spread-spectrum modulation. The power control scheme in a CDMA system is a requirement for digital cellular application. However, it was a challenging task and has been solved. Before describing the structure of the system, we list the key terms of CDMA systems.

Terms of CDMA Systems:

Active set: The set of pilots associated with the CDMA channels containing forward traffic channels assigned to a particular mobile station (MS).

CDMA channel number: An 11-bit number corresponding to the center of the CDMA frequency assignment.

Code channel: A sub channel of a forward CDMA channel. A forward CDMA channel contains 64 code channels. Certain code channels are assigned to different logic channels.

Code channel zero: Pilot channel.

Code channels 1 through 7: Either paging channels or traffic channels.

Code channel 32: A sync channel or a traffic channel.

The remaining code channels are traffic channels.

Code symbol: The output of an error-correcting encoder.

Dim-and-burst: A frame in which the primary traffic is multiplexed with either secondary traffic or signal traffic. It is equivalent to the blank-and-burst function in AMPS.

Forward CDMA channel: Contains one or more code channels.

Frame: A basic timing interval in the system. For the access channel, paging channel, and traffic channel, a frame is 20-ms long. For the sync channel, a frame is 26.666-ms long.

Frame offset: A time skewing of traffic channel frames from system time in integer multiples of 1.25ms. The maximum frame offset is 18.75ms.

GPS (Global Position System): System used for providing location and time information to the CDMA system.

Handoff (HO): The act of transferring communication with a mobile station from one base station to another.

Hard HO: Occurs when (1) the MS is transferred between disjoint active sets, (2) the CDMA

frequency assignment changes, (3) the frame offset changes, and (4) the MS is directed from a CDMA traffic channel to an analog voice channel but not vice versa.

Soft HO: HO from CDMA cell to CDMA cell at the same CDMA frequency.

Idle HO: Occurs when the paging channel is transferred from one base station (BS) to another.

Layering: A method of organization for communication protocols. A layer is defined in terms of its communication protocol to a peer layer.

Layer 1: Physical layer presents a frame by the multiplex sub layer and transforms it an over-the-air waveform.

Layer 2: Provides for the correct transmission and reception of signaling messages.

Layer 3: Provides the control of the cellular telephone system. The signaling messages

originate and terminate at layer 3.

Long code: A PN (pseudo noise) sequence with period using a tapped n-bit shift register.

Modulation symbol: The output of the data modulator before spreading. There are 64 modulation symbols on the reverse traffic channel, 64-ary orthogonal modulation is used, and six code symbols are associated with one modulation symbol. On the forward traffic channel, each code symbol (data rate is 9600 bps) or each repeated code symbol (data rate is less than 9600 bps) is 1 modulation symbol.

Multiplex option: The ability of the multiplex sub layer and lower layers to be tailored to provide special capabilities. A multiplex option defines the frame format and the rate decision rules.

Multiplex sublayer: One of the conceptual layers of the system that multiplexes and demultiplexes primary traffic, secondary traffic, and signaling traffic.

Nonslotted mode: An operating mode of an MS in which the MS continuously monitors the paging channel.

Null traffic data: A frame of sixteen 1’s followed by eight 0’s sent at the 1200 bps rate. Null traffic channel data serve to maintain the connectivity between MS and BS when no service is active and no signaling message is being sent.

Paging channel: A code channel in a forward CDMA channel used for transmission of (1) control information and (2) pages from BS to MS. The paging channel slot has a 200-ms interval.

Power control bit: A bit sent in every 1.25 ms interval on the forward traffic channel to the MS that increases or decreases its transmit power.

Primary CDMA channel: A pre assigned frequency used by the mobile station for initial acquisition.

Primary paging channel: The default code channel (code channel 1) assigned for paging.

Primary traffic: The main traffic stream between MS and BS on the traffic channel.

Reverse traffic channel: Used to transport user and signaling traffic from a single MS to one or more BSs.

Shared Secret Data (SSD): A 128-bit pattern stored in the MS.

SSD is a concatenation of two 64-bit subsets.

SSD-A is used to support the authentication.

SSD-B serves as one of the inputs to generate the encryption mask and private long code.

Secondary CDMA channel: A pre assigned frequency (one of two) used by the mobile station for initial acquisition.

Secondary traffic: An additional traffic stream carried between the MS and the BS on the traffic channel.

Slotted mode: An operation mode of MS in which the MS monitors only selected slots on the paging channel.

Sync channel: Code channel 32 in the forward CDMA channel which transports the synchronization message to the MS.

Pilot channel: An un modulated, direct-sequence (DS) signal transmitted continuously by each CDMA BS. The pilot channel allows a mobile station to acquire the timing of the forward CDMA channel, provides a phase reference for coherent demodulation, and provides a means for signal strength comparisons between base stations for determining when to hand off.

System time: The time reference used by the system. System time is synchronous to universal time coordination (UTC) time and uses the same time origin as GPS time. All BSs use the same

system time. MSs use the same system time, offset by the propagation delay from the BS to the MS.

Time reference: A reference established by the MS that is synchronous with the earliest arriving multipath component that is used for demodulation. The time reference establishes transmit time

and the location of zero in PN space.

Walsh chip: The shortest identifiable component of a 64-walsh function. On the forward CDMA channel, one chip equals 1/1.2288 MHz or 813.802 ns. On the reverse CDMA channel, one chip equals 4/1.2288 MHz or 3255 ns.