blank-and-burst in AMPS and replaces speech data when used. It carries call release instructions, MAHO, and mobile status requests.

1.4.2 Personal Digital Cellular System

The PDC system, established in Japan, employs TDMA technique. It uses three time slots per frequency channel and has a frame duration of 20 ms. It can support three users at full-rate speech and six halfrate speech users similar to IS-54. It has a channel spacing of 25 kHz and uses π/4 DQPSK modulation. It supports a frequency-reuse plan with cluster size four and uses MAHO.

1.4.3Code Division Multiple Access Digital Cellular System (Interim Standard-95)

This CDMA digital system uses CDMA as a multiple access technique and occupies the same frequency band as that occupied by AMPS; that is, the forward-link frequency band is from 869 to 894 MHz and the reverse-link band is from 824 to 849 MHz. Forward-link and reverse-link carrier frequencies are separated by 45 MHz.

Each channel in IS-95 occupies a 1.25-MHz bandwidth and this is shared by many users. The users are separated from each other by allocating 1 of 64 orthogonal spreading sequences (Walsh functions).

The user data are grouped into 20-ms frames and are transmitted at a basic user rate of 9600 bps. This is spread to a channel chip rate of 1.2288 Mchip/s giving a spreading factor of 128.

RAKE receivers are used at both base station and mobiles to resolve and combine multipath components. During handoff the standard allows for base station diversity whereby a mobile keeps link with both the base stations and combines signals from both the stations to improve signal quality as it would combine multipath signals.

In forward-link a base station transmits simultaneously to all users using 1 of 64 spreading sequences for each user once the user data are encoded using a half-rate convolution code and are interleaved. All signals in a cell are also scrambled using a PN sequence of length 215 to reduce the co-channel interference. During the scrambling process the orthogonality between different users is preserved.

The forward channel consists of 1 pilot channel, 1 synchronization channel, up to 7 paging channels, and up to 63 traffic channels. The pilot channel transmits higher power than other channels and is used by mobiles to acquire timing for forward channel and to compare signal strength of different base stations. It also provides phase reference for coherent detection.

The synchronization channel operates at 1200 bps and broadcasts a synchronization message to mobiles. The paging channels are used to transmit paging messages from the base station to mobiles and to operate at 9600, 4800, or 2400 bps. The traffic channels support variable data rate operating at 9600, 4800, 2400, and 1200 bps.

On reverse channels, mobiles transmit asynchronously to the base, and orthogonally between different users in a cell is not guaranteed. A strict control is applied to the power of each mobile so that a base station receives constant power from each user, thus eliminating the near–far problem. Power control command is sent by the base to mobiles at a rate of 800 bps. The reverse channels are made up of access channels and reverse traffic channels.

The reverse channels contain up to 32 access channels per paging channel, operate at 4800 bps, and are used by mobiles to initiate communication with base and to respond to paging messages. The reverse traffic channel is a variable data rate channel and operates similar to the forward channels at 9600, 4800, 2400, and 1200 bps.

1.4.4 Pan European Global System for Mobile Communications

The “Groupe Special Mobile” was established in 1982 to work toward the evolution of digital system in Europe, and its work now has become the Global System for Mobile (GSM) Communications system. Two frequency bands have been allocated for this system. The primary band is at 900 MHz and the

© 2002 by CRC Press LLC

15

26

Frequency

(kHz)

200

Time (ms)

FIGURE 1.4 GSM multiple access.

secondary band is at 1800 MHz. The description here mainly concerns the primary band. It has been divided into two subbands of 25 MHz each, separated by 20 MHz. The lower band is used for uplink and the upper band is used for downlink. Operators are assigned a portion of the spectrum for their use.

The carrier frequencies are separated by 200 kHz. This gives the total number of frequency channels over the 25-MHz band as 124. The first carrier is at 890.2 MHz, the second one is at 890.4 MHz, and so on. These carriers are numbered as 0, 1, 2 and so on, respectively. Similarly, 374 different carriers are allocated in the secondary band, which is 75 MHz wide.

1.4.5.1 Multiple Access Scheme

GSM employs a combination of TDMA and FDMA schemes with slow frequency hopping. GSM transmission takes place by modulating a bundle of about 100 b known as a burst. A burst occupies a finite duration in time and frequency plane. The center frequency of these bursts is 200 kHz apart and these are 15/26 ms in duration.

The duration of these bursts is the time unit and is referred to as burst period (BP). Thus, time is measured in BP. When this burst is combined with slow frequency hopping, a typical transmission appears as shown in Fig. 1.4. The hopping sequence is selected randomly using a PN sequence.

A channel is defined by specifying which time slot it may use for transmit burst. That means specifying time instant and specific frequency. Time slots of a channel are not contiguous in time. All time slots are numbered and the description of a channel sent to the mobile by the base refers to this numbering scheme. The numbering is cyclic and each time slot is uniquely identified in this cycle, which is about 3.5 h (3 h 28 min 53 s, and 760 ms).

Many types of channels are defined in GSM, and each are cyclic. The simplest cycle is of 8 BP. This cycle of eight time slots is also called a slot, which is 60/13 ms in duration. The duration of the BP is chosen such that 26 slots equal to 120 ms, which is a multiple of 20 ms to obtain synchronization with other networks such as the Integrated Services Digital Network (ISDN).

A full dedicated channel is thus cyclic in 120 ms and uses 26 slots. Note that each slot is made up of eight time slots of 15/26 ms known as BP. Out of these 26 slots, 24 slots are used for traffic burst, 1 slot is used for control burst, and 1 slot is not used.

The transmission between the uplink and the downlink is not independent. Transmission in uplink follows the downlink reception by 3 BP later. When the mobile is far from the base, the mobile advances its transmission from the reception to compensate the propagation delay. Hopping sequences in the uplink and the downlink are also related. The hopping sequence in the uplink direction is derived from the one in the downlink direction in by adding 45 MHz.

© 2002 by CRC Press LLC