Electromagnetic Propagation Path

The electromagnetic propagation takes on an energy propagation mode. During the propagation, the electric field is vertical to the magnetic field, both vertical to the propagation direction. Through interaction between the electric field and the magnetic field, the energy is propagated to the distance, 

just like propagation of water waves. 

When the radio wave propagates in the air, the electric field direction changes regularly. This phenomenon is known as polarization of radio wave. The electric field direction of radio wave is known as radio wave polarization direction. 

- If the electric field direction of radio wave is vertical to the ground, the radio wave is vertical polarization wave. 

- If the electric field direction of radio wave is parallel with the ground, the radio wave is horizontal polarization wave.

Radio wave can be propagated from the transmitting antenna to the receiving antenna in many ways: perpendicular incidence wave or ground refraction wave, diffraction wave, troposphere reflection wave, ionosphere reflection wave, as shown in the diagram. As for radio wave, the most simple propagation mode between the transmitter and the receiver is free space propagation.  One is perpendicular incidence wave; the other is ground reflection wave. The result of overlaying  the perpendicular incidence wave and the reflection wave may strengthen the signal, or weaken the signal, which is known as multi-path effect. Diffraction wave is the main radio wave signal source for shadow areas such building interior. The strength of the diffraction wave is much dependent of the propagation environment. 

The higher the frequency is, the weaker the diffraction signal will be. The troposphere reflection wave derives from the troposphere. The heterogeneous media in the troposphere changes from time to time for weather reasons. Its reflectance decreases with the increase of height. This slowly changing reflectance causes the radio wave to curve. The troposphere mode is applicable to the wireless communication where the wavelength is less than 10m.

Ionosphere reflection propagation: When the wavelength of the radio wave is less than 1m (frequency is greater than 300MHz), the ionosphere is the reflector. There may be one or multiple hops in the radio wave reflected from the ionosphere, so this propagation is applicable to long-distance communication. Like the troposphere, the ionosphere also presents the continuous fluctuation feature.

In a typical cellular mobile communication environment, a mobile station is always far shorter than a BTS. The direct path between the transmitter and the receiver is blocked by buildings or other objects. Therefore, the communication between the cellular BTS and the mobile station is performed via many other paths than the direct path. In the UHF band, the electromagnetic wave from the transmitter to the receiver is primarily propagated by means of scattering, namely, the electromagnetic wave is reflected from the building plane or refracted from the man-made or natural objects. 

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Characteristics of WCDMA FDD

The DS-CDMA system with a bandwidth of approximately 1MHz, i.e. IS-95, is generally named as narrowband CDMA system. But WCDMA owns a chip rate of 3.84Mcps, bringing approximately 5MHz carrier bandwidth.This feature enables the system to support higher bit rate, and at the same time brings other benefits, for example, increasing of multi-path diversity.

Gold sequence is used as scrambling code in WCDMA

In IS-95, only coherence detection is used in the downlink, but in WCDMA, coherence detection based on pilot symbol or common pilot is adopted for both the uplink and the downlink, increasing the coverage scope and the capacity of the uplink. IS-95 only uses closed loop power control in the uplink, while WCDMA uses this in both of the uplink and the downlink.With the closed loop power control used in the downlink, link performance and downlink capacity is improved.
IS-95 system mainly aims at macro cell. Because BTS synchronization is necessary, BTS is generally placed on the roof, etc. for the sake of receiving GPS signal. In this case, a global time reference can be used. But this application is difficult to carry out in the places where it is hard to receive the GPS signals. WCDMA system supports asynchronous BTS operation, and it may not use the global reference, thus it is different with the IS-95 system requiring BTS synchronization operating mode. Thus, the application of indoor cell and microcell is much simpler. This makes that the handover of WCDMA is slightly different with that of IS-95.

- Channel bandwidth: 5MHz

- Chip rate: 3.84Mcps

- Frame length: 10ms

- Voice coding: AMR (Adaptive Multi-Rate)

- Uplink and downlink modulation: QPSK/QPSK 

- Coherence demodulation aided with pilot

- Fast closed loop power control:  1500Hz

- Handover: soft/hard handover

- Support synchronous and asynchronous NodeB operation 

- Satisfy the minimum performance requirement of IMT2000

- Compatible with GSM-MAP core network

- Comparatively steady version R99 has been released

- Support open loop and closed loop transmit diversity mode

- Support Common Packet Channel(CPCH) and Downlink Share Channel, adapt to Internet data access mode

- Support macro diversity, selection diversity of NodeB location

- Support different fast power control algorithms and open loop, out loop power control

- Fully support UE locating services

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Multiple Access Technology

There are 3 types of multiple access technology

1.       Time division multiple access (TDMA)

2.       Frequency division multiple access (FDMA)

3.       Code division multiple access (CDMA)

Time Division Multiple Access means that the wireless carrier of one bandwidth is divided into multiple time division channels in terms of time (or called timeslot). Each user occupies a timeslot and receives/transmits signals within this specified timeslot. Therefore, it is called time division multiple access. This multiple access mode is adopted in both a digital cellular system and a GSM. TDMA is a complex architecture and the simplest case is that a single channel carrier is divided into many different timeslots, each of which transmits one-way burst-oriented information. The key part in TDMA is the user part, in which each user is allocated with one timeslot (allocated when a call begins). The user communicates with a base station in a synchronous mode and counts the timeslot. When his own timeslot comes, the mobile station starts a receiving and demodulation circuit to decode the burst-oriented information sent from the base station. Likewise, when a user wants to send any information, he should first cache the information and waits for his timeslot to come. After a timeslot begins, the information is transmitted at a double rate and next burst-oriented transmission begins to be accumulated.

Frequency Division Multiple Access:  frequency division, sometimes called channelization, means dividing the whole available spectrum into many single radio channels (transmit/receive carrier pair). Each channel can transmit one-way voice or control information. Under the control of the system, any user can be accessed to any of these channels. Analog cellular system is a typical example of FDMA structure. Similarly, FDMA can also be used in a digital cellular system, except that pure frequency division is not adopted. For example, FDMA is adopted in GSM and CDMA.

CDMA is a multiple access mode implemented by Spread Spectrum Modulation. Unlike FDMA and TDMA, both of which separate the user information in terms of time and frequency, CDMA can transmit the information of multiple users on a channel at the same time. That is to say,mutual interference between users is permitted. The key is that every information before transmission should be modulated by different Spread Spectrum Code-Sequence to broadband signal, then all the signals should be mixed and send. The mixed signal would be demodulated by different Spread Spectrum Code-Sequence at the different receiver.Because all the Spread Spectrum Code-Sequence is orthogonal,only the information that was be demodulated by same Spread Spectrum Code-Sequence can be reverted in mixed signal.  

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