Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks

1. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology • 4. Basic Operation of Mobile Communication System • 4.1 Basic principle of CDMA • A spread-spectrum modulation technique must fulfill two criteria: • The transmission bandwidth must be much larger than the information bandwidth. • The resulting radio-frequency bandwidth is determined by a function other than the information being sent (so the bandwidth is statistically independent of the information signal). This excludes modulation techniques like frequency modulation (FM) and phase modulation (PM).

2. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology The ratio of transmitted bandwidth to information bandwidth is called processing gain Gp of the spread-spectrum system, Gp = Bt / Bi (1) Where Bt is the transmission bandwidth and Bi is the bandwidth of the information-bearing signal. Because of the coding and the resulting enlarged bandwidth, SS signals have a number of properties that differ from the properties of narrowband signals. The most interesting from the communication systems point of view are discussed below:

3. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology 1 1 2 2 Multiple access capability If multiple users transmit a spread-spectrum signal at the same time, the receiver will still able to distinguish between the users provided each user has a unique code that has a sufficiently low cross-correlation with the other codes.

4. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology 1 & 2 1 2 Figure 4.a1: Principle of spread-spectrum multiple access

5. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology I S S S I Figure 4.a2: Interference rejection • Protection against multipath interference • Privacy • Interference rejection • Anti-jamming capability

6. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology • Low probability of interception (LPI) • Because of its low power density, the spread-spectrum signal is difficult to detect and intercept by a hostile listener. 4.2 Spread-Spectrum Multiple Access 4.2.1 Direct Sequence Figure 4.a3: Block diagram of a DS-SS transmitter

7. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology Figure 4.a4: Generation of a BPSK-modulated SS signal

8. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology Figure 4.a5: Receiver of a DS-SS signal

9. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology The binary data signal modulates a RF carrier. The modulated carrier is then modulated by the code signal. This code signal consists of a number of code bits called “chips” that can be either +1 or –1. To obtain the desired spreading of the signal, the chip rate of the code signal must be much higher than the chip rate of the information signal. The rate of the code signal is called the chip rate; one chip denotes one symbol when referring to spreading code signals. In figure 4a4, 10 code chips per information symbol are transmitted (the code chip rate is 10 times the data rate) so the processing gain is equal to 10.

10. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology 4.2.2. Frequency hopping Figure 4.a6: Block diagram of an FH-CDMA transmitter and receiver

11. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology Example: Spreading of user data A, B & C CH code for A In Air

12. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology Example: Despreading of user data A, B & C Same for user B & C

13. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology 4.2 Spreading Codes Spreading codes can be divided into pseudo-noise(PN) codes and orthogonal codes. PN codes are pseudo-random codes generated by a feedback shift register. The most commonly considered PN codes for DS-CDMA systems are generated using linear shift registers. The cross-correlation between orthogonal codes is zero for a synchronous transmission. Orthogonal codes such as Walsh sequences are typically used for channel separation in DS-CDMA systems. 4.2.1 Basic Properties of Spreading Codes In a DS-CDMA transmitter, the information signal is modulated by a spreading code and in the receiver it is correlated with a replica of the same code. Thus, low cross-correlation between the desired and interfering users is important to suppress the multiple access interference. Good autocorrelation properties are required for reliable initial synchronization, since large sidelobes of the autocorrelation function might lead to erroneous code synchronization decisions.

14. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology Furthermore, good autocorrelation properties are important to reliably separate the multipath components. Since the autocorrelation function of a spreading code should resemble, as much as possible, the autocorrelation function of white Gaussian noise, the DS code sequences are also called pseudo-noise (PN) sequences. The autocorrelation and cross-correlation functions are connected in such a way that it is not possible to achieve good autocorrelation and cross-correlation values simultaneously. Figure 4.b1 illustrates auto and cross-correlation functions for a 31 chip length M-sequence.

15. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology 31 bit (5,3) autocorrelation 31 bit (5,3) and 31 bit (5,4,3,2) crosscorrelation Figure 4.b1: Auto and cross-correlation functions

16. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology Typically, PN sequences are generated with a feedback shift register generator, depicted in Figure 4.b2. The output of the shift register cells are connected through a linear function formed by exclusive-or (XOR)-type logic gates into the input of the shift register. Linear Feedback 1 2 3 N Figure: 4.b2 Feedback Shift Register

17. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology + Stage 1 Stage 2 Stage 3 4.2.2 Case Study Let us begin with a particular example shown in Figure 4b.3. The shift register is initiated with contents 111. Figure 4b.3: Shift generator for m=3

18. Clock Pulse Stage 1 Stage 2 Stage 3 Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology 0 1 1 1 1 0 1 1 2 0 0 1 3 1 0 0 4 0 1 0 5 1 0 1 6 1 1 0 7 1 1 1 We start from clock timing 1 and proceed. The status at different timing is summarized in Table 4.2.1. Contents of shift register states

19. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology Note that at clock pulse 7, the state contents are the same as that for clock pulse 0, implying that a new period begins. Remark 1 1. The stage contents include all possible three-tuple except the all-zero vector. 2. The last column 1 1 1 0 0 1 0 represents a PN sequence. 3. The length of produced PN sequence is equal 23 – 1=7 4. All the column contain the same elements and furthermore, one is a cyclic- shifted version of another. Each linear feedback shift register has its special feedback connections, which can be represented by a polynomial. For the relation shown in the shift register above can be expressed as x3 = x + 1 (1)

20. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology Or equivalently we can define a polynomial f(x) = x3 + x + 1 (2) This is the generator polynomial. In this course, we adopt the convention that the highest order is on the leftmost. Note here, all additions are modulo-2, a negative sign has the same effect as a positive sign. A general polynomial can be written as xm = am-1 zm-1+………+a1z+1 (3) Its implementation is illustrated in Figure 4b.4. The coefficients a’s can take on the value of one or zero.

21. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology + + + am-1 a2 a1 Figure 4b.4: Implementation of a polynomial

22. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology • 4.2.2 Three Types of Codes used in 3G system • a) Scrambling Code : Ways to separate cells and users • b) Channelisation Code : Ways to separate different channels • c) Spreading Code : Ways to separate transmissions • Network • Common Information Dedicated Information • (Spreading Code = ( Spreading Code = • Scrambling Code ) Scrambling Code x CH code) • Terminal (UE)

23. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology 4.3 Power Control In the uplink of a DS-CDMA system, the requirement for power control is the most serious negative point. The power control problem arises because of the multiple access interference. All users in a DS-CDMA system transmit the messages by using the same bandwidth at the same time and therefore users interfere with one another. Due to the propagation mechanism, the signal received by the base station from a user terminal close to the base station will be stronger than the signal received from another terminal located at the cell boundary. Hence, the distant users will be dominated by the close user. This is called the near-far effect. To achieve a considerable capacity, all signals, irrespective of distance, should arrive at the base station with the same mean power. A solution to this problem is power control, which attempts to achieve a constant received mean power for each user. Therefore, the performance of the transmitter power control (TPC) is one of the several dependent factors when

24. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology deciding on the capacity of a DS-CDMA system. In contrast to the uplink, in the downlink all signals propagate through the same channel and thus are received by a mobile station with equal power. Therefore, no power control is required to eliminate near-far problem. The power control is, however, required to minimize the interference to other cells and to compensate against the interference from other cells. The worst case situation for a mobile station occurs when the mobile station is at the cell edge, equidistant from three base station. However, the interference from other cells does not very very abruptly. In addition to being useful against interfering users, power control improves the performance of DS-CDMA against fading channel by compensating the fading dips. If it followed the channel fading perfectly, power control would turn a fading channel into AWGN channel by eliminating the fading dips completely. There exists two types of power control principles: open loop and closed loop. The open loop power control measures the interference conditions for the channel and adjusts the transmission accordingly to meet the desired frame error rate (FER) target.

25. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology channel and adjusts the transmission accordingly to meet the desired frame error rate (FER) target. However, since the fast fading does not correlate between uplink and downlink, open loop power control will achieve the right power target only on average. Therefore, closed loop power control is required. The closed loop power control measures the signal-to-interference ratio (SIR) and sends commands to the transmitter on the other end to adjust the transmission power. IS-95 has three different power control mechanisms. In the uplink, both open loop and fast closed loop power control are employed. In the downlink, a relatively slow power control loop controls the transmission power

26. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology 4.3.1 Open Loop Power Control Figure 4.b5 : Uplink Open Loop Power Control Principle

27. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology The open loop power control has two main functions: it adjusts the initial access channel transmission power of the mobile station and compensates large abrupt variations in the pathloss attenuation. The mobile station determines an estimate of the pathloss between the base station and mobile station by measuring the received signal strength at the mobile using an automatic gain control (AGC) circuitry, which gives a rough estimate of the propagation loss for each user. The smaller the received power, the larger the propagation loss, and vice-versa. The transmit power of mobile station is determined from the equation: Mean output power (dBm) = -mean input power (dBm) + offset power + parameters (4) The offset power for the 800-MHz band mobiles (band class 0) is –73dB and for the 1900-MHz band mobiles (band class 1) –76dB. The parameters are used to adjust the open-loop power control for different cell sizes and different cell effective radiated power (ERP) and receiver sensitivities. These parameters are initially transmitted on the synchronization channel.

28. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology 4.3.2 Closed Loop Power Control Since the IS-95 uplink and downlink have a frequency separation of 20 MHz, their fading process are not strongly correlated. Even though the average power is approximately the same, the short term power is different, and therefore, the open loop power control cannot compensate for the uplink fading. To account for the independent of the Rayleigh Fading in the uplink and downlink, the base station also controls the mobile station transmission power. Figure 4.b6 illustrate the closed loop power control.

29. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology Figure 4.b6: Closed Loop Power Control Principle

30. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology 4.3.3 Downlink Slow Power Control The base station controls its transmission power to a given mobile station according to the pathloss and interference situation. The main purpose of the slow downlink power control is to improve the performance of mobile stations at a cell edge where the signal is weak and the interfering base station are strong. The downlink power control mechanism is as follows. The base station periodically reduces the transmitted power to the mobile station. The mobile station measures the frame error ratio (FER). When the FER exceeds a predefined limit, typically 1%, the mobile station request additional power from the base station. This adjustment occurs every 15 to 20 ms. The dynamic range of the downlink power control is only ± 6 dB. Both periodic and threshold reporting may be enabled simultaneously, either one of them may be enabled, or both forms of reporting may be disabled at any given time.

31. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology • 4.3.4 Power Control Criteria • As discussed earlier, all wideband CDMA proposals use open and fast closed loop power control methods. Depending on the power control criteria, several different algorithms can be derived. Most typical criteria are: • Path loss based power control; • Quality-based power control. • Normally, the power control algorithm is a combination of these two basic criteria. Quality can be measured through Signal to Interference Ratio (SIR). Since different SIRs correspond to the same Frame Error Rate (FER) in different radio environments, we need to have a function that maps the desired FER into the required SIR target. This is performed by continuously measuring the FER and SIR, and then adjusting the SIR target to produce the desired FER.

32. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology 4.3.5 Power Control Step Size Power control step size defines how much a power control command changes the transmission power. Either a simple up/down adjustment or several power adjustment levels can be used. Typical step size are between 0.5 and 1 dB. It should be noted that power control adjustment is relative to the previous power setting, since an absolute power setting would require extremely accurate, and thus expensive, power control circuitry.

33. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology 4.4 Soft Handover In soft handover a mobile station is connected to more than one base station simultaneously. Soft handover is used in CDMA to reduce the interference into other cells and to improve performance through macro diversity. Softer handover is a soft handover between two sectors of a cell. A separate pilot channel is usually used for the signal strength measurements for handover purposes. Figure 4.b7 illustrate the soft handoff principle with two base stations involved. In the uplink the mobile station signal is received by the two base stations, which, after demodulation and combining, pass the signal forward to the combining point, typically to the base station controller (BSC). In the downlink the same information is transmitted via both base station, and the mobile station receives the information from two base stations as separate multipath signals and can therefore combine them.

34. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology Figure 4.b7 Principle of soft handover with two base station transceivers (BTS).

35. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology Summary: • Make Before Break • Soft Handoff between neighbouring cells controlled by same • switch • Softer handover between sectors of the same cell

36. Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology • In IS-95, the handover decision is based on the pilot strength measurements of the downlink only. In wideband CDMA for third generation systems with asymmetric traffic, more decision parameters are needed. At least the following parameters can be identified: • Distance attenuation; • Uplink interference; • Downlink interference.